担当区分：最終著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Japan Association for Earthquake Engineering  

DOI： 10.5610/jaee.21.3_119

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担当区分：最終著者   記述言語：英語   出版者・発行元：SEISMOLOGICAL SOC AMER  

DOI： 10.1785/0120200291

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担当区分：最終著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：SEISMOLOGICAL SOC AMER  

Picking of P and S waves is a fundamental process in seismology, and various kinds of picking techniques have been developed. Seismic waveforms change dramatically depending on the magnitude, the mechanism of the earthquake, and the positional relationship between the hypocenter and the seismic station.The availability of various picking techniques is supposed to be helpful for appropriately dealing with a variety of seismic records. Hence, in addition to the revision of conventional techniques, the development of new picking techniques is worthwhile. In the present study, we developed a new stochastic technique to detect P and S waves based on the statistical amplitude distribution in the seismic record amplitude.In the proposed method, the probabilistic density function (PDF) of the amplitude is calculated for each segment of seismic records, and the similarity between the PDF of the amplitude and that of the Rayleigh or Gaussian distribution is evaluated by divergence. Because Rayleigh and Gaussian distributions are typically found in amplitude distributions of highly random waves, such as coda waves, the divergence indicates the randomness of the seismic records. P and S waves are found by tracing the temporal change of the divergence.We tested the proposed method using local seismic records for a series of seismic events that occurred before and after the 2016 Kumamoto earthquake. The mean absolute errors for picking P and S waves are 2.72 x 10(-2) and 7.38 x 10(-2) s, respectively. The proposed method is a simple and new statistical picking method that enables automatic detection of P- and S-wave arrivals.

DOI： 10.1785/0120190078

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担当区分：最終著者   記述言語：日本語   掲載種別：研究論文（学術雑誌）  

DOI： 10.5610/jaee.19.4_156

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：SPRINGER HEIDELBERG  

Seismic wave propagation from shallow subduction-zone earthquakes can be strongly affected by 3D heterogeneous structures, such as oceanic water and sedimentary layers with irregular thicknesses. Synthetic waveforms must incorporate these effects so that they reproduce the characteristics of the observed waveforms properly. In this paper, we evaluate the accuracy of synthetic waveforms for small earthquakes in the source area of the 2011 Tohoku-Oki earthquake (M-JMA 9.0) at the Japan Trench. We compute the synthetic waveforms on the basis of a land-ocean unified 3D structure model using our heterogeneity, oceanic layer, and topography finite-difference method. In estimating the source parameters, we apply the first-motion augmented moment tensor (FAMT) method that we have recently proposed to minimize biases due to inappropriate source parameters. We find that, among several estimates, only the FAMT solutions are located very near the plate interface, which demonstrates the importance of using a 3D model for ensuring the self-consistency of the structure model, source position, and source mechanisms. Using several different filter passbands, we find that the full waveforms with periods longer than about 10 s can be reproduced well, while the degree of waveform fitting becomes worse for periods shorter than about 10 s. At periods around 4 s, the initial body waveforms can be modeled, but the later large-amplitude surface waves are difficult to reproduce correctly. The degree of waveform fitting depends on the source location, with better fittings for deep sources near land. We further examine the 3D sensitivity kernels: for the period of 12.8 s, the kernel shows a symmetric pattern with respect to the straight path between the source and the station, while for the period of 6.1 s, a curved pattern is obtained. Also, the range of the sensitive area becomes shallower for the latter case. Such a 3D spatial pattern cannot be predicted by 1D Earth models and indicates the strong effects of 3D heterogeneity on short-period (less than or similar to 10s) waveforms. Thus, it would be necessary to consider such 3D effects when improving the structure and source models.

DOI： 10.1186/s40623-018-0871-z

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：ELSEVIER  

Basal conditions of the Greenland Ice Sheet (GrIS) are a key research topic in climate change studies. The recent construction of a seismic network has provided a new opportunity for direct, real-time, and continuous monitoring of the GrIS. Here we use ambient noise surface wave data from seismic stations all over Greenland for a 4.5-year period to detect changes in Rayleigh-wave phase velocity between seismic station pairs. We observe clear seasonal and long-term velocity changes for many pairs, and propose a plausible mechanism for these changes. Dominant factors driving the velocity changes might be seasonal and long-term pressurization/depressurization of the GrIS and shallow bedrock by air and ice mass loading/unloading. However, heterogeneity of the GrIS basal conditions might impose strong regionalities on the results. An interesting feature is that, even at adjacent two station pairs in the inland GrIS, one pair shows velocity decrease while another shows velocity increase as a response to the high air and snow pressure. The former pair might be located on a thawed bed that decreases velocity by increased meltwater due to pressure melting, whereas the latter pair might be located on a frozen bed that increases velocity by compaction of ice and shallow bedrock. The results suggest that surface waves are very sensitive to the GrIS basal conditions, and further observations will contribute to a more direct and quantitative estimation of water balance in the Arctic region.

DOI： 10.1016/j.pepi.2017.10.010

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担当区分：最終著者   記述言語：日本語   掲載種別：研究論文（学術雑誌）  

DOI： 10.5610/jaee.18.1_77

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：SPRINGER HEIDELBERG  

We investigate the three-dimensional P- and S-wave attenuation (Q(P)(-1) and Q(S)(-1)) structures of the crust around the source region of the 2016 Kumamoto earthquakes, Japan. To estimate the attenuation structures, the path-averaged attenuation factor t* is estimated from the amplitude decay rate of the P- and S-wave spectra corrected for the source spectrum. The Q(P)(-1) and Q(S)(-1) structures are estimated by tomography using t* for the P- and S-waves, respectively. Several features are found in the attenuation structures as follows: In the source region, two high-Q(P) and high-Q(S) zones exist along the Futagawa and the Hinagu fault segments in the upper crust. The high-Q(P) and high-Q(S) zone along the Futagawa fault segment is found to include the large-slip area of the mainshock obtained from a source inversion study. In the lower crust, the low Q(P) is distributed beneath the entire source region. A low-Q(P) and low-Q(S) zone also exists beneath the Kuju and Aso volcanoes, which is consistent with the shallow limited depth extent of the seismogenic zone due to high temperature. The western edge of this zone adjoins the eastern edge of the high-Q(P) and high-Q(S) area, including the large-slip area.

DOI： 10.1186/s40623-017-0683-6

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：SPRINGER HEIDELBERG  

Seismic activity occurred off western Kyushu, Japan, at the northern end of the Okinawa Trough on May 6, 2016 (14:11 JST), 22 days after the onset of the 2016 Kumamoto earthquake sequence. The area is adjacent to the Beppu-Shimabara graben where the 2016 Kumamoto earthquake sequence occurred. In the area off western Kyushu, a M7.1 earthquake also occurred on November 14, 2015 (5:51 JST), and a tsunami with a height of 0.3 m was observed. In order to better understand these seismic activity and tsunamis, it is necessary to study the sources of, and strong motions due to, earthquakes in the area off western Kyushu. For such studies, validation of synthetic waveforms is important because of the presence of the oceanic water layer and thick sediments in the source area. We show the validation results for synthetic waveforms through nonlinear inversion analyses of small earthquakes (similar to M5). We use a land-ocean unified 3D structure model, 3D HOT finite-difference method ("HOT" stands for Heterogeneity, Ocean layer and Topography) and a multi-graphic processing unit (GPU) acceleration to simulate the wave propagations. We estimate the first-motion augmented moment tensor (FAMT) solution based on both the long-period surface waves and short-period body waves. The FAMT solutions systematically shift landward by about 13 km, on average, from the epicenters determined by the Japan Meteorological Agency. The synthetics provide good reproductions of the observed full waveforms with periods of 10 s or longer. On the other hand, for waveforms with shorter periods (down to 4 s), the later surface waves are not reproduced well, while the first parts of the waveforms (comprising P- and S-waves) are reproduced to some extent. These results indicate that the current 3D structure model around Kyushu is effective for generating full waveforms, including surface waves with periods of about 10 s or longer. Based on these findings, we analyze the 2015 M7.1 event using the cross-correlations between the observed and synthetic waveforms. The result suggests a rupture propagation toward the NNE, with a major radiation about 25 km north of the onset point.

DOI： 10.1186/s40623-017-0672-9

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担当区分：筆頭著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：SPRINGER HEIDELBERG  

A simple and efficient finite-difference scheme is developed to calculate seismic wave propagation in a partial spherical shell model of a three-dimensionally (3-D) heterogeneous global Earth structure for modeling on regional or sub-global scales where the effects of the Earth's spherical geometry cannot be ignored. This scheme solves the elastodynamic equation in the quasi-Cartesian coordinate form similar to the local Cartesian one, instead of the spherical polar coordinate form, with a staggered-grid finite-difference method in time domain (FDTD) that is one of the most popular numerical methods in seismic-motion simulations for local-scale models. The proposed scheme may be a local-friendly approach for modeling on a sub-global scale to link regional-scale and local-scale simulations. It can be easily implemented using an available 3-D Cartesian FDTD local-scale modeling code by changing a very small part of the code. We implement the scheme in an existing Cartesian FDTD code and demonstrate the accuracy and validity of the present scheme and the feasibility to apply it to real large simulations through numerical examples.

DOI： 10.1186/s40623-017-0651-1

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記述言語：英語   出版者・発行元：SPRINGER HEIDELBERG  

DOI： 10.1186/s40623-016-0597-8

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：NATURE PUBLISHING GROUP  

Long-period ground motions in plain and basin areas on land can cause large-scale, severe damage to structures and buildings and have been widely investigated for disaster prevention and mitigation. However, such motions in ocean-bottom areas are poorly studied because of their relative insignificance in uninhabited areas and the lack of ocean-bottom strong-motion data. Here, we report on evidence for the development of long-period (10-20 s) motions using deep ocean-bottom data. The waveforms and spectrograms demonstrate prolonged and amplified motions that are inconsistent with attenuation patterns of ground motions on land. Simulated waveforms reproducing observed ocean-bottom data demonstrate substantial contributions of thick low-velocity sediment layers to development of these motions. This development, which could affect magnitude estimates and finite fault slip modelling because of its critical period ranges on their estimations, may be common in the source areas of subduction earthquakes where thick, low-velocity sediment layers are present.

DOI： 10.1038/srep16648

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担当区分：最終著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：SPRINGER  

We present numerical modeling of SH-wave propagation for the recently proposed whole Moon model and try to improve our understanding of lunar seismic wave propagation. We use a hybrid PSM/FDM method on staggered grids to solve the wave equations and implement the calculation on a parallel PC cluster to improve the computing efficiency. Features of global SH-wave propagation are firstly discussed for a 100-km shallow and 900-km deep moonquakes, respectively. Effects of frequency range and lateral variation of crust thickness are then investigated with various models. Our synthetic waveforms are finally compared with observed Apollo data to show the features of wave propagation that were produced by our model and those not reproduced by our models. Our numerical modeling show that the low-velocity upper crust plays significant role in the development of reverberating wave trains. Increasing frequency enhances the strength and duration of the reverberations. Surface multiples dominate wavefields for shallow event. Core-mantle reflections can be clearly identified for deep event at low frequency. The layered whole Moon model and the low-velocity upper crust produce the reverberating wave trains following each phases consistent with observation. However, more realistic Moon model should be considered in order to explain the strong and slow decay scattering between various phases shown on observation data.

DOI： 10.1007/s11589-015-0121-4

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：ELSEVIER SCIENCE BV  

We calculate regional synthetic seismograms for a realistic structure model beneath Greenland, including surface topography and ice sheet thickness, for observations of the multinational GreenLand Ice Sheet monitoring Network (GLISN). The thick and heterogeneous Greenland ice sheet can cause distortion of the seismic waveforms observed at the GLISN stations on ice. We developed a numerical technique that calculates accurate regional seismic wavefields with low computational requirements. Here, we calculate the elastic wave propagation up to 2 Hz for four structural models of the Greenland ice sheet from a seismic source at various depths and with different mechanisms. Our computations for a realistic ice sheet model, the near-surface seismic source produced a very characteristic wave train with a group velocity smaller than the S-wavespeed in the ice, considered to be an ice-sheet guided S wave, developed by the superposition of post-critical reflections between the free surface and the ice bed. We named this wave "Le", analogous to the Lg wave, a crustally guided S wave. Furthermore, computation for a deeper seismic source resulted in reinforcement of the crustal Sg-coda wave with a group velocity range of similar to 3.1-2.6 km/s, which agrees with the characteristic waveform observed on the Greenland ice sheet. (C) 2014 Elsevier B.V. and NIPR. All rights reserved.

DOI： 10.1016/j.polar.2014.12.001

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：SPRINGER  

Seismic wave amplifications were investigated using strong-motion data obtained from the ground's surface (K-net) on the Kii peninsula (southwestern Japan) and from the network of twenty seismic stations on the seafloor (DONET) located off the peninsula near the Nankai trough. Observed seismograms show that seismic signals at DONET stations are significantly larger than those at K-net stations, independent of epicentral distances. In order to investigate the cause of such amplifications, seismic wavefields for local events were simulated using the finite-difference method, in which a realistic 3D velocity structure in and around the peninsula was incorporated. Our simulation results demonstrate that seismic waves are significantly amplified at DONET stations in relation to the presence of underlying low-velocity sediment layers with a total thickness of up to 10 km. Our simulations also show considerable variations in the degree of amplification among DONET stations, which is attributed to differences in the thickness of the sediment layers. The degree of amplification is relatively low at stations above thin sediment layers near the trough axis, but seismic signals are much more amplified at stations closer to the Kii peninsula, where sediment layers are thicker than those at the trough axis. Simulation results are consistent with observations. This study, based on seafloor observations and simulations, indicates that because seismic signals are amplified due to the ocean-specific structures, the magnitude of earthquakes would be overestimated if procedures applied to data observed at land stations are used without corrections.

DOI： 10.1007/s11001-014-9211-2

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：SPRINGER BASEL AG  

We use the finite difference method to simulate seismic wavefields at broadband land and seafloor stations for a given terrestrial landslide source, where the seafloor stations are located at water depths of 1,900-4,300 m. Our simulation results for the landslide source explain observations well at the seafloor stations for a frequency range of 0.05-0.1 Hz. Assuming the epicenter to be located in the vicinity of a large submarine slump, we also model wavefields at the stations for a submarine landslide source. We detect propagation of the Airy phase with an apparent velocity of 0.7 km/s in association with the seawater layer and an accretionary prism for the vertical component of waveforms at the seafloor stations. This later phase is not detected when the structural model does not consider seawater. For the model incorporating the seawater, the amplitude of the vertical component at seafloor stations can be up to four times that for the model that excludes seawater; we attribute this to the effects of the seawater layer on the wavefields. We also find that the amplification of the waveform depends not only on the presence of the seawater layer but also on the thickness of the accretionary prism, indicating low amplitudes at the land stations and at seafloor stations located near the trough but high amplitudes at other stations, particularly those located above the thick prism off the trough. Ignoring these characteristic structures in the oceanic area and simply calculating the wavefields using the same structural model used for land areas would result in erroneous estimates of the size of the submarine landslide and the mechanisms underlying its generation. Our results highlight the importance of adopting a structural model that incorporates the 3D accretionary prism and seawater layer into the simulation in order to precisely evaluate seismic wavefields in seafloor areas.

DOI： 10.1007/s00024-013-0717-3

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担当区分：最終著者   記述言語：英語   掲載種別：研究論文（学術雑誌）  

グリーンランド氷床は地球規模の気候変動に伴って融解が進行している.近年,氷床融解の過程で末端部の氷河が移動する際に,「氷河地震」と呼ばれる地震動が発生することが知られるようになり,地震観測による氷床モニタリングに関心が集まっている.2009 年に発足した「グリーンランド氷床の地震モニタリング観測網(GLISN)」は,氷床モニタリングを目的として国際共同でグリーンランドや周辺の島々に広帯域地震計を展開するプロジェクトである.日本はGLISN 発足時からの参加国として,2011 年から観測隊を派遣しており,米国と共同で氷床上に観測点1 点を新設したほか,他の観測点のメンテナンスにも従事している.これらの観測点から得られた長周期地震波形データは,全球地震波伝播モデリングによる理論波形との比較でチェックを行い,設置場所によるノイズの影響が少ない,良質なデータであることを確認した.

DOI： 10.15094/00009722

CiNii Article

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担当区分：最終著者   記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：公益社団法人 日本地震学会  

DOI： 10.4294/zisin.67.25

CiNii Article

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：OXFORD UNIV PRESS  

The whole-Moon model was recently proposed by re-analyzing of lunar seismic data, which presumes efficient propagation of seismic waves within the Moon. However, common seismic body-wave or surface-wave phases observed for earthquake are absent or very weak in lunar seismic signals characterized by strong reverberations and coda. Hence, the process of seismic wave propagation in the Moon's interior is not well understood from limited lunar seismic data. We present numerical simulations of seismic body wave propagation in the whole Moon based on recently published whole-Moon model. Seismic wave equations are solved in a 2-D cross-section of spherical Moon with a staggered grid pseudospectral and finite difference hybrid method. Our simulation results provide the processes of seismic body wave propagation in the whole Moon for both deep and shallow moonquakes with sequential wavefield snapshots and synthetic waveforms. Effects of lateral heterogeneity on seismic wave propagation were investigated by simulations for a Moon tomographic model. Comparisons with the observed Apollo seismograms show that simulations predicted the arrivals of direct P and S waves and reproduced the reverberating nature of both direct and secondary waves. However, modelling with only the 1-D or tomographic model does not seem to be enough to produce the slow decay of energy in observations if other possible factors, such as scattering, are not considered in the model. Comparisons between different focal mechanisms suggest that great differences can be seen for the direct waves, but the structure model contributes significantly to the overall characteristics of synthetic seismograms. Numerical simulations demonstrated efficient propagation and interactions with various interfaces of seismic body waves within the whole Moon. Seismic energy propagating in the near surface low-velocity layer formed trapped waves, which propagate along the surface and appear as reverberations in the waveforms. Strength of the reverberations increases with dominant frequency and decreasing focal depth. Detectability of lunar seismic body-wave phases should be enhanced with deployment of very broad band seismometer network in the future lunar seismic experiment, which will further improve our knowledge of the Moon's interior.

DOI： 10.1093/gji/ggs094

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担当区分：最終著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：PERGAMON-ELSEVIER SCIENCE LTD  

Full-waveform seismic response of horizontally layered media can be calculated by semi-analytical methods. However, for gradient velocity and randomly heterogeneous structures the semi-analytical methods face difficulties. In such cases, numerical methods such as the finite-difference (FD) method have to be used. We develop an efficient numerical scheme to calculate plane-wave response of vertically heterogeneous attenuative media by applying Radon transform to the three-dimensional wave equation. The scheme employs fourth-order FD operator in space and second-order FD operator in time to solve the wave equation. In order to facilitate applicability of the scheme we introduce the FORTRAN code FDTD3C which implements the algorithm and provides multi-component response of the media to oblique incident P-, SV-, and SH-waves incoming from arbitrary azimuth. The calculated components are three particle velocity components in three Cartesian directions, and divergence and rotation of the wavefield. The code is extremely efficient and is capable of incorporating highly fluctuating subsurface velocity and attenuation models. This program is intended for all FD users who are concerned with full-waveform seismic modelling and inversion. Wide range of applicability of the code is demonstrated with a set of numerical examples. (c) 2012 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.cageo.2012.07.022

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記述言語：英語   掲載種別：論文集(書籍)内論文   出版者・発行元：Springer International Publishing  

We adopted the GPU (graphics processing unit) to accelerate the large-scale finite-difference simulation of seismic wave propagation. We describe the main part of our implementation: the memory optimization, the three-dimensional domain decomposition, and overlapping communication and computation. With our GPU program, we achieved a very high single-precision performance of about 61 TFlops by using 1,200 GPUs and 1.5 TB of total memory, and a scalability nearly proportional to the number of GPUs on TSUBAME–2.0, the recently installed GPU supercomputer in Tokyo Institute of Technology, Japan. In a realistic application by using 400 GPUs, only a wall clock time of 2,068 s (including the times for the overhead of snapshot output) was required for a complex structure model with more than 13 billion unit cells and 20,000 time steps. We therefore conclude that GPU computing for large-scale simulation of seismic wave propagation is a promising approach.

DOI： 10.1007/978-3-642-16405-7_24

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

Ground motion amplification in sedimentary basins has been observed in some moderate or large earthquakes, such as the 1994 Northridge and 1999 Chi-Chi event. Many numerical studies with simplified 2D models have shown significant effects of the vertical velocity gradient of sediment on basin amplification. However, we need to consider a more realistic 3D model and solve wave equations with 3D numerical methods in order to improve our understanding of basin amplification. In this study, we extend a 2D pseudospectral and finite difference hybrid method to a 3D case and investigate the effects of the vertical velocity gradient for a 3D basin model. Numerical simulations were performed for four basin models with increasing vertical velocity gradients on a PC cluster using 64 processors for 67 108 864 discretized grids. The results show that the vertical velocity gradient enhances basin amplification through strong secondary surface waves and basin trapped waves. The 3D geometry of the basin causes a wave-front focusing effect that contributes significantly to a localized strong amplification with the maximum peak ground velocity in the basin. The results of this study suggest that it is important to consider the detailed properties of sedimentary basins in seismic ground motion studies. © 2012 Sinopec Geophysical Research Institute.

DOI： 10.1088/1742-2132/9/6/761

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

Feasibility studies on simulating seismic-wave propagation in media for a suboceanic earthquake, including both land and ocean-bottom topographies and a seawater layer, are scarce. Some of the conventional staggered-grid finite-difference method (FDM) simulations use a simplified structure model without a seawater layer and with a flat ocean-bottom topography. In this study, we apply our heterogeneity, oceanic layer, and topography scheme (HOT)-FDM and a 3D structure model including land and ocean-bottom topographies, a seawater layer, and a fluid-solid boundary condition to an aftershock (Mw 5.8) of the 2009 Suruga Bay earthquake.We attempt to reproduce observations at seismic stations near the coast and then simulate waveforms in the ocean-bottom stations. Our results show that a large difference between the cases with and without topographies can be seen in the coda part after the S wave in the simulated waveforms in terms of amplitudes and elongations. The synthetic waveforms in the model with topographies are in agreement with the observed waveforms. Our results also show that a significant difference in the amplification of the coda part between the cases with and without the seawater layer can be found at the ocean-bottom stations. This coda part is the S- and Rayleigh-wave propagation associated with the ocean and the underlying sediment layers. Our results show that a realistic model with topographies and a seawater layer is needed in FDM simulations in order to precisely reproduce observed waveforms or predict seismic motion for a suboceanic earthquake.

DOI： 10.1785/0120110356

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

We have been developing an accurate and efficient numerical scheme, which uses the finite-difference method (FDM) in spherical coordinates, for the computation of global seismic wave propagation through laterally heterogeneous realistic Earth models. In the field of global seismology, traditional axisymmetric modeling has been used widely as an efficient approach since it can solve the 3-D elastodynamic equation in spherical coordinates on a 2-D cross-section of the Earth, assuming structures to be invariant with respect to the axis through the seismic source. However, it has the severe disadvantages that asymmetric structures about the axis cannot be incorporated and the source mechanisms with arbitrary shear dislocation have not been attempted for a long time. Our scheme is based on the framework of axisymmetric modeling but has been extended to treat asymmetric structures, arbitrary moment-tensor point sources, anelastic attenuation, and the Earth center which is a singularity of wave equations in spherical coordinates. All these types of schemes which solve 3-D wavefields on a 2-D model cross-section are classified as 2.5-D modeling, so we have named our scheme the spherical 2.5-D FDM. In this study, we compare synthetic seismograms calculated using our FDM scheme with three-component observed long-period seismograms including data from stations newly installed in Antarctica in conjunction with the International Polar Year (IPY) 2007-2008. Seismic data from inland Antarctica are expected to reveal images of the Earth's deep interior with enhanced resolution because of the high signal-to-noise ratio and wide extent of this region, in addition to the rarity of sampling paths along the rotation axis of the Earth. We calculate synthetic seismograms through the preliminary reference earth model (PREM) including attenuation using a moment-tensor point source for the November 9, 2009 Fiji earthquake. Our results show quite good agreement between synthetic and observed seismograms, which indicates the accuracy of observations in the Antarctica, as well as the feasibility of the spherical 2.5-D modeling scheme. © 2012 Elsevier B.V. and NIPR.

DOI： 10.1016/j.polar.2012.06.001

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担当区分：最終著者   記述言語：英語   掲載種別：研究論文（学術雑誌）  

We propose a method for modeling global seismic wave propagation through an attenuative Earth model including the center. This method enables accurate and efficient computations since it is based on the 2.5-D approach, which solves wave equations only on a 2-D cross section of the whole Earth and can correctly model 3-D geometrical spreading. We extend a numerical scheme for the elastic waves in spherical coordinates using the finite-difference method (FDM), to solve the viscoelastodynamic equation. For computation of realistic seismic wave propagation, incorporation of anelastic attenuation is crucial. Since the nature of Earth material is both elastic solid and viscous fluid, we should solve stress-strain relations of viscoelastic material, including attenuative structures. These relations represent the stress as a convolution integral in time, which has had difficulty treating viscoelasticity in time-domain computation such as the FDM. However, we now have a method using so-called memory variables, invented in the 1980s, followed by improvements in Cartesian coordinates. Arbitrary values of the quality factor (Q) can be incorporated into the wave equation via an array of Zener bodies. We also introduce the multi-domain, an FD grid of several layers with different grid spacings, into our FDM scheme. This allows wider lateral grid spacings with depth, so as not to perturb the FD stability criterion around the Earth center. In addition, we propose a technique to avoid the singularity problem of the wave equation in spherical coordinates at the Earth center. We develop a scheme to calculate wavefield variables on this point, based on linear interpolation for the velocity-stress, staggered-grid FDM. This scheme is validated through a comparison of synthetic seismograms with those obtained by the Direct Solution Method for a spherically symmetric Earth model, showing excellent accuracy for our FDM scheme. As a numerical example, we apply the method to simulate seismic waves affected by hemispherical variations of P-wavespeed and attenuation in the top 300. km of the inner core. © 2012 Elsevier B.V..

DOI： 10.1016/j.pepi.2012.03.010

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.5772/1400

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担当区分：最終著者   記述言語：英語   掲載種別：研究論文（学術雑誌）  

We present a scheme to simulate SH-wave propagation in a whole-Earth model with arbitrary lateral heterogeneities employing the Fourier pseudospectral method. Wave equations are defined in two-dimensional cylindrical coordinates and the model is taken through a great circle of the Earth. Spatial derivatives in the wave equations are calculated in the wavenumber domain by multiplication, and the transformation between spatial and wavenumber domains is performed via fast Fourier transformation. Because of the high accuracy and high speed of the Fourier pseudospectral method, the scheme enables us to calculate a short-wavelength global SH-wavefield with accurate waveforms and arrival times for models with heterogeneities that can be approximated as azimuthally symmetric. Comparing with two-dimensional simulation methods based on an axisymmetric model, implementing the seismic source in the present scheme is more convenient. We calculated the global SH-wavefield for the preliminary reference Earth model to identify the generation, reflection and refraction of various seismic phases propagating in the Earth. Applications to a heterogeneous global model with low-velocity perturbation above the core-mantle boundary were conducted to analyze the effect of lateral heterogeneity on global SH-wave propagation. © 2011 Science China Press and Springer-Verlag Berlin Heidelberg.

DOI： 10.1007/s11430-011-4244-8

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記述言語：日本語   出版者・発行元：公益社団法人 日本地震学会  

DOI： 10.4294/zisin.63.189

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担当区分：最終著者   記述言語：英語   掲載種別：研究論文（学術雑誌）  

The pseudospectral method has been applied to the simulation of seismic wave propagation in 2-D global Earth model. When a whole Earth model is considered, the center of the Earth is included in the model and then singularity arises at the center of the Earth where r = 0 since the 1/r term appears in the wave equations. In this paper, we extended the global seismic wavefield simulation algorithm for regular grid mesh to staggered grid configuration and developed a scheme to solve the numerical problems associated with the above singularity for a 2-D global Earth model defined on staggered grid using pseudospectral method. This scheme uses a coordinate transformation at the center of the model, in which the field variables at the center are calculated in Cartesian coordinates from the values on the grids around the center. It allows wave propagation through the center and hence the wavefield at the center can be stably calculated. Validity and accuracy of the scheme was tested by compared with the discrete wavenumber method. This scheme could also be suitable for other numerical methods or models parameterized in cylindrical or spherical coordinates when singularity arises at the center of the model.

DOI： 10.1007/s11589-010-0001-x

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担当区分：最終著者   記述言語：英語   掲載種別：論文集(書籍)内論文   出版者・発行元：World Scientific Publishing Co.  

We propose a very efficient scheme for modeling three-component seismic plane waves in vertically heterogeneous attenuative media using the finitedifference (FD) method in the time domain. The scheme is able to represent P, SV, and SH waves incoming from different direction. The algorithm can also calculate the plane-wave responses of media for different incident angels. In the algorithm, neglecting lateral heterogeneity, the wave equation is rewritten for plane waves by applying a 2D Radon transform to the 3D general wave equation. QP and QS are incorporated via generalized Zener body rheological models for viscoelasticity to represent anelastic attenuation. Our FD scheme uses a 1D grid, which leads to a significant reduction in computation time and memory requirements as compared to the corresponding 3D or 2D computations. It may be an efficient tool for pre- or post-analysis of local structural effects including anelastic attenuation, before or after the large-scale seismic wave simulation. To demonstrate the ability and efficiency of the scheme, we calculate a synthetic vertical seismic profile (VSP) for a borehole with a highly heterogeneous velocity model and frequency dependent attenuation model in the Kanto area of Japan.

DOI： 10.1142/9789812838186_0020

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Springer Berlin  

We adopted the GPU (graphics processing unit) to accelerate the large-scale finite-difference simulation of seismic wave propagation. The simulation can benefit from the high-memory bandwidth of GPU because it is a "memory intensive" problem. In a single-GPU case we achieved a performance of about 56 GFlops, which was about 45-fold faster than that achieved by a single core of the host central processing unit (CPU). We confirmed that the optimized use of fast shared memory and registers were essential for performance. In the multi-GPU case with three-dimensional domain decomposition, the non-contiguous memory alignment in the ghost zones was found to impose quite long time in data transfer between GPU and the host node. This problem was solved by using contiguous memory buffers for ghost zones. We achieved a performance of about 2.2 TFlops by using 120 GPUs and 330 GB of total memory: nearly (or more than) 2200 cores of host CPUs would be required to achieve the same performance. The weak scaling was nearly proportional to the number of GPUs. We therefore conclude that GPU computing for large-scale simulation of seismic wave propagation is a promising approach as a faster simulation is possible with reduced computational resources compared to CPUs. Copyright © The Society of Geomagnetism and Earth, Planetary and Space Science (SGEPSS).

DOI： 10.5047/eps.2010.11.009

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担当区分：最終著者   記述言語：英語   掲載種別：研究論文（学術雑誌）  

Despite the broad use of the heterogeneous finite-difference (FD) method for seismic waveform modeling, accurate treatment of material discontinuities inside the grid cells has been a serious problem for many years. One possible way to solve this problem is to introduce effective grid elastic moduli and densities (effective parameters) calculated by the volume harmonic averaging of elastic moduli and volume arithmetic averaging of densities in grid cells. This scheme enables us to place a material discontinuity in an arbitrary position in the spatial grids. Standard Earth models have made a significant contribution to synthetic seismogram calculations with a variety of numerical procedures such as the FD method. For the FD computation of seismic waveform with these models, we must first ensure accurate treatment of material discontinuities in the radius (or depth). The present paper introduces a FORTRAN subroutine ACE which calculates effective parameters analytically for an arbitrary spatial region in either the radius or depth direction for four major standard Earth models, namely, the PREM, IASP91, SP6, and AK135. This program is intended for all FD users who are concerned with seismic wave simulation for these models. © 2008 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.cageo.2008.05.005

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担当区分：最終著者   記述言語：英語   掲載種別：論文集(書籍)内論文   出版者・発行元：World Scientific Publishing Co.  

We study strong effect of near-source structure on teleseismic body waveforms from two well-recorded aftershocks of the 2006 Java tsunami earthquake. We assume a two-dimensional, heterogeneous model of the Java trench structure on the basis of recent seismic surveys. We compute synthetic waveforms by an efficient “2.5-dimensional” (2.5D) finite-difference method. A “waveform relocation technique” which combines a non-linear inversion of source parameters with a grid search procedure is applied in order to correct possible systematic bias in hypocentral parameters. Simulation of inversion shows that the resolution of the source position obtained by using 2.5D Green’s functions is better than that for 1D Green’s functions. The synthetic waveforms obtained by the inversion of true data with 2.5D Green’s functions reproduce well the observed large later phases, and retrieved moment tensors are similar to those of Global CMT. Thus, the effect of the near-source structure can be reproduced by 2.5D model, and with the 2.5D synthetics we can obtain improved source parameters at the trench regions where only teleseismic data are available.

DOI： 10.1142/9789812836182_0015

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担当区分：最終著者   記述言語：英語   掲載種別：研究論文（学術雑誌）  

A scheme for the non-standard finite-difference method in the time-domain (NS-FDTD), 2-Box scheme, is proposed for elastic wave simulations in two dimensions (P-SV). The method improves the accuracy and efficiently reduces grid dispersion and anisotropy. The proposed non-standard scheme is based on two main operations. The first operation replaces spatial grid spacing and time step by their frequency optimized counterparts, called the correction functions, and the second operation introduces an optimum grid stencil for the finite-difference operator of the 2-D Laplacian. The optimal stencil is obtained by introducing two optimization parameters estimated for a design frequency. Error analysis of the proposed scheme (2-Box scheme) shows that specifying the maximum frequency as the design frequency leads to a significant reduction of the grid dispersion over a wide frequency band. We derive the formulations of grid dispersion and stability condition for the scheme. The grid dispersion is investigated, and it is shown that the proposed scheme reduces not only the grid dispersion but also the grid anisotropy significantly. The grid dispersion is insensitive to the Poisson's ratio and size of the time step within the stability limit. Since the wide spatial stencil of the 2-Box scheme might become difficult to implement at the computational domain boundaries, two additional non-standard schemes - 1-Box and 0-Box schemes - are also introduced. The 1-Box scheme uses narrower stencil than the 2-Box scheme, and the 0-Box scheme uses the same stencil as the standard FDTD. Numerical experiments of elastic wave propagation demonstrate the significant superiority of the proposed non-standard schemes over the commonly used standard one. With six grid spacings per minimum wavelength, the 2-Box and 1-Box schemes represent excellent results and the 0-Box scheme has higher accuracy than the standard scheme with seven grid spacings per minimum wavelength. © 2009 The Authors, Journal compilation © 2009 RAS.

DOI： 10.1111/j.1365-246X.2009.04101.x

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担当区分：最終著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：TERRA SCIENTIFIC PUBL CO  

We first incorporate the large effect of near-source heterogeneity on teleseismic body waveforms in the inversion of the slip distribution of the 2006 Java tsunami earthquake. We incorporate the effect by computing the response of an assumed "2.5D" model structure of the Java trench by a 2.5D finite-difference method. Based on a simulation of inversion, we suggest that intense smearing is possible when we apply 1D Green's functions in the analysis, and that it may obscure the slip pattern. In the inversion of real data, we confirm macroscopic features, such as a long duration (similar to 165 s) and a slow rupture velocity (similar to 1.25 km/s). The region of the initial rupture is found to be isolated from the eastern broad region in which we further identify a heterogeneous slip distribution. Most of these regions are likely to be at the sedimentary plate interface where the accreted sediment and the subducting plate are in contact. In particular, the nearly "isolated" feature of a shallow slip region suggests a possible faulting in the shallowest part of the sedimentary plate interface without being strongly enforced by the rupture propagated from the deeper part of the fault. Such heterogeneity suggests a highly variable frictional behavior at the sedimentary plate interface.

DOI： 10.1186/BF03352919

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担当区分：筆頭著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：TERRA SCIENTIFIC PUBL CO  

The rupture process at the beginning stage of the 2007 Chuetsu-oki, Niigata, Japan, earthquake (M(w), 6.6) is investigated by analyzing P-wave records from local strong-motion stations. The P-wave portion of the near-source strong-motion records shows about 2 s of small but increasing amplitude arrival (so-called "initial rupture phase") followed by the onset of the main energy release ("main rupture phase"), Two issues are addressed in this paper: (I) where the initial rupture process occurred and (2) where the seismic energy corresponding to the main rupture phase was released at the primary stage of the main rupture. The first Issue is addressed by locating the main rupture onset position, and the second issue is then approached by introducing a method for mapping the wave energy onto plausible fault planes. Eventually, the following were revealed, The rupture initiated and propagated oil the NW-dipping plane, which is a nodal plane of the focal mechanism Solution. At 2.1 s after rupture initiation the subsequent main rupture started at a position of approximately 4 km away. southwestward and updipward from the hypocenter. The main rupture at this stage has two possible rupture planes: the same plane as the initial rupture plane, and the Conjugated plane, which shares the main rupture onset point with the initial rupture plane. Although it is difficult to determine which plane was actually ruptured at the primary stage of the main rupture, we found that the possible areas radiating strong wave energy oil the two possible planes, which could correspond to the first asperity of this earthquake, are located between the hypocenter and the Kashiwazaki-Kariwa nuclear power plant.

DOI： 10.1186/BF03352908

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担当区分：最終著者   記述言語：英語   掲載種別：研究論文（学術雑誌）  

The stochastic method for finite faults is applied to simulate the ground motion of the 12 October 1992, m b = 5.9, Dahshour earthquake. The method includes discritization of the fault plane into certain number of subfaults, and a ω-squared spectrum is assigned to each of them. Contributions from all subfaults are then empirically attenuated to the observation sites, where they are summed to produce the synthetic acceleration time-history. The method is first tested against its ability of reproducing the recording at Kottamya station. The calibrated model is then applied to calculate the synthetics at a large number of grid points covering the area around the fault plane. Simulated peak values are subsequently used to produce the synthetic peak horizontal acceleration map for the area. We compare the peak horizontal acceleration with the attenuation laws proposed for Egypt as well as the macroseismic intensity map of the 1992 Dahshour earthquake. The peak horizontal acceleration contours estimated using the calibrated model are mostly consistent with the observed intensity values and evidences of strong ground motions. Our results encourage the application of the approach as a supplementary tool for site-specific strong ground motion prediction. © Versita Warsaw and Springer-Verlag Berlin Heidelberg 2009.

DOI： 10.2478/s11600-009-0012-y

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担当区分：最終著者   記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：公益社団法人 日本地震学会  

DOI： 10.4294/zisin.62.137

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担当区分：筆頭著者   記述言語：英語   掲載種別：研究論文（学術雑誌）  

We propose a mathematical representation to qualitatively describe the spatio-temporal slip evolution during earthquake rupture in an efficient and easy-to-use manner for numerical simulations of strong ground motion. It is based on three basis functions and associated expansion coefficients. It is an extension of the approach of Ide and Takeo, (J Geophys Res, 102:27379-27391, 1997). We compare our approach and theirs using simple kinematic source models to illustrate differences between the two approaches, and show that our approach more accurately represents the spatio-temporal slip evolution. We also propose a technique based on our representation for extracting a spatio-temporal slip velocity function from a kinematic source model obtained by the conventional source inversion. We then demonstrate the feasibility of our procedure with application to an inverted source model of the 26 March 1997 Northwestern Kagoshima, Japan, earthquake (MW 6.1). In the simulations for actual earthquakes, source models obtained from kinematic source inversions are commonly employed. Our scheme could be used as an interpolation method of slip time functions from relatively coarse finite-source models obtained by conventional kinematic source inversions. © Springer Science+Business Media B.V. 2008.

DOI： 10.1007/s10950-007-9087-6

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記述言語：日本語   出版者・発行元：Japanese Society for Active Fault Studies  

The Umi fault was first described by Ikeda et al. (2004) as an active fault'of about 9km in length. Fukuoka Prefectural Government organized a research committee for examination and investigation of the Umi fault in 2005. The present study is based on air-photograph analysis, geological and topographical field surveys, soundreflection surveys in the marine areas, drilling and trenching, and tephrochronological studies. Some characters of the Umi fault are clarified through this intensive study.<BR>The Umi fault extends father north and south than was previously known, but it does not extend as far north as Hakata Bay and the Genkainada Sea. The length of the fault is 17km at least. The average vertical slip rate in the late Quaternary is estimated to be 0.02-0.03m/kyr (Activity Rank C in Matsuda 1975). This slip rate of the Umi fault is lower than those of the neighboring Kego and Nishiyama faults. The last activity of the fault was confirmed to be younger than 4300y. B. P. by the trenching study at Yamaura.<BR>The average recurrence interval of the activity is estimated at 29 kyr. This estimation is based on the assumption that the observed slip at the Yamaura trench (0.6m) occurred in one earthquake event and that the slip at Uenohara (1.9m), which is observed on middle terrace surface, formed immediately after the Aso-4 pyroclastic flow, is a result of repeated earthquake events of the same size slip.

DOI： 10.11462/afr1985.2008.29_59

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記述言語：英語   出版者・発行元：SOC EXPLORATION GEOPHYSICISTS  

DOI： 10.1190/1.2817659

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担当区分：最終著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：SOC EXPLORATION GEOPHYSICISTS  

We propose an efficient algorithm for modeling seismic plane-wave propagation in vertically heterogeneous viscoelastic media using a finite-difference tirne-domain (FDTD) technique. In the algorithm, the wave equation is rewritten for plane waves by applying a Radon transform to the 2D general wave equation. Arbitrary values of the quality factor for P- and S-waves (Q(p) and Q(s)) are incorporated into the wave equation via a generalized Zener body rheological model. An FDTD staggered-grid technique is used to numerically solve the derived plane-wave equations. The scheme uses a 1 D grid that reduces computation time and memory requirements significantly more than corresponding 2D or 3D computations. Comparing the finite-difference solutions to their corresponding analytical results, we find that the methods are sufficiently accurate. The proposed algorithm is able to calculate synthetic waveforms efficiently and represent viscoelastic attenuation even in very attenuative media. The technique is then used to estimate the plane-wave responses of a sedimentary system to normal and inclined incident waves in the Kanto area of Japan via synthetic vertical seismic profiles.

DOI： 10.1190/1.2732555

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担当区分：最終著者   記述言語：英語   掲載種別：研究論文（学術雑誌）  

An anomalous seismic velocity structure is revealed inside the subducted Pacific plate at northern Mariana near the deepest earthquake foci. We observe anomalous later phases in the P wave coda for a small cluster of deep earthquakes and an anomalous differential P wave slowness between a few pairs of the earthquakes belonging to the cluster. We interpret these observations with a 5% low velocity wedge which is nearly 25 km wide at the depth of 590 km and is truncated at 630 km. The wedge is bounded by two steeply dipping (60°-70°) transition layers with the thickness of about 4 km. The highly localized seismic anomaly inside the slab represents a wedge of metastable olivine (α) embedded in dry ringwoodite (γ). Deep earthquakes occur across only one of the boundaries of the metastable wedge, with some of them outside the wedge. The mechanism of deep earthquakes is discussed based on the spatial pattern of seismicity relative to the wedge. © 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.epsl.2007.03.035

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担当区分：最終著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Terra Scientific Pub. Co.  

DOI： 10.1186/BF03352593

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担当区分：筆頭著者   記述言語：英語   掲載種別：研究論文（学術雑誌）  

Near-source strong-motion records of the 2005 West Off Fukuoka Prefecture earthquake have two remarkable features. One is the presence of a relatively long period pulse with large amplitude on the fault-normal component of the velocity and displacement records, which is the result of the forward rupture directivity. The other is that the records show several seconds of small but increasing amplitude arrival (‘ initial rupture phase‘ ) followed by the onset of the main energy release (‘ main rupture phase‘ ). We first determined the precise geometry of the fault plane of this earthquake by examining the horizontal particle motion of the main S-wave portion on the records, as follows: the strike is N304°E, and the dip angle is 87°. The dip direction of the fault plane is northeast, and the surface intersection of the fault plane passes by the eastern coast of Genkai Island. We also obtained the relative location of the onset of the main rupture with respect to the hypocenter, and the mean rupture velocity between them. The distance between them is 5.1 km, the onset of the main rupture is located southeast above the hypocenter. The mean rupture velocity along the straight path is 1.4 km/s. It is found that the main rupture began 3.6 sec later from the origin time, at the central point between the hypocenter and Genkai Island. Our results suggest that Genkai Island directly suffered the strong effects of the forward rupture directivity during the earthquake. Copyright © The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS)
The Seismological Society of Japan
The Volcanological Society of Japan
The Geodetic Society of Japan
The Japanese Society for Planetary Sciences
TERRAPUB. © 2006, The Seismological Society of Japan, Society of Geomagnetism and Earth, Planetary and Space Sciences, The Volcanological Society of Japan, The Geodetic Society of Japan, The Japanese Society for Planetary Sciences. All rights reserved.

DOI： 10.1186/BF03351917

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担当区分：最終著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Springer Berlin  

In this study we examine the effects of anisotropy on the seismic wavefield in a fault zone from computation of the synthetic seismograms for a simple fault zone model and a variety of seismic wave sources. The fault zone is modeled by a homogeneous vertical layer with transverse isotropy, induced by cracks, sandwiched between isotropic half-spaces (host rocks). The symmetry axis of the transverse isotropy is horizontal and perpendicular to the fault zone strike. We calculate the synthetic seismograms for this anisotropic fault zone model using a semi-analytical method, the propagator matrix method. The synthetic seismograms show a later phase arriving after the main shear-wave in the horizontal component perpendicular to the fault zone strike at most stations near the fault zone. It is the slower shear-wave (q S2) and its reverberation. The amplitude of this phase and the time delay from the main shear-wave arrival are proportional to the degree of anisotropy, which suggests that observing such phase in field measurements may imply the presence of an anisotropic fault zone. We also perform the shear-wave splitting measurements by applying the cross-correlation method to the synthetic seismograms for various sources. For a strike-slip source, the synthetic seismograms show that the wavefield is more affected by the velocity structure than by the degree of anisotropy, which makes it difficult to estimate the anisotropic (shear-wave splitting) parameters. For normal and dip-slip fault sources with the strike parallel to or striking against the fault zone, the effects of anisotropy is so dominant that the anisotropic fault zone can be detected. These results suggest that the determination of the anisotropic properties in the fault zone would require an appropriate station deployment and the source type information. Copyright © The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS)
The Seismological Society of Japan
The Volcanological Society of Japan
The Geodetic Society of Japan
The Japanese Society for Planetary Sciences
TERRAPUB.

DOI： 10.1186/BF03351954

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1029/2004gl022180

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担当区分：最終著者   記述言語：日本語   出版者・発行元：日本地震学会  

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担当区分：責任著者   記述言語：日本語   出版者・発行元：日本地震学会  

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担当区分：最終著者   記述言語：日本語   出版者・発行元：日本地震学会  

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記述言語：日本語   出版者・発行元：日本地震学会  

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Springer Berlin  

The 2000 Tottori-ken Seibu earthquake (Mw6.6) occurred in Tottori prefecture, western Japan on October 6, 2000. We conducted aftershock observation and analyzed S-wave anisotropy using the aftershocks Mjma2.0 to 3.5 observed at two stations near the aftershock region. The fast S-waves are polarized to almost E-W direction, which corresponds to the P-axis direction of the mainshock. The delay time of the split S-waves ranges between 20 and 100 ms. The crack density inside and outside the aftershock area is estimated from the delay time. The distribution of crack density shows a strong spatial variation and depends on the ratio of the path length inside the aftershock region against the whole path length. Assuming a uniform distribution of anisotropy inside and outside the aftershock region, the crack densities inside and outside are 0.017 and 0.007, respectively. It thus seems probable that in the aftershock region the distribution of cracks is intensive and cracks are opened due to the presence of fluids in seismogenic layers. This considerable spatial variation of anisotropy between inside and outside the aftershock region suggests that the aftershock region may have different mechanical properties from the surrounding area. Copyright © The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS)
The Seismological Society of Japan
The Volcanological Society of Japan
The Geodetic Society of Japan
The Japanese Society for Planetary Sciences
TERRAPUB.

DOI： 10.1186/BF03351884

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担当区分：最終著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：SEISMOLOGICAL SOC AMER  

In 1997, two moderate earthquakes (M-JMA 6.5 on March 26 and MJMA 6.3 on May 13) occurred in northwestern Kagoshima prefecture, Japan. Using the records of aftershocks with focal depths from 5 to 10 km, recorded at a station located above the center of the aftershock zone, attenuation values for shear waves (Q(S)(-1)) were determined for the focal region. This region consists of the aftershock zones, including the rupture zones of the two mainshocks. A coda normalization method was applied for the measurement of Q(S)(-1) and determined as a function of frequency in five frequency bands centered at 1.5, 3, 6, 12, and 24 Hz. Before applying the method, a correction for the effect of the radiation pattern of shear waves on the spectral amplitude was made for frequency bands centered at 3 Hz and lower. Estimated Q(S)(-1)'s have significantly larger values compared to other previous studies in the crust. From our results, Q(S)(-1) can be approximately represented as a function of frequency f by Q(S)(-1)(f) = (9.93 X 10(-2))f(-0.95).
In order to check the significance of the large values of Q(S)(-1) in the focal region, we also determined the attenuation in the surrounding area, using shear waves that propagate through both the focal region and the surrounding region. Our results show that the estimated Q(S)(-1) outside the focal region can be approximately represented as a function of frequency by Q(S)(-1)(f) = (1.03 X 10(-2))f(-0.59). This is similar to the crustal Q(S)(-1) in the surrounding area obtained by Kato (1999), suggesting that the values of Q(s)(-1) in the focal region are correctly estimated. Comparison of the values of Q(S)(-1) demonstrates that attenuation of S waves in the focal region is several times larger than in the surrounding area.

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Wiley-Blackwell  

In 1999, a marine seismic structure survey was conducted in the northernmost Okinawa Trough. Outcrops of the acoustic basement through the sedimentary layers are seen in the obtained structures. Velocities of lower layer in upper crust are varying laterally from 5.7 to 6.2 km/s and thickness of 6.0-km/s layer ranges from 3 to 9 km. The lower crust has uniform thickness of about 15 km. The Pn velocity is 7.7-7.8 km/s and the total crustal thickness is about 25 km including sedimentary layers. The structure is in a first approximation similar to that of island arcs. However, the large heterogeneity of upper crust can be interpreted as a result of crustal thinning. The low Pn velocity and the outcrops of the acoustic basement relate to igneous activity in this area. These results suggest that the northernmost part of the Okinawa Trough is in the incipient stage of a back arc rifting.

DOI： 10.1029/2003gl018928

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担当区分：筆頭著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：ELSEVIER SCIENCE BV  

It has been empirically recognized that high-frequency components of S waves radiated from earthquake do not show a radiation pattern predicted from a double-couple source, while the low frequency components show the standard double-couple radiation pattern. In this paper, we investigate this frequency dependence of radiation pattern using the high quality single-station records from the aftershock events of the 26 March 1997 and 13 May 1997 Northwestern Kagoshima, Japan, earthquakes. We focus on frequency dependence of energy partition between SH and SV waves in the direct S-phase. From the variation of the SH energy fraction with frequency we find that the energy fraction can be well predicted by theoretical radiation coefficients for a double-couple source at frequencies lower than 2 Hz, while at 3.5 Hz or higher it cannot be predicted and shows stochastic behavior. The intermediate frequencies can be seen as a transition from deterministic to stochastic. This supports the assumption that the radiation patterns of SH and SV waves are purely stochastic for high frequencies. Analyses of events which are located close to each other suggest that this stochastic nature is ascribed to strong SH and SV mixing or coupling induced by local structural effects near the sources. (C) 2003 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0031-9201(03)00006-2

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：AMER GEOPHYSICAL UNION  

[1] We analyzed broadband waveforms recorded by the newly deployed Japan-Indonesia Seismic Network to determine the depths of the 410 km and 660 km discontinuities and thickness of the mantle transition zone beneath Indonesia by the receiver function method. The discontinuities have an undulation with a maximum difference of 40 km with respect to the Iasp91 model. The thickness of the transition zone is well correlated with seismicity and seismic velocity anomalies, suggesting that the topography is caused by temperature anomalies associated with the subducted slabs beneath Indonesia.

DOI： 10.1029/2001GL013629

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記述言語：日本語   掲載種別：学位論文（博士）  

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担当区分：最終著者  

DOI： 10.4294/zisin1948.42.3_391

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記述言語：日本語   掲載種別：速報，短報，研究ノート等（大学，研究機関紀要）  

DOI： 10.18926/ESR/63515

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担当区分：最終著者   記述言語：日本語   掲載種別：速報，短報，研究ノート等（大学，研究機関紀要）   出版者・発行元：岡山大学理学部地球科学教室  

The 2016 Kumamoto earthquake sequence occurred on April 14 (MJMA 6.5) and April 16 (MJMA 7.3). Seismic intensity of 7 on the Japan Meteorological Agency (JMA) scale was observed in Mashiki Town, Kumamoto Prefecture for the both events and in Nishihara Village, Kumamoto Prefecture for the April-16 event. We estimate the displacement waveforms from these acceleration records. Since the acceleration seismograms include the long-period noise due to tilting of the ground and instrumental effects, the baseline corrections are required to derive the accurate velocity and displacement waveforms. We apply a median filter to the velocity waveforms to identify the linear trends on them due to the steplike noise on the acceleration records, and determine the time at which baseline shifts take place and the step value of each shift for the baseline correction through trial and error. Our baseline correction can successfully reconstruct the velocity and displacement waveforms from the acceleration records. The displacement waveforms show the static components consistent with the geodetic data.

DOI： 10.18926/ESR/61959

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担当区分：筆頭著者   記述言語：日本語   掲載種別：速報，短報，研究ノート等（大学，研究機関紀要）   出版者・発行元：岡山大学理学部地球科学教室  

We extend the computational code of Takenaka and Sasatani (2000) for synthetic nearfield seismograms for horizontally layered elastic media, based on the reflection/transmission matrices and the discrete wavenumber summation method, to calculate seismic motion and its spatial derivatives at a subsurface position in the attenuative media. In this paper we describe the theory of this extension and show some numerical examples to verify the extended code.

DOI： 10.18926/ESR/61956

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記述言語：日本語   出版者・発行元：海洋出版  

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記述言語：日本語   出版者・発行元：大成学術財団  

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担当区分：最終著者   記述言語：日本語   掲載種別：速報，短報，研究ノート等（大学，研究機関紀要）   出版者・発行元：岡山大学理学部地球科学教室  

We have developed a time-domain staggered-grid finite-difference code for modeling non-linear response of a one-dimensionally inhomogeneous subsurface structure to a SH plane-wave incidence. It employs the velocity-stress formulation of elastodynamic equation for the linear part, and adopts a elastoplastic rheology model for the non-linear relation between the stress and strain. In this paper, we apply this code to four constitutive models from linear-elastic to nonlinear: (1) linear elastic model, (2) linear viscoelastic model, (3) elastoplastic model, and (4) viscoelastoplastic model, which simulate shallow sand and clay structures and are vibrated by a vertically incident SH plane-wave of Ricker wavelet, to compare the linear and the non-linear soil behaviors including low strains damping (viscoelastic effect) and/or hysteretic attenuation (non-linear effect). We also apply it to a local strong-motion record of the 2000 Western-Tottori earthquake (MW6.8). We then simulate characteristics of non-linear site response such as reduction of the spectral amplitude in the high frequency band and shift of the peak frequencies to lower frequencies.

DOI： 10.18926/ESR/58574

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記述言語：日本語   出版者・発行元：物理探査学会  

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記述言語：日本語   出版者・発行元：岡山大学理学部地球科学教室  

Simulated annealing (SA) is known as one of the efficient methods for global optimization. It allows for getting an optimal solution by jumping out of local minimum. In this paper, we apply the simulated annealing to determination of focal mechanism solution. To investigate the feasibility of the application, we determine focal mechanism solution of the 14 March 2014 Iyo-Nada intermediatedepth earthquake. The computation of SA method is then 1278 times faster than that of the grid search method.

DOI： 10.18926/ESR/54116

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記述言語：日本語   出版者・発行元：海洋出版  

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記述言語：日本語   出版者・発行元：日本科学者会議 ; 1966-  

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記述言語：日本語   出版者・発行元：東北大学サイバーサイエンスセンター  

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記述言語：日本語  

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記述言語：日本語   出版者・発行元：海洋出版  

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記述言語：日本語   出版者・発行元：東京工業大学学術国際情報センター  

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記述言語：日本語   出版者・発行元：北海道大学大学院理学研究院自然史科学部門（地球物理学）  

DOI： 10.14943/gbhu.73.185

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その他リンク： http://hdl.handle.net/2115/44473

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記述言語：日本語   出版者・発行元：海洋出版  

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記述言語：日本語   出版者・発行元：独立行政法人防災科学技術研究所  

地震調査研究推進本部地震調査委員会による地震動予測地図作成に資するため、各種地震計による観測網からのデータを最大限に活用し、ある一定期間内にある地域が強い地震動に襲われる確率を予測した確率論的地震動予測地図や、特定の断層を想定し、それが活動した場合をモデル化して震源断層周辺域の地盤の揺れの分布を予測したシナリオ地震による地震動予測地図を作成する。また、高精度の強震動予測を行い、地震のメカニズムや断層面などを推定するとともに、断層面上での詳細な破壊過程を分析する。さらに、地震災害を引き起こす強震動を予測し、震災被害予測を行う。これらの成果は、地震調査研究推進本部委員会において進めている「全国を概観した地震動予測地図」の作成(平成16年末を目途)に活用されている。

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記述言語：日本語   出版者・発行元：九州大学大学院理学研究院  

The Okinawa Trough is located in the・ back-arc region of the Ryukyu Island arc, and is considered to be an active spreading area. In fall of 1999, seismic survey using ocean bottom seismometers was conducted to obtain the detailed seismic structure in the northernmost part of the Okinawa Trough. The total crust thickness is 25 km including a sedimentary layer. The velocity of the upper crust changes from 5.7-6.0 km/s in the upper part to 6.1-6.2 km/s in the lower part. And its thickness changes from 3 km to 9 km. The lower crust of which velocity slightly changes from 6.5-6.7 km/sin the upper part to 6.8-6.9 km/sin the lower part has uniform thickness of about 15 km. The Pn velocity is determined to be 7.7-7.8 km/s. The seismic structure suggests that the northernmost part of the Okinawa Trough is in the incipient stage of a backarc rifting.

DOI： 10.15017/4493142

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その他リンク： http://hdl.handle.net/2324/4493142

記述言語：英語   出版者・発行元：ELSEVIER SCIENCE BV  

We delineate shallow structures of the Mozumi-Sukenobu fault, central Japan, using fault zone waves generated by near-surface explosions and detected by a seismometer array. Two explosive sources, S1 and S2, were placed at a distance of about 2 km from the array, and the other two, S3 and S4, were at a distance of about 4 km. Fault zone head waves and fault zone trapped waves following direct P wave arrivals were clearly identified in the seismograms recorded by a linear seismometer array deployed across the fault in a research tunnel at a depth of 300 in. Synthetic waveforms generated by a 3-D finite-difference (3-D FD) method were compared with observed fault zone waves up to 25 Hz. The best fitting model indicates a 200-m-wide low-velocity zone extending at least to shot site S1 located 2 kin east of the seismic array with a 20% decrease in the P wave velocity relative to the wall rock. The width of the low-velocity zone is consistent with the fault zone defined by direct geological observation in the research tunnel. However, the low-velocity zone should disappear just to the east of the site S1 to explain the observed fault zone waves for shot S3 and S4 located 4 km east of the seismometer array. Yet the observation and the simulation show notable trapped wave excitation even though shots S3 and S4 are outside the fault zone. These results indicate that (1) the effective waveguide for seismic waves along the fault does not exist east of source site S1 although the surface traces of the fault are observed in this region, and (2) considerable trapped waves can be excited by sources well outside the fault zone. These results highlight the along-strike variability in fault zone structure. (C) 2003 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.tecto.2003.09.008

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記述言語：日本語   出版者・発行元：独立行政法人防災科学技術研究所  

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記述言語：日本語   出版者・発行元：独立行政法人防災科学技術研究所  

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記述言語：英語   出版者・発行元：九州大学  

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DOI： 10.15017/1470410

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記述言語：英語   出版者・発行元：九州大学  

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記述言語：日本語   出版者・発行元：九州大学大学院理学研究院  

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記述言語：英語   出版者・発行元：TERRA SCIENTIFIC PUBL CO  

Fault zone waves have the potential to be a powerful tool to reveal the fine structure of a fault zone down to the seismogenic depth. Seismic fault zone waves include head waves, trapped waves and direct body waves propagating in the fault zone. 3-D numerical simulation is necessary to interpret the waveforms in the presence of low-velocity zones with relatively complex fault structure. We computed finite difference (FD) synthetic seismograms to fit the seismograms of explosions, which contain frequencies up to 25 Hz, recorded by a linear seismometer array across the Mozumi-Sukenobu fault, central Japan. We find fault zone head waves, direct P waves propagating within the low-velocity zone and wave trains following the direct P waves associated with the fault for both observed and synthetic waveforms. Thus, modelling of fault zone waves is expected to determine details of complex fault zone structure.

DOI： 10.5636/eps.54.11_1055

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記述言語：英語   出版者・発行元：九州大学  

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記述言語：英語  

DOI： 10.1046/j.1365-246X.2001.01413.x

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記述言語：英語   出版者・発行元：TERRA SCIENTIFIC PUBL CO  

Pseudospectral method is an efficient and high-accuracy numerical modeling technique that has been applied to the simulation of seismic wavefields in realistic models with complex structures such as sedimentary basin. However, if the sedimentary layers are included in the model, high velocity contrasts arise because of the very low S-wave velocities of the sediments as compared with the bedrock. The grid spacing then has to be very small, which leads to spatial oversampling in regions with higher velocities. As a result, we also have to use very small time interval for time marching, so that the computation requires much CPU time. A treatment of this problem is use of a multidomain approach that can employ different grid spacings in different regions, i.e. "discontinuous grid". In this study, we propose a scheme for the Fourier pseudospectral method using discontinuous grid, which uses an interpolation by the FFT. This scheme can reduce the computation time and the required computer memory as compared with the conventional one. The accuracy and efficiency of this approach are demonstrated in the present paper through examples of 2-D elastic wave modeling.

DOI： 10.5636/eps.53.149

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記述言語：英語  

We present a method for modelling seismic wave propagation in a whole-earth model by solving the elastodynamic equations in 2-D cylindrical coordinates (r, 0) using the Fourier pseudospectral method (PSM). In solving the 2-D cylindrical elastodynamic equations for a whole-earth model, a singularity arises at the centre (r = 0) of the earth. To avoid the singularity, we develop a scheme that uses extension of field variables in the radial direction, with which computation of the wavefield at the centre is avoided, so that the wave propagation through the centre can be calculated. The time interval used in the calculation is determined by the smallest lateral grid spacing around the centre in the model. In a cylindrical coordinate system, the smallest lateral grid spacing is generally so small that the calculation is too time-consuming to be realistically carried out even on a supercomputer. We adopt a multidomain scheme to increase the smallest lateral grid spacing and avoid the oversampling of the physical domain around the centre of the earth. A smoothing scheme in the wavenumber domain is also proposed, which enables us to use a large enough time interval to allow the calculation for the whole-earth model on a desktop workstation. The waveforms calculated by the present method are compared with those obtained by the Direct Solution Method (DSM) to demonstrate their high accuracy. This method significantly reduces the computer memory and computation time required and makes it possible to study the effects of small-wavelength heterogeneities that can be approximated as azimuthally symmetric on wave propagation in the earth. We apply the present method to study the effects of local heterogeneity in the earth by adding a low-velocity perturbation above the core-mantle boundary (CMB) to the IASP91 earth model.

DOI： 10.1046/j.1365-246X.2001.01413.x

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DOI： 10.5636/eps.53.149

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記述言語：日本語   出版者・発行元：海洋出版  

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記述言語：日本語   出版者・発行元：公益社団法人 日本地震学会  

The needs for installing broadband seismometers in inconvenient places are increasing. A simple and easy installation method is necessary for this purpose because it is difficult to make seismic observatory consuming much time and money. However only a few researches have been done for this purpose. In this paper we try to find the best way to install seismometers in inconvenient places. To decrease background noises under simple installation, we tested several ways to install broadband seismometers in shallow vaults. We made two vaults with concrete blocks whose sizes are about 1.5m³ and install a broadband seismometer in it. One block was set on the ground, and the other was settled below the ground. We checked background noise level of vertical component for three cases of insulation stuff: sand, styrofoam, and none. The noise level of the seismometers below the ground is lower than that on the ground for all three cases. The noise level is the lowest when the seismometer is installed in the underground vault filled with sand. The order of the amplitude of noise level is in proportion to the temperature fluctuation near the seismometer. Noise levels show higher correlation with temperature than wind speed, precipitation and atmospheric pressure. These suggest that the inhibition of temperature change near seismometers is the key to decrease background noise level. Therefore the installation of seismometer in the underground vault filled with sand achieves the lowest noise level, though the level is about 10dB higher than that in deep tunnel at permanent observatory near the vault.

DOI： 10.4294/zisin1948.54.1_9

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記述言語：英語   出版者・発行元：AMER GEOPHYSICAL UNION  

A simultaneous inversion for three-dimensional velocity structure and hypocentral parameters is used to determine the tomographic images of the backarc of Kyushu, southwestern Japan. The data used in the imaging are the first arrival times of 2736 local earthquakes recorded at 305 stations in a wide region from western Japan to eastern China. Additional data are provided by including arrival times of 357 teleseismic earthquakes. We got the high resolution of the uppermost mantle images for P waves at least in the depth range to 150 km, the latitude range of 30 degrees-34 degrees N and the longitude range of 128 degrees-132 degrees E. Results of inversion reveal a major zone of strong low velocity under west off Kyushu Island in the depth from 40 km to 100 km. This feature may result from partially melting in relation to the upwelling of hot mantle around the northeastern edge of the extending Okinawa trough.

DOI： 10.1029/1999GL008385

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記述言語：英語  

DOI： 10.1029/1999GL008385

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記述言語：英語   出版者・発行元：TERRA SCIENTIFIC PUBL CO  

The natural sedimental deposits in basins show strong vertical heterogeneity in their material parameters. The aim of this paper is to investigate the effects of such vertical heterogeneity, especially vertical velocity gradient, inside basin on the seismic ground motion through the parametric study on the response of a two-dimensional semi-cylindrical sediment-filled basin to a vertical incidence of plane SV wave using the pseudospectral method. This numerical study has tried to find the effects caused by vertical velocity gradient through the use of synthetic seismograms, wavefield snapshots and surface amplitude distribution. Simulation results clearly demonstrate the derailed character of wave propagation phenomena in basins with vertical velocity gradient, which produces characteristic amplification pattern of the surface motion caused mainly by the generation of strong Rayleigh wave induced at the basin edge associated with large lateral velocity change across the basin edge. Amplification pattern at the surface strongly depends on both the vertical velocity gradient in the basin and the predominant frequency of the incident wave. Although similar phenomena on wave propagation and surface motion found in previous studies for homogeneous basin models have also been observed in this study, it has been found that the vertical velocity gradient enhances such phenomena. The results suggest that it is important to represent the vertical velocity profiles accurately when we construct a structural model for realistic modelling of ground motion.

DOI： 10.5636/eps.52.13

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記述言語：英語  

DOI： 10.5636/eps.52.3

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記述言語：英語   出版者・発行元：TERRA SCIENTIFIC PUBL CO  

We have developed an imaging algorithm for subsurface faults on the basis of the steepest descent method in our previous paper. In this paper we introduce two new techniques, the multi-event stacking and multi-master stacking, to improve detectability for subsurface faults. The multi-event stacking is a technique using data from plural earthquakes with different incident angles. The multi-master stacking is a technique using data of a single earthquake, in which the images obtained by employing different master receiver are stacked. We examined the feasibility of our approach using several trial models and synthetic data with different signal to noise ratios, different pulse durations and different incident angles in the two dimensional SH cases. In the numerical experiments, we could recover the image of the subsurface fault for all the trial models used, which indicates our method has a potential for sensing real subsurface faults.

DOI： 10.5636/eps.52.3

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記述言語：日本語   出版者・発行元：物理探査学会  

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記述言語：日本語   出版者・発行元：九州大学大学院理学研究院  

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記述言語：日本語   出版者・発行元：公益社団法人 日本地震学会  

DOI： 10.4294/zisin1948.53.2_185

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記述言語：英語   掲載種別：速報，短報，研究ノート等（学術雑誌）   出版者・発行元：TERRA SCIENTIFIC PUBL CO  

Magmatic reflectors have been detected beneath some volcanic regions by analysing some reflection phases of seismograms. In this letter we consider a problem to determine the location and tangential plane of such a reflector from the ray parameter, back-azimuth and travel time of a reflection phase, and present a scheme to solve it. It is then shown that the problem is essentially a one-dimensional root-finding problem whose independent variable is only the depth of the reflection point, and thus it can be solved by a line search method such as the one-dimensional grid search. While homogeneous velocity structure models have been used in the conventional analyses, the present scheme employs a vertically inhomogeneous one and can also easily treat converted phases at the reflector.

DOI： 10.5636/eps.52.299

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記述言語：英語  

DOI： 10.5636/eps.52.13

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記述言語：英語   出版者・発行元：九州大学  

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記述言語：英語   出版者・発行元：Springer Berlin  

Magmatic reflectors have been detected beneath some volcanic regions by analysing some reflection phases of seismograms. In this letter we consider a problem to determine the location and tangential plane of such a reflector from the ray parameter, back-azimuth and travel time of a reflection phase, and present a scheme to solve it. It is then shown that the problem is essentially a one-dimensional root-finding problem whose independent variable is only the depth of the reflection point, and thus it can be solved by a line search method such as the one-dimensional grid search. While homogeneous velocity structure models have been used in the conventional analyses, the present scheme employs a vertically inhomogeneous one and can also easily treat converted phases at the reflector.

DOI： 10.1186/BF03351639

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記述言語：英語   出版者・発行元：BLACKWELL SCIENCE LTD  

The seismoacoustic scattering due to an irregular fluid-solid interface (i.e. the ocean bottom) must be considered when modelling seismic wave propagation in oceanic regions. In this paper, we present an accurate method to treat this problem. The method is an extension of the reflection/transmission matrix approach that was developed to model scattering due to an irregular interface between two elastic solids. This approach uses the discrete wavenumber representation of the wavefield and the Rayleigh ansatz; the wavefield is decomposed into a set of plane waves, and: we assume that the scattered wavefield due to an incident plane wave can be expanded in terms of only the waves moving away from the interface. The plane waves at:both sides of the interface are then connected to each other based on this hypothesis and proper boundary conditions. The elements of the reflection/transmission matrices describe these connection coefficients.
As the fluid-solid interface, the direct application of the above method is difficult because the tangential displacement at the interface is not necessarily continuous. To avoid this problem, we use pressure as the variable inside,the fluid layer. The reflection/transmission matrices between the pressure wave (i.e. the acoustic wave) and the elastic waves are then developed by using the standard boundary conditions at the fluid-solid interface (i.e. normal stress continuity, zero tangential stress, and normal displacement continuity) and the Rayleigh ansatz.
To demonstrate the validity acid feasibility of our method, we compare the results of our method with those of other methods. First, we examine the reflection of plane elastic waves by a periodic sinusoidal interface for a single frequency. The interface profile is characterized by its periodicity or wavelength (D) and peak to trough height (h). The behaviour of the reflection calculated by our method shows almost complete agreement with that calculated by a boundary element method (BEM) up to a relatively steep interface whose height to wavelength ratio (h/D) is 0.5.
Second, we calculate the synthetic time domain waveforms for a plane vertically incident P wave into basin-like fluid layers. We compare the waveforms calculated by the present method with those calculated by the finite difference method (FDM). In general, the waveforms calculated by the two methods match well. A quantitative study shows that the degree of agreement indicated by the RMS residuals between the waveforms of the two methods becomes better when we use a smaller grid interval in the FDM calculation. For the models treated here, the cases of 30 grid-points per wavelength results in RMS residuals less than about 10-15 per cent. This grid interval is smaller than that usually applied (more than 5-10 grid-points per wavelength). Since, in general, the accuracy of the FDM calculation becomes better as the grid interval decreases, these results indicate the validity of the present method.

DOI： 10.1046/j.1365-246x.1999.00959.x

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記述言語：英語  

DOI： 10.1046/j.1365-246x.1999.00959.x

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記述言語：英語   出版者・発行元：ELSEVIER SCIENCE BV  

Ray tracing between two fixed points is a boundary value problem. An initial path connecting two points, the source and the receiver, is algorithmically perturbed until it converges to a solution giving a minimum travel-time path. In multi-pathing cases, it is quite possible for an algorithm to converge to a ray path with local minimum travel time. Furthermore, when the velocity structure has discontinuities, there is another serious problem, i.e., how to determine the correct intersection between the ray and the surface of discontinuity. In this paper, a new approach for two-point ray tracing is presented, which uses genetic algorithms (GAs) to overcome these problems. Since GAs are efficient global search techniques, the proposed method guarantees to find a global minimum travel-time path and also the correct intersections. A micro-GA implementation is adopted to further enhance computational efficiency. This approach is suitable for tracing the first arriving seismic waves through a 3-D heterogeneous medium with discontinuities. The method can also find the minimum time paths of later arrivals, such as reflected and refracted waves, by constraining the ray to hit a prescribed interface. The accuracy of the method is demonstrated on the numerical examples. (C) 1999 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0031-9201(99)00011-4

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記述言語：英語  

DOI： 10.1016/S0031-9201(99)00011-4

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記述言語：日本語  

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記述言語：日本語   出版者・発行元：九州大学理学部  

Orientation of seismometers at 12 observatories of the K-NET in western Kagoshima region (KGSOO1～KGS012) was measured in this study. We found that stations KGS006 and KGS009 have rather large misorientation greater than 15°．We quickly check the effects of such instrument misorientation on waveform using the records of the 1997 northwestern Kagoshima earthquakes.

DOI： 10.15017/4494281

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記述言語：日本語   出版者・発行元：九州大学  

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In 1997, two moderate earthquakes occurred in the northwestern part of Kagoshima Prefecture (Mjma 6.5 on March 26 and Mjma 6.3 on May 13), and a large number of aftershocks occurred in the southern part of the second event focal region. We started the observation of the aftershocks from April 1, 1998, at Kushikino. We used a broadband strong-motion velocity-type seismograph and an accelerograph in order to record the full waveforms of the strong ground motions. Up to April 25, 1999, we observed 74 events, and 3 events were recorded by both seismographs. In this paper, we report the detail of observational conditions and show some waveforms we obtained.

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記述言語：英語  

DOI： 10.5636/eps.51.73

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記述言語：英語   出版者・発行元：TERRA SCIENTIFIC PUBL CO  

The pseudospectral method is a high-accuracy numerical modelling technique that requires less computer memory and computation time than the traditional techniques such as the finite-difference method. These advantages of the pseudospectral method have enabled us to practically apply this method to modelling realistic problems that have complex structure and source models. However, a major drawback of such numerical schemes for discrete grid models is that even for rather a simple structural model they require as much computational requirements (e.g. computation time and memory) as for an entirely complex structural model with the same size of the simple one. We actually need to employ idealised simple models, such as a model with geometrical symmetry, to investigate basic phenomena of seismic waves, to develop new techniques, or to choose optimal values of some computational parameters for more complex modelling. In this paper we propose an efficient approach of an economical pseudospectral method for calculation of wavefields in models symmetric with respect to a vertical plane or two orthogonal vertical planes. Using this approach, the wavefields only need to be computed in a half or quarter domain of the models, so that the computer memory and computation time can be reduced ideally by half or quarter, respectively, as compared with the calculation of the entire models.

DOI： 10.5636/eps.51.73

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記述言語：英語  

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記述言語：日本語   出版者・発行元：海洋出版  

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記述言語：英語  

DOI： 10.1190/1.1444322

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記述言語：英語   出版者・発行元：SOC EXPLORATION GEOPHYSICISTS  

Three-dimensional pseudospectral modeling for a realistic scale problem is still computationally very intensive, even when using current powerful computers. To overcome this, we have developed a parallel pseudospectral code for calculating the 3-D wavefield by concurrent use of a number of processors. The parallel algorithm is based on a partition of the computational domain, where the field quantities are distributed over a number of processors and the calculation is concurrently done in each subdomain with interprocessor communications. Experimental performance tests using three different styles of parallel computers achieved a fairly good speed up compared with conventional computation on a single processor: maximum speed-up rate of 26 using 32 processors of a Thinking Machine CM-5 parallel computer, 1.6 using a Digital Equipment DEC-Alpha two-CPU workstation, and 4.6 using a cluster of eight Sun Microsystems SPARC-Station 10 (SPARC-10) workstations connected by an Ethernet. The result of this test agrees well with the performance theoretically predicted for each system. To demonstrate the feasibility of our parallel algorithm, we show three examples: 3-D acoustic and elastic modeling of fault-zone trapped waves and the calculation of elastic wave propagation in a 3-D syncline model.

DOI： 10.1190/1.1444322

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記述言語：英語   出版者・発行元：九州大学  

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記述言語：英語   出版者・発行元：九州大学  

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記述言語：日本語  

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記述言語：英語   掲載種別：速報，短報，研究ノート等（学術雑誌）   出版者・発行元：CENTER ACADEMIC PUBL JAPAN  

DOI： 10.4294/jpe1952.45.455

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記述言語：英語   掲載種別：速報，短報，研究ノート等（学術雑誌）  

DOI： 10.4294/jpe1952.45.147

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記述言語：英語   掲載種別：速報，短報，研究ノート等（学術雑誌）  

DOI： 10.4294/jpe1952.45.455

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記述言語：日本語   出版者・発行元：公益社団法人 東京地学協会  

Twin earthquakes (the Amami-Oshima-Kinkai Earthquakes) occurred at the southeast from Kikai Island on October 18 (M_JMA 6.6) and 19 (M_JMA 6.5), 1995. We observed the aftershocks at Kikai Island from October 21, 1995 to February 18, 1996 using the broadband strong-motion seismometer (VSE11C/12C). After all, we observed 40 events that include the second largest aftershock, the event of November 1 (M_JMA 5.6).<BR>In this paper we model the strong-motion records of six events among the observed aftershocks, which have large amplitudes and rather simple waveforms to estimate their focal parameters (focal depth, strike, dip, rake, seismic moment and rise time), by try and error. The results suggest the following : <BR>(1) The strikes of the mainshocks of October 18 (the first mainshock) and 19 (the second mainshock) might be about N230&deg;E and N185&deg;E, respectively;<BR>(2) The hypocenter of the first mainshock might locate near the north edge of the fault which was ruptured from north to south along the fault strike, while the hypocenter of the second one might locate near the south edge of the fault which was ruptured from south to north.

DOI： 10.5026/jgeography.106.4_525

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記述言語：英語   掲載種別：速報，短報，研究ノート等（学術雑誌）   出版者・発行元：CENTER ACADEMIC PUBL JAPAN  

DOI： 10.4294/jpe1952.45.147

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記述言語：英語   出版者・発行元：OXFORD UNIV PRESS  

In order to provide a quantitative analysis of real seismic records from complex regions we need to be able to calculate the wavefields in three dimensions. However, full 3-D modelling of seismic-wave propagation is still computationally intensive. An economical approach to the modelling of seismic-wave propagation which includes many important aspects of the propagation process is to examine the 3-D response of a model where the material parameters vary in two dimensions. Such a configuration, in which a 3-D wavefield is calculated for a 2-D medium, is called the '2.5-D problem'. Recently, Takenaka & Kennett (1996) proposed a 2.5-D time-domain elastodynamic equation for seismic wavefields in models with a 2-D variation in structure but obliquely incident plane waves in the absence of source. This approach is useful even for non-plane waves. In the presence of source a new 2.5-D elastodynamic equation for general anisotropic media can be derived in the time domain based on the Radon transform over slowness in the direction with constant medium properties. The approach can also be formulated in terms of velocity-stress, a representation which is well suited to the use of numerical techniques for 2-D time-domain problems such as velocity-stress finite-difference or velocity-stress pseudospectral techniques.

DOI： 10.1111/j.1365-246X.1996.tb04716.x

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記述言語：英語  

DOI： 10.1111/j.1365-246X.1996.tb04716.x

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記述言語：英語   出版者・発行元：SEISMOLOGICAL SOC AMER  

Finite-difference seismograms are calculated for the 1990 Odawara earthquake to model transverse and radial components of strong-motion data. Using a basin structure derived from a geotechnical survey and double-couple solution determined from P first-motion data and SH/SV amplitude ratio, two-dimensional SH and P/SV finite-difference calculation, along with correction for soft surface layers, reproduces the observed spatial variation in amplitude and waveform of the transverse and radial records at the Ashigara Valley blind prediction sites for frequencies up to 2 Hz. Comparison of the synthetic and the observed seismograms suggests that the observed spatial variation in the SH amplitude is attributed to the source radiation and amplification due to the soft surface layers, while the SV amplitude variation is controlled by the amplification due to the sedimentary basin as well as the source and surface-layer effects.

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記述言語：英語  

DOI： 10.1111/j.1365-246X.1996.tb00001.x

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記述言語：英語   出版者・発行元：OXFORD UNIV PRESS  

Full 3-D modelling of seismic wave propagation is still computationally intensive. Recently, as a compromise between realism and computational efficiency, two-and-a-half-dimensional (2.5-D) methods for calculating 3-D elastic wavefields in media varying in two dimensions have been developed. Such 2.5-D methods are an economical approach for calculating 3-D wavefields, and require a storage capacity only slightly larger than those of the corresponding 2-D calculations.
In this paper, a 2.5-D elastodynamic equation in the time domain is constructed for seismic wavefields in models with a 2-D variation in structure but obliquely incident plane waves. The approach does not require wavenumber summation, and as a result requires much less computation time than in previous techniques. The modelling of such seismic wavefields for a 2.5-D situation with an incident plane wave is of considerable practical importance: for example, this approach can be applied to the modelling of the local response of an irregular basin structure, or to teleseismic body waveforms from shallow earthquakes occurring in subduction zones, where the laterally heterogeneous medium can have a large effect on the waveform.
All the variables in the 2.5-D time-domain elastodynamic equation are real-valued, so the propagation characteristics can be efficiently calculated using 2-D time-domain numerical techniques such as the finite-difference method or the pseudospectral method with less computation time and memory than for other implementations.

DOI： 10.1111/j.1365-246X.1996.tb00001.x

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記述言語：英語  

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記述言語：英語   出版者・発行元：SEISMOLOGICAL SOC AMER  

The Aki-Larner method is one of the cheapest methods for synthetic seismograms in irregularly layered media. In this article, we propose a new approach for a two-dimensional SH problem, solved originally by Aki and Larner (1970). This new approach is not only based on the Rayleigh ansatz used in the original Aki-Larner method but also uses further information on wave fields, i.e., the propagation invariants. We reduce two coupled integral equations formulated in the original Aki-Larner method to a single integral equation. Applying the trapezoidal rule for numerical integration and collocation matching, this integral equation is discretized to yield a set of simultaneous linear equations. Throughout the derivation of these linear equations, we do not assume the periodicity of the interface, unlike the original Aki-Larner method. But the final solution in the space domain implicitly includes it due to use of the same discretization of the horizontal wavenumber as the discrete wavenumber technique for the inverse Fourier transform from the wavenumber domain to the space domain.
The scheme presented in this article is more efficient than the original Aki-Larner method. The computation time and memory required for our scheme are nearly half and one-fourth of those for the original Aki-Larner method. We demonstrate that the band-reduction technique, approximation by considering only coupling between nearby wavenumbers, can accelerate the efficiency of our scheme, although it may degrade the accuracy.

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記述言語：英語   出版者・発行元：BLACKWELL SCIENCE LTD  

A full treatment of topographic effects on the seismic wavefield requires a 3-D treatment of the topography and a 3-D calculation for the wavefield. However, such full 3-D calculations are still very expensive to perform. An economical approach, which does not require the same level of computational resources as full 3-D modelling, is to examine the 3-D response of a model in which the heterogeneity pattern is 2-D (the so-called 2.5-D problem). Such 2.5-D methods can calculate 3-D wavefields without huge computer memory requirements, since they require storage nearly equal to that of the corresponding 2-D calculations.
In this paper, we consider wave propagation from a point source in the presence of 2-D irregular topography, and develop a computational method for such 2.5-D wave-propagation problems. This approach is an extension to the 2.5-D case of the discrete wavenumber-boundary integral equation method introduced by Bouchon (1985) and Gaffet & Bouchon (1989) to study 2-D topographic problems. One of the most significant advantages of the 2.5-D calculations is that calculations are performed for a point source and so it is possible for us to take into account the 3-D radiation pattern from the source. We demonstrate that this discrete wavenumber-boundary integral equation procedure, coupled with a Green's function decomposition into P-and S-wave contributions, provides a flexible and effective means of evaluating the wavefield.

DOI： 10.1111/j.1365-246X.1996.tb05635.x

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記述言語：英語  

DOI： 10.1111/j.1365-246X.1996.tb05640.x

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記述言語：英語   出版者・発行元：BLACKWELL SCIENCE LTD  

The pseudospectral method is a high-accuracy numerical modelling technique which requires less memory and CPU time than traditional numerical modelling techniques, such as finite difference. However, full 3-D modelling is still computationally intensive. As we must compromise between realism and computational efficiency, we have developed a 2.5-D pseudospectral scheme for calculating 3-D elastic wavefields in media varying in two dimensions. To demonstrate the feasibility of the scheme described in this paper, we use data from a refraction survey carried out in 1984 by the Explosion Seismology Group in the southeastern foothills of the Hidaka Mountains, Hokkaido, Japan.

DOI： 10.1111/j.1365-246X.1996.tb05640.x

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記述言語：英語  

DOI： 10.1111/j.1365-246X.1996.tb05635.x

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記述言語：英語   出版者・発行元：CENTER ACADEMIC PUBL JAPAN  

It is important for disaster prevention and public safety design to know whether subsurface faults exist, not only in Kobe, which was heavily damaged by the 1995 Hyogo-ken Nanbu earthquake, but also in any urban area which is tectonically active. However, there is still little information on subsurface faults. In this paper, we propose a new approach for sensing subsurface faults using teleseismic-wave array records. Our new method employs an imaging technique similar to the reflection method, but is much less expensive and can be used in urban areas. The target of our method is not the velocity structure itself, but rather the position and size of a subsurface fault. The imaging algorithm is based on the steepest descent method, which is also used in waveform inversion of reflection data. However, since iteration is not required by our approach, only one forward computation together with reverse-time computation of the wavefield is required. The method proposed in this paper consists of the following steps: 1) pre-processing observed records, 2) forward calculation, 3) calculation of the steepest descent direction of the model parameters, and 4) image analysis and identification of faults. The feasibility of the method is demonstrated using SH synthetic data.

DOI： 10.4294/jpe1952.44.529

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