Faculty Profiles - TAKENAKA Hiroshi

Papers - TAKENAKA Hiroshi

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FAMT Analysis of Suboceanic Earthquakes off the Okinawa Island Estimated by Using a Land-Ocean Unified 3D Structure Model Reviewed

KOMATSU Masanao, TAKENAKA Hiroshi, OKAMOTO Taro, NAKAMURA Takeshi

Zisin (Journal of the Seismological Society of Japan. 2nd ser.) 76 17 - 30 2023.5

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：SEISMOLOGICAL SOCIETY OF JAPAN

In this study, we conduct First-motion Augmented Moment Tensor (FAMT) analysis to estimate the source parameters of small-to-moderate suboceanic earthquakes (M_JMA 4.5-5.8) occurring off the Okinawa island, Japan, from October 2007 to April 2020. We calculate Green’s functions for a land-ocean unified 3D structure model that includes non-flat complicated land and seafloor topographies, the sediment layers, the oceanic plate and Moho interfaces obtained by compiling with recent seismic survey results. For the inversion, we use velocity waveform data in two period ranges of 4-40 s and 10-40 s and reproduce both the body and surface waves at K-NET and F-net stations. The results indicate that two of ten events are located in the oceanic mantle of the subducting Philippine Sea (PHS) slab, and eight ones are located in the oceanic crust of the PHS slab. The focal mechanisms of the former two events are primarily strike-slip and/or normal-fault type. The latter crustal events have mechanisms of reverse-fault type with the extension axis of NW-SE direction, which is consistent with the subducting direction of the slab. It is found that the depth difference between the JMA unified hypocenter catalog in travel time data and our estimated FAMT solution is large for events near the Ryukyu trench. Our results show that the offshore events in the trench side are located around the submarine active faults and are the same strike direction as the orientation of the faults.

DOI： 10.4294/zisin.2022-7

Deep Subsurface S-Wave Velocity Structure in the Amami Islands, Tokara Islands, and Yakushima Island, Japan Reviewed

YAMADA Nobuyuki, TAKENAKA Hiroshi

Journal of Japan Association for Earthquake Engineering 23 ( 5 ) 5_21 - 5_34 2023

Authorship：Last author Language：Japanese Publishing type：Research paper (scientific journal) Publisher：JAPAN ASSOCIATION FOR EARTHQUAKE ENGINEERING

In this study, we estimated the S-wave velocity structure of the deep subsurface using microtremor array data in the Amami Islands, the Tokara Islands, and Yakushima Island. The depth of the top of the seismic basement is about 2 km with the largest depth at Kikaishima Island, and the depths at the other islands are 0.2 to 0.6 km. And also shown are the mean Vs values for each layer and the layer boundary depths at each location for the four-layer model, which were 0.69, 1.02, 2.18, and 3.49 km/s, respectively. These mean values are close to those of the Yaeyama Islands, Miyako Islands and Okinawa Islands. These results can be used as basic data for improving the S-wave velocity structure model for the Nansei Islands.

DOI： 10.5610/jaee.23.5_21

High‐Resolution Fault‐Rupture Imaging by Combining a Backprojection Method With Binarized MUSIC Spectral Image Calculation Reviewed

Mitsutaka Oshima, Hiroshi Takenaka, Makoto Matsubara

Journal of Geophysical Research: Solid Earth 127 ( 11 ) 2022.11

Enhancement of direct waves based on the probability density function of seismic wave amplitudes Reviewed

Mitsutaka Oshima, Hiroshi Takenaka

Geophysical Journal International 231 ( 1 ) 327 - 354 2022.6

Authorship：Last author Language：English Publishing type：Research paper (scientific journal) Publisher：Oxford University Press (OUP)

SUMMARY

The enhancement of direct waves in seismic records is desirable in various seismological situations, where the accurate picking of seismic phases is necessary for hypocentre location, spectral inversion, waveform inversion and seismic tomography. In this study, we developed a new method to enhance direct waves in seismic waveforms based on the statistical distribution (probability density function, PDF) of the amplitudes of waveforms. The seismic waveform changes from a direct wave to a coda wave as time proceeds, which also causes the PDF of its amplitude to vary with time. The PDF becomes similar to a Gaussian or Rayleigh distribution with time, and hence, direct waves can be enhanced using the dissimilarity between the PDF of the amplitude of the seismic waveform and the Gaussian or Rayleigh distribution. In the proposed method, the Kullback–Leibler divergence is used to measure dissimilarity and it is applied as a weight. We confirmed that the method can effectively enhance direct P and S waves by applying it to seismic records of the 2016 Kumamoto earthquake, synthetic waveforms with noise and volcanic events. The method was also applied to waveforms of the 2008 Iwate–Miyagi Nairiku earthquake, which revealed that direct waves were enhanced, and an overview of the wave field and rupture process was obtained. The proposed method can detect direct waves even from subevents, thereby producing detailed information on the earthquake rupture process. We detected two significant pulses and determined their source location and rupture times. The estimated pulse sources were consistent with parts of the rupture scenario proposed by the previous studies. In addition, for local and teleseismic event records, the proposed method was compared with other methods to study its merits and demerits. The present method can enhance seismic phases on records of local and teleseismic events. Our method detects or enhances direct waves based neither on their amplitudes nor on their temporal variations but on the statistical distribution of their amplitudes, which has both advantages and disadvantages. Although polarization filtering is a powerful method that can enhance seismic phases, it requires three-component waveforms. The proposed method can be applied even to a single component record, which is a significant advantage.

DOI： 10.1093/gji/ggac193

Other Link： https://academic.oup.com/gji/article-pdf/231/1/327/44160493/ggac193.pdf

Estimation of Seismic Attenuation of the Greenland Ice Sheet Using 3-D Waveform Modeling Reviewed

Genti Toyokuni, Masanao Komatsu, Hiroshi Takenaka

JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH 126 ( 4 ) 2021.4

Authorship：Last author Language：English Publishing type：Research paper (scientific journal) Publisher：AMER GEOPHYSICAL UNION

We estimated the seismic attenuation (Q factor) of the Greenland Ice Sheet (GrIS) by comparing observed and theoretical Rayleigh waveforms. Observed waveforms are obtained by interfering noise waveforms in vertical-component seismograms between stations belonging to the latest broadband seismic network distributed throughout Greenland (GLISN network). Theoretical waveforms are calculated by parallel computation using the latest 3-D seismic waveform modeling method. Comparing the observed waveforms with the theoretical waveforms at different Q factors reveals that the GrIS has a low Q of 10 <= Q(P), Q(S) <= 50, indicating very high attenuation of seismic waves due to the ice. This study is the first to establish the low Q factor of ice sheets via ultra-long-distance propagation (350-1,000 km). The Q factors obtained in this study are indispensable for estimating the thermal state and density distribution of the GrIS, as well as for interpreting the characteristics of seismic waveform that propagates through the GrIS.Plain Language Summary Seismic anelastic attenuation refers to the energy loss caused by anelastic processes or internal friction during wave propagation and is quantified by the quality (Q) factor. The Q factor of ice sheets and glacial ice is especially important because it is highly sensitive to the thermal state and density distribution. We estimated the Q factor of the Greenland Ice Sheet (GrIS) by comparing observed and theoretical surface (Rayleigh) waveforms. Observed waveforms are extracted by the latest broadband seismic network distributed throughout Greenland (GLISN network). Theoretical waveforms are calculated using the latest 3-D seismic waveform modeling method. The results showed an extremely low Q (10-50) for the GrIS, indicating very high attenuation of seismic waves. Although previous studies have obtained similar results, they all used high frequency (>= 30 Hz) seismic waves with short propagation distances (<similar to 10 km). This study is the first to infer the large-scale, bulk Q property of the GrIS using low-frequency (0.1-0.3 Hz) waves and long (350-1,000 km) propagation distances. The Q factors obtained in this study are essential for estimating the internal state of the GrIS, as well as for interpreting the characteristics of seismic waveforms that propagate through it.

DOI： 10.1029/2021JB021694

DEEP SUBSURFACE S-WAVE VELOCITY STRUCTURE OF OKINAWA ISLANDS, JAPAN—FOR NUMERICAL MODELING Reviewed

Nobuyuki YAMADA, Hiroshi TAKENAKA

Journal of Japan Association for Earthquake Engineering 21 ( 3 ) 3_119 - 3_134 2021

A P- and S-Wave Picking Technique Based on the Probabilistic Density Function of Seismic Waveform Amplitude (vol 110, pg 763, 2020) Reviewed

Mitsutaka Oshima, Hiroshi Takenaka

BULLETIN OF THE SEISMOLOGICAL SOCIETY OF AMERICA 110 ( 6 ) 3211 - 3212 2020.12

A P- and S-Wave Picking Technique Based on the Probabilistic Density Function of Seismic-Waveform Amplitude Reviewed

Mitsutaka Oshima, Hiroshi Takenaka

BULLETIN OF THE SEISMOLOGICAL SOCIETY OF AMERICA 110 ( 2 ) 763 - 782 2020.4

Authorship：Last author Language：English Publishing type：Research paper (scientific journal) Publisher：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

Deep Subsurface S-wave Structure of Okinawa Islands, Japan Reviewed

Nobuyuki YAMADA, Hiroshi TAKENAKA

Journal of Japan Association for Earthquake Engineering 19 ( 4 ) 4_156 - 4_169 2019.8

Evaluation of accuracy of synthetic waveforms for subduction-zone earthquakes by using a land-ocean unified 3D structure model Reviewed

Taro Okamoto, Hiroshi Takenaka, Takeshi Nakamura

EARTH PLANETS AND SPACE 70 2018.6

Language：English Publishing type：Research paper (scientific journal) Publisher：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

Changes in Greenland ice bed conditions inferred from seismology Reviewed

Genti Toyokuni, Hiroshi Takenaka, Ryota Takagi, Masaki Kanao, Seiji Tsuboi, Yoko Tono, Dean Childs, Dapeng Zhao

PHYSICS OF THE EARTH AND PLANETARY INTERIORS 277 81 - 98 2018.4

Language：English Publishing type：Research paper (scientific journal) Publisher：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

Deep Subsurface S-wave Structure of Sakishima (Miyako and Yaeyama) Islands, Southwestern Japan Reviewed

Nobuyuki YAMADA, Hiroshi TAKENAKA

Journal of Japan Association for Earthquake Engineering 18 ( 1 ) 1_77 - 1_88 2018.2

Three-dimensional P- and S-wave attenuation structures around the source region of the 2016 Kumamoto earthquakes Reviewed

Masanao Komatsu, Hiroshi Takenaka, Hitoshi Oda

EARTH PLANETS AND SPACE 69 2017.7

Language：English Publishing type：Research paper (scientific journal) Publisher：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

FDM simulation of earthquakes off western Kyushu, Japan, using a land-ocean unified 3D structure model Reviewed

Taro Okamoto, Hiroshi Takenaka, Takeshi Nakamura, Tatsuhiko Hara

EARTH PLANETS AND SPACE 69 2017.7

Language：English Publishing type：Research paper (scientific journal) Publisher：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

Quasi-Cartesian finite-difference computation of seismic wave propagation for a three-dimensional sub-global model Reviewed

Hiroshi Takenaka, Masanao Komatsu, Genti Toyokuni, Takeshi Nakamura, Taro Okamoto

EARTH PLANETS AND SPACE 69 2017.5

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal) Publisher：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

Special issue "The 2015 Gorkha, Nepal, earthquake and Himalayan studies: First results" PREFACE

Hiroe Miyake, Soma Nath Sapkota, Bishal Nath Upreti, Laurent Bollinger, Tomokazu Kobayashi, Hiroshi Takenaka

EARTH PLANETS AND SPACE 69 2017.1

Long-period ocean-bottom motions in the source areas of large subduction earthquakes Reviewed

Takeshi Nakamura, Hiroshi Takenaka, Taro Okamoto, Michihiro Ohori, Seiji Tsuboi

SCIENTIFIC REPORTS 5 2015.11

Language：English Publishing type：Research paper (scientific journal) Publisher：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

Global SH-wave propagation in a 2D whole Moon model using the parallel hybrid PSM/FDM method Reviewed

Xianghua Jiang, Yanbin Wang, Yanfang Qin, Hiroshi Takenaka

EARTHQUAKE SCIENCE 28 ( 3 ) 163 - 174 2015.6

Authorship：Last author Language：English Publishing type：Research paper (scientific journal) Publisher：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

Numerical modeling of seismic waves for estimating the influence of the Greenland ice sheet on observed seismograms Reviewed

Genti Toyokuni, Hiroshi Takenaka, Masaki Kanao, Seiji Tsuboi, Yoko Tono

POLAR SCIENCE 9 ( 1 ) 80 - 93 2015.3

Language：English Publishing type：Research paper (scientific journal) Publisher：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

Anomalously large seismic amplifications in the seafloor area off the Kii peninsula Reviewed

Takeshi Nakamura, Masaru Nakano, Naoki Hayashimoto, Narumi Takahashi, Hiroshi Takenaka, Taro Okamoto, Eiichiro Araki, Yoshiyuki Kaneda

MARINE GEOPHYSICAL RESEARCH 35 ( 3 ) 255 - 270 2014.9

Language：English Publishing type：Research paper (scientific journal) Publisher：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

Seismic Wavefields in the Deep Seafloor Area from a Submarine Landslide Source Reviewed

Takeshi Nakamura, Hiroshi Takenaka, Taro Okamoto, Yoshiyuki Kaneda

PURE AND APPLIED GEOPHYSICS 171 ( 7 ) 1153 - 1167 2014.7

Language：English Publishing type：Research paper (scientific journal) Publisher：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

Monitoring of the Greenland ice sheet using a broadband seismometer network: the GLISN project Reviewed

Genti Toyokuni, Masaki Kanao, Yoko Tono, Tetsuto Himeno, Seiji Tsuboi, Dean Childs, Kent Anderson, Hiroshi Takenaka

Antarctic Record 58 ( 1 ) 1 - 18 2014.3

Authorship：Last author Language：English Publishing type：Research paper (scientific journal)

Global climate change is currently causing melting of the Greenland ice sheet. Recently, a new type of seismic event, referred to as a "glacial earthquake", has been recognized. Such earthquakes are generated by the movements of large masses of ice within the terminal regions of glacier, and represent a new approach for monitoring ice sheet dynamics. In 2009, the multinational GreenLand Ice Sheet monitoring Network (GLISN), a large broadband seismological network in and around Greenland, was initiated to monitor these events. Japan, a partner country of the GLISN project, has been sending a field team to Greenland each year since 2011, when a joint USA and Japanese team first established a dual seismic-GPS station (station code: ICESG-GLS2) on the Greenland ice sheet. In 2012, the same team contributed to the maintenance of ICESG-GLS2, as well as two other stations (NUUK and DY2G-GLS1). The quality of the long-period seismic waveform data obtained by these stations has been checked by comparing the data with global synthetic seismograms. Results indicate that the data from the three stations have not been substantially affected by noise, and that the quality is well controlled.

DOI： 10.15094/00009722

Exploration of Deep S-Wave Velocity Structure in Fukuoka Area Reviewed

YAMADA Nobuyuki, TAKENAKA Hiroshi

Zisin (Journal of the Seismological Society of Japan. 2nd ser.) 67 ( 1 ) 25 - 34 2014

Modelling two-dimensional global seismic wave propagation in a laterally heterogeneous whole-Moon model Reviewed

Yanbin Wang, Hiroshi Takenaka, Xianghua Jiang, Jianshe Lei

GEOPHYSICAL JOURNAL INTERNATIONAL 192 ( 3 ) 1271 - 1287 2013.3

Language：English Publishing type：Research paper (scientific journal) Publisher：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

FDTD3C-A FORTRAN program to model multi-component seismic waves for vertically heterogeneous attenuative media Reviewed

Arash JafarGandomi, Hiroshi Takenaka

COMPUTERS & GEOSCIENCES 51 314 - 323 2013.2

Authorship：Last author Language：English Publishing type：Research paper (scientific journal) Publisher：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

Accelerating large-scale simulation of seismic wave propagation by multi-GPUs and three-dimensional domain decomposition Reviewed

Taro Okamoto, Hiroshi Takenaka, Takeshi Nakamura, Takayuki Aoki

Lecture Notes in Earth System Sciences ( 9783642164040 ) 375 - 389 2013

Language：English Publishing type：Part of collection (book) Publisher：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

Seismic ground motion amplification in a 3D sedimentary basin: The effect of the vertical velocity gradient Reviewed

Yanfang Qin, Yanbin Wang, Hiroshi Takenaka, Xianbing Zhang

Journal of Geophysics and Engineering 9 ( 6 ) 761 - 772 2012.12

Language：English Publishing type：Research paper (scientific journal)

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

FDM simulation of seismic-wave propagation for an aftershock of the 2009 Suruga bay earthquake: Effects of ocean-bottom topography and seawater layer Reviewed

Takeshi Nakamura, Hiroshi Takenaka, Taro Okamoto, Yoshiyuki Kaneda

Bulletin of the Seismological Society of America 102 ( 6 ) 2420 - 2435 2012.12

Language：English Publishing type：Research paper (scientific journal)

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

Comparison of global synthetic seismograms calculated using the spherical 2.5-D finite-difference method with observed long-period waveforms including data from the intra-Antarctic region Reviewed

Genti Toyokuni, Hiroshi Takenaka, Masaki Kanao, Douglas A. Wiens, Andrew Nyblade

Polar Science 6 ( 2 ) 155 - 164 2012.7

Language：English Publishing type：Research paper (scientific journal)

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

Accurate and efficient modeling of global seismic wave propagation for an attenuative Earth model including the center Reviewed

Genti Toyokuni, Hiroshi Takenaka

Physics of the Earth and Planetary Interiors 200-201 45 - 55 2012.6

Authorship：Last author Language：English Publishing type：Research paper (scientific journal)

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

Quasi-axisymmetric Finite-difference Method for Realistic Modeling of Regional and Global Seismic Wavefield - Review and Application - Reviewed

Toyokuni, G, M. Kanao, H. Takenaka

In: Kanao, M., H. Takenaka, Y. Murai, J. Matsushima and G. Toyokuni (Eds.), Seismic Waves – Research and Analysis - ISBN 978-953-307-944-8, ( Rijeka, Croatia, InTech. ) Publisher, pp.85-112, - doi:10.5772/1400 2012

SH-wavefield simulation for a laterally heterogeneous whole-Earth model using the pseudospectral method Reviewed

YanBin Wang, Hiroshi Takenaka

Science China Earth Sciences 54 ( 12 ) 1940 - 1947 2011.12

Authorship：Last author Language：English Publishing type：Research paper (scientific journal)

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

A Study of the Finite Difference Solution for 3D Seismic Wavefields near a Fluid-Solid Interface Reviewed

NAKAMURA Takeshi, TAKENAKA Hiroshi, OKAMOTO Taro, KANEDA Yoshiyuki

Zisin (Journal of the Seismological Society of Japan. 2nd ser.) 63 ( 3 ) 189 - 196 2011

A scheme to treat the singularity in global seismic wavefield simulation using pseudospectral method with staggered grids Reviewed

Yanbin Wang, Hiroshi Takenaka

Earthquake Science 23 ( 2 ) 121 - 127 2010.4

Authorship：Last author Language：English Publishing type：Research paper (scientific journal)

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

Three component 1D viscoelastic FDM for plane-wave oblique incidence Reviewed

Arash Jafargandomi, Hiroshi Takenaka

Advances in Geosciences: Volume 20: Solid Earth (SE) 299 - 312 2010.1

Authorship：Last author Language：English Publishing type：Part of collection (book) Publisher：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

Accelerating large-scale simulation of seismic wave propagation by multi-GPUs and three-dimensional domain decomposition Reviewed

Taro Okamoto, Hiroshi Takenaka, Takeshi Nakamura, Takayuki Aoki

Earth, Planets and Space 62 ( 12 ) 939 - 942 2010

Language：English Publishing type：Research paper (scientific journal) Publisher：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

ACE-A FORTRAN subroutine for analytical computation of effective grid parameters for finite-difference seismic waveform modeling with standard Earth models Reviewed

Genti Toyokuni, Hiroshi Takenaka

Computers and Geosciences 35 ( 3 ) 635 - 643 2009.3

Authorship：Last author Language：English Publishing type：Research paper (scientific journal)

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

Effect of near-source trench structure on teleseismic body waveforms: An application of a 2.5D FDM to the Java trench Reviewed

Taro Okamoto, Hiroshi Takenaka

Advances in Geosciences: Volume 13: Solid Earth (SE) 215 - 230 2009.1

Authorship：Last author Language：English Publishing type：Part of collection (book) Publisher：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

Waveform inversion for slip distribution of the 2006 Java tsunami earthquake by using 2.5D finite-difference Green's function Reviewed

Taro Okamoto, Hiroshi Takenaka

EARTH PLANETS AND SPACE 61 ( 5 ) E17 - E20 2009

Authorship：Last author Language：English Publishing type：Research paper (scientific journal) Publisher：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

Rupture process at the beginning of the 2007 Chuetsu-oki, Niigata, Japan, earthquake Reviewed

Hiroshi Takenaka, Yosuke Yamamoto, Hirokazu Yamasaki

EARTH PLANETS AND SPACE 61 ( 2 ) 279 - 283 2009

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal) Publisher：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

Stochastic ground motion simulation of the 12 October 1992 Dahshour earthquake Reviewed

Sayed S.R. Moustafa, Hiroshi Takenaka

Acta Geophysica 57 ( 3 ) 636 - 656 2009

Authorship：Last author Language：English Publishing type：Research paper (scientific journal)

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

Non-standard FDTD for elastic wave simulation: Two-dimensional P-SV case Reviewed

Arash JafarGandomi, Hiroshi Takenaka

Geophysical Journal International 178 ( 1 ) 282 - 302 2009

Authorship：Last author Language：English Publishing type：Research paper (scientific journal)

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

Source Modeling of the 2007 Niigataken Chuetsu-oki Earthquake Using the Empirical Green's Function Method Reviewed

YAMAMOTO Yosuke, TAKENAKA Hiroshi

Zisin (Journal of the Seismological Society of Japan. 2nd ser.) 62 ( 2 ) 137 - 138 2009

A compact representation of spatio-temporal slip distribution on a rupturing fault Reviewed

Hiroshi Takenaka, Yushiro Fujii

Journal of Seismology 12 ( 2 ) 281 - 293 2008.4

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal)

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

A study of characters of the Umi fault along the eastern margin of the Fukuoka Plain, southwest Japan Reviewed

Shimoyama Shoichi, Matsusue Kazuyuki, Matsuyama Hisanori, Yamamori Kunio, Iso Nozomi, Chida Noboru, Okamura Makoto, Matsuoka Hiromi, Ikeda Yasutaka, Matsuda Tokihiko, Takenaka Hiroshi, Ishimura Daisuke

Active Fault Research 2008 ( 29 ) 59 - 70 2008

Language：Japanese Publisher：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

Efficient FDTD algorithm for plane-wave simulation for vertically heterogeneous attenuative media (vol 72, pg H43, 2007)

Arash Jafar-Gandomi, Hiroshi Takenaka

GEOPHYSICS 72 ( 6 ) Y7 - Y7 2007.11

Efficient FDTD algorithm for plane-wave simulation for vertically heterogeneous attenuative media Reviewed

Arash JafarGandomi, Hiroshi Takenaka

GEOPHYSICS 72 ( 4 ) H43 - H53 2007.7

Authorship：Last author Language：English Publishing type：Research paper (scientific journal) Publisher：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

Evidence for a metastable olivine wedge inside the subducted Mariana slab Reviewed

Satoshi Kaneshima, Taro Okamoto, Hiroshi Takenaka

Earth and Planetary Science Letters 258 ( 1-2 ) 219 - 227 2007.6

Authorship：Last author Language：English Publishing type：Research paper (scientific journal)

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

FDM computation of seismic wavefield for an axisymmetric earth with a moment tensor point source Reviewed

Genti Toyokuni, Hiroshi Takenaka

EPS 58 ( 12 ) e29 - e32 2006.12

Precise location of the fault plane and the onset of the main rupture of the 2005 West Off Fukuoka Prefecture earthquake Reviewed

Hiroshi Takenaka, Takeshi Nakamura, Yosuke Yamamoto, Genti Toyokuni, Hiroshi Kawase

Earth, Planets and Space 58 ( 1 ) 75 - 80 2006

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal)

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

A numerical analysis of seismic waves for an anisotropic fault zone Reviewed

Takeshi Nakamura, Hiroshi Takenaka

Earth, Planets and Space 58 ( 5 ) 569 - 582 2006

Authorship：Last author Language：English Publishing type：Research paper (scientific journal) Publisher：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

Quasi-spherical approach for seismic wave modeling in a 2-D slice of a global Earth model with lateral heterogeneity Reviewed International journal

Toyokuni, G., Takenaka, H., Wang, Y., Kennett, B. L. N.

Geophysical Research Letters 32 ( 9 ) L09305 2005.5

Fluid-Solid Boundary Implementation in the Velocity-Stress Finite-Difference Method Reviewed

OKAMOTO Taro, TAKENAKA Hiroshi

Zisin : Journal of the Seismological Society of Japan 57 ( 3 ) 355 - 364 2005.2

A Three-dimensionally Heterogeneous Structure Model for Seismic Wavefield Simulation in Northwestern Kagoshima Region Reviewed

FUJII Yushiro, TAKENAKA Hiroshi, MIYAMACHI Hiroki

Zisin : Journal of the Seismological Society of Japan 57 ( 3 ) 365 - 375 2005.2

An Efficient FDTD Solution for Seismic Plane-wave Responses of Vertically Heterogeneous Media Reviewed

TANAKA Hiroki, TAKENAKA Hiroshi

Zisin : Journal of the Seismological Society of Japan 57 ( 3 ) 343 - 354 2005.2

Influence of Anisotropy in the Fault Zone on the Seismic Waves Reviewed

NAKAMURA Takeshi, TAKENAKA Hiroshi

Zisin : Journal of the Seismological Society of Japan 57 ( 3 ) 331 - 342 2005.2

Strong S-wave anisotropy in the aftershock region of the 2000 Tottori-ken Seibu, Japan, earthquake (Mw6.6) Reviewed

Takeshi Nakamura, Hiroshi Takenaka, Sadaomi Suzuki

Earth, Planets and Space 57 ( 11 ) 1055 - 1062 2005

Language：English Publishing type：Research paper (scientific journal) Publisher：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

Strong attenuation of shear waves in the focal region of the 1997 Northwestern Kagoshima earthquakes, Japan Reviewed

Y Mamada, H Takenaka

BULLETIN OF THE SEISMOLOGICAL SOCIETY OF AMERICA 94 ( 2 ) 464 - 478 2004.4

Authorship：Last author Language：English Publishing type：Research paper (scientific journal) Publisher：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.

Seismic structure of the crust and uppermost mantle in the incipient stage of back arc rifting - northernmost Okinawa Trough Reviewed

Kazuo Nakahigashi, Masanao Shinohara, Sadaomi Suzuki, Ryota Hino, Hajime Shiobara, Hiroshi Takenaka, Minoru Nishino, Takeshi Sato, Shinji Yoneshima, Toshihiko Kanazawa

Geophys. Res. Lett. 31 ( 2 ) n/a - n/a 2004.1

Language：English Publishing type：Research paper (scientific journal) Publisher：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

Near-source effect on radiation pattern of high-frequency S waves: strong SH-SV mixing observed from aftershocks of the 1997 Northwestern Kagoshima, Japan, earthquakes Reviewed

H Takenaka, Y Mamada, H Futamure

PHYSICS OF THE EARTH AND PLANETARY INTERIORS 137 ( 1-4 ) 31 - 43 2003.5

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal) Publisher：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

Transition zone thickness beneath Indonesia as inferred using the receiver function method for data from the JISNET regional broadband seismic network Reviewed

T Saita, D Suetsugu, T Ohtaki, H Takenaka, K Kanjo, Purwana, I

GEOPHYSICAL RESEARCH LETTERS 29 ( 7 ) 2002.4

Language：English Publishing type：Research paper (scientific journal) Publisher：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

不規則成層構造における地震波動場の理論的研究

竹中博士

北海道大学学位論文 1990

近地地震波の伝播に関する理論 Reviewed

纐纈一起, 竹中博士

地震. 2輯 42 ( 3 ) 391 - 403 1989

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