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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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