Faculty Profiles - TAKENAKA Hiroshi

Papers - TAKENAKA Hiroshi

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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

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