研究者詳細 - 竹中　博士

論文 - 竹中　博士

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

Sayed S.R. Moustafa, Hiroshi Takenaka

Acta Geophysica 57 ( 3 ) 636 - 656 2009年

担当区分：最終著者記述言語：英語掲載種別：研究論文（学術雑誌）

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

DOI： 10.2478/s11600-009-0012-y

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

Arash JafarGandomi, Hiroshi Takenaka

Geophysical Journal International 178 ( 1 ) 282 - 302 2009年

担当区分：最終著者記述言語：英語掲載種別：研究論文（学術雑誌）

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

DOI： 10.1111/j.1365-246X.2009.04101.x

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経験的グリーン関数法を用いた 2007年新潟県中越沖地震の震源のモデル化査読

山本容椎, 竹中博士

地震第2輯 62 ( 2 ) 137 - 138 2009年

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

Hiroshi Takenaka, Yushiro Fujii

Journal of Seismology 12 ( 2 ) 281 - 293 2008年4月

担当区分：筆頭著者記述言語：英語掲載種別：研究論文（学術雑誌）

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

DOI： 10.1007/s10950-007-9087-6

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福岡平野東縁部に位置する宇美断層の特徴について査読

下山正一, 松末和之, 松山尚典, 山盛邦生, 磯望, 千田昇, 岡村眞, 松岡裕美, 池田安隆, 松田時彦, 竹中博士, 石村大輔

活断層研究 2008 ( 29 ) 59 - 70 2008年

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

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

DOI： 10.11462/afr1985.2008.29_59

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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年11月

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

Arash JafarGandomi, Hiroshi Takenaka

GEOPHYSICS 72 ( 4 ) H43 - H53 2007年7月

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

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

DOI： 10.1190/1.2732555

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

Satoshi Kaneshima, Taro Okamoto, Hiroshi Takenaka

Earth and Planetary Science Letters 258 ( 1-2 ) 219 - 227 2007年6月

担当区分：最終著者記述言語：英語掲載種別：研究論文（学術雑誌）

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

DOI： 10.1016/j.epsl.2007.03.035

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

Earth, planets and space 58 ( 12 ) e29 - e32 2006年12月

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

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

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

担当区分：筆頭著者記述言語：英語掲載種別：研究論文（学術雑誌）

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

DOI： 10.1186/BF03351917

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

Takeshi Nakamura, Hiroshi Takenaka

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

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

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

DOI： 10.1186/BF03351954

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Quasispherical approach for seismic wave modeling in a 2-D slice of a global Earth model with lateral heterogeneity 査読国際誌

TOYOKUNI G.

Geophys. Res. Lett. 32 ( 9 ) L09305 2005年5月

速度・応力型差分法での固体・流体境界の扱いについて査読

岡元太郎, 竹中博士

地震. 2輯 57 ( 3 ) 355 - 364 2005年2月

地震波動シミュレーションのための鹿児島県北西部における3次元速度構造モデル査読

藤井雄士郎, 竹中博士, 宮町宏樹

地震. 2輯 57 ( 3 ) 365 - 375 2005年2月

鉛直方向任意不均質弾性媒質における平面波入射問題の時間領域差分解法査読

田中宏樹, 竹中博士

地震. 2輯 57 ( 3 ) 343 - 354 2005年2月

断層破砕帯の異方性が地震波形に及ぼす影響の検討査読

中村武史, 竹中博士

地震. 2輯 57 ( 3 ) 331 - 342 2005年2月

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

Takeshi Nakamura, Hiroshi Takenaka, Sadaomi Suzuki

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

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

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

DOI： 10.1186/BF03351884

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

Y Mamada, H Takenaka

BULLETIN OF THE SEISMOLOGICAL SOCIETY OF AMERICA 94 ( 2 ) 464 - 478 2004年4月

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

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

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

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年1月

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

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

DOI： 10.1029/2003gl018928

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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 査読

H Takenaka, Y Mamada, H Futamure

PHYSICS OF THE EARTH AND PLANETARY INTERIORS 137 ( 1-4 ) 31 - 43 2003年5月

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

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

DOI： 10.1016/S0031-9201(03)00006-2

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