巻号一覧
57 巻 (2004 - 2005)
- 4 号 p. 409-
- 3 号 p. 245-
- 2 号 p. 73-
- 1 号 p. 1-
前身誌
57 巻, 3 号
選択された号の論文の17件中1～17を表示しています
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上原 大二郎, 角田 地文, 工藤 健, 梅田 浩司, 小川 康雄, 棚瀬 充史, 武田 祐啓, 千葉 昭彦, 菊池 晃, 鍵山 恒臣2005 年 57 巻 3 号 p. 245-255
発行日: 2005/02/20
公開日: 2010/03/11
ジャーナル フリーGravity interpretation and magnetotelluric (MT) survey were carried out in the southern part of the Kii peninsula, where many anomalous high temperature hot springs are distributed, in order to estimate the geothermal structure in the crust at depths up to nearly 30km. The results and their prospects of this survey are as follows: (1) A high-density and resistive body exists in the central region of the survey area. It extends from the southernmost part of the survey area toward the north. This body may geologically coincide with the pluton of Omine acidic rocks; (2) An extremely conductive layer is located at the depth of 5 to 15km in the western region of the survey area. It implies a reservoir of hypersaline water; (3) The lower crust appears as conductive layer. This feature implies that the conductive lower crust is saturated with water brought by the subducting Philippine Sea plate; (4) Hot springs around the Hongu town are located along the extension of the crustal boundary between the high-density and resistive body and the conductive layer. This fact suggests that the fluid migrated from the lower crust through the fractured zone between the Shimanto super group and Tertiary acidic rocks cause high temperature and high 3He emanation associated with springs in this region.抄録全体を表示PDF形式でダウンロード (2094K) -
1990年小田原地震 (MJ5.1) における小田原市久野地区の強震記録を用いて大堀 道広2005 年 57 巻 3 号 p. 257-273
発行日: 2005/02/20
公開日: 2010/03/11
ジャーナル フリーTo obtain appropriate source parameters from waveform inversions using near-field strong motion records at only one or two stations, we examined the 1990 Odawara earthquake (MJ5.1) that has been well studied by many researchers. We used strong motion data recorded at two stations in the Kuno area of Odawara City, and released by the Japanese National Working Group on the Effects of Surface Geology on Seismic Motion (JESG). These stations were located at distances of 7-8km from the epicenter. We estimated the seismic moment, strike, dip, and rake of a double-couple point source by applying a grid search technique to the displacement seismograms bandpass-filtered between 2 and 10s. The data from KR1 and KD2 were used for each single-station inversion and for the two-station inversion. The first attempt with no constraint to the unknown parameters yielded reverse-faulting mechanisms similar to those of previous studies. Because the difference in the strike and rake was significant, we investigated the dependence of error between the observed and calculated waveforms on unknown parameters. We found that the strike of the low-dip nodal plane is the most appropriate parameter to be constrained. Constraining the strike within the range of the previous solutions, we carried out one- and two-station inversions to obtain reasonable source parameters compared with the previous solutions. The agreement between the observed and the synthetic waveforms is satisfactory even for the constrained solution. Moreover, through numerical experiments, we confirmed that constraining the strike could be effective in estimating source parameters of the target event for stations of different azimuth.抄録全体を表示PDF形式でダウンロード (1913K) -
茂木 清夫2005 年 57 巻 3 号 p. 275-278
発行日: 2005/02/20
公開日: 2010/03/11
ジャーナル フリーPDF形式でダウンロード (342K) -
笠原 稔2005 年 57 巻 3 号 p. 279-280
発行日: 2005/02/20
公開日: 2010/03/11
ジャーナル フリーPDF形式でダウンロード (215K) -
山田 知朗, 篠原 雅尚, 金沢 敏彦, 平田 直, 金田 義行, 高波 鐵夫, 三ヶ田 均, 末廣 潔, 酒井 慎一, 渡邊 智毅, 植平 ...2005 年 57 巻 3 号 p. 281-290
発行日: 2005/02/20
公開日: 2010/03/11
ジャーナル フリーThe 2003 Tokachi-oki earthquake (Mj 8.0) occurred in the area off the southeastern coast of Hokkaido in northern Japan at 19:50 UTC, 25 September 2003. Five days after the mainshock, we constructed a dense seismic array using pop-up type ocean bottom seismometers (OBSs) to obtain a detailed aftershock distribution. The observation lasted one and half months. We deployed in total 47 OBSs at 38 sites. The observation array covered a 150km×100km area, where a high aftershock activity was estimated from a land seismic network. For accurate determination of aftershock distributions, we deployed OBSs with a spacing of 15km near the trench in contrast to 20km in the landward region. Hypocenters were determined by using a three-dimensional velocity structure based on the seismic refraction study. From an epicentral distribution, aftershocks occurred within the small slip region of the mainshock rather than the large slip region. In addition, the epicentral distribution of aftershocks except near the large slip region, is similar to that of the earthquakes occurring before the mainshock. The hypocentral distribution forms a dipping plane toward the land and we infer that this plane shows the upper boundary of the Pacific Plate. The position of the plate boundary estimated by the aftershock distribution is consistent with those estimated by the past seismic surveys.抄録全体を表示PDF形式でダウンロード (1333K) -
高波 鐵夫, 村井 芳夫, 町田 祐弥, 斉藤 市輔, 牧野 由美, 勝俣 啓, 山口 照寛, 西野 実2005 年 57 巻 3 号 p. 291-303
発行日: 2005/02/20
公開日: 2010/03/11
ジャーナル フリーNine OBSs (Ocean Bottom Seismographs) were deployed just before the 2003 Tokachi-oki earthquake (M8.0) in the source region, off Hokkaido island, Japan for the period from 7 August to 21 September 2003. The observation area was located about 50-100km landward of the southern Kuril trench where the Pacific plate is subducted toward Hokkaido island. The main shock occurred on 26 September 2003, just several days after this 50-days observation by OBS. Until this big earthquake, the seismic activity in this area has been extremely low in contrast with the adjacent trench areas. No big earthquake has occurred since the 1952 Tokachi-oki earthquake (M8.2). From the phase-reading data, 186 micro- and ultra micro-earthquakes were located in the regular seismic active zones. The b-value of 0.82 from the earthquakes with magnitude from -0.6 to 2.9 could support the fore-shocks of the 2003 Tokachi-oki earthquake. Several interesting phenomena were found in this study as follows. (1) Many clusters of events took place simultaneously in the surrounding source region after a short quiescence period of several days. (2) An enormous number of curious burst signals, which oscillated for only a few seconds, continued to appear at almost OBSs. In particular, at two OBSs DHH and DK being the shortest distance to the Kuril trench, over 3500 bursts of events with amplitudes of 10-7m/s were counted during the observation period. On the other side, OBS DD, which was close to the epicenter of main event and far away from the trench, around the beginning of September 2003, the rate of occurrence abruptly rose to over twice of the previous rate, which has kept the low rate until then. The start time of activation was almost the same time of the appearance of many widespread clusters of earthquakes. These results will be applied to a research project of the great earthquakes occurring along the trenches.抄録全体を表示PDF形式でダウンロード (3450K) -
杉田 裕志, 横田 崇2005 年 57 巻 3 号 p. 305-314
発行日: 2005/02/20
公開日: 2010/03/11
ジャーナル フリーThe 2003 Tokachi-oki earthquake with Japan Meteorological Agency magnitude (Mj) 8.0 occurred at 4:50 on September 26, 2003 (JST), off Tokachi region of Hokkaido. The 6 lower intensity was observed in a wide area of Hidaka, Tokachi and Kushiro sub-prefecture. JMA issued a tsunami warning at 4:56 to the middle and eastern part of the Pacific coast of Hokkaido.
The tsunami higher than 1m was observed at several places by the tide gauge. The highest was 2.5m at Tokachi port, Hiroo town. The tsunami run-up reached 4 m at Hyakunin-hama, Cape Erimo, and Mabiro, Mabiro town.
Two people went missing and 847 people were injured. It is noted that the remarkable excitation of long period motion was observed, and it resulted in the fire of oil tank at Tomakomai city. Surface soil liquefaction was also observed in Sapporo, about 250km away from the hypocenter. The co-seismic subsidence between 0.1m and 0.5m was recorded by tide gauge.
The aftershock activity was very high soon after the main shock. Number of aftershocks with Mj≥2.5 was 1, 669, including 7 events with Mj≥6, until the end of April, 2004. The largest aftershock (Mj7.1) occurred at 6:08, September 26, 2003, 1 hour and 20 minutes after the main shock. The hypocenter was about 30km west-southwest of the main shock. After April of 2004, the activity decreased, a few events counted in a day. Though the aftershock activity has been high in the region northeast of the main shock and near the trench axis, it has been low near the main shock and largest aftershock hypocenters.
Small earthquakes clustered after the main shock in several regions along the volcanic front. The seismic activity also increased in the region out of the Kurile trench.抄録全体を表示PDF形式でダウンロード (2879K) -
川崎 一朗2005 年 57 巻 3 号 p. 315-316
発行日: 2005/02/20
公開日: 2010/03/11
ジャーナル フリーPDF形式でダウンロード (273K) -
安芸 敬一2005 年 57 巻 3 号 p. 317-319
発行日: 2005/02/20
公開日: 2010/03/11
ジャーナル フリーLooking back at the last 100 years of seismology, we are most impressed by the great contrast between the complexity of the first observed seismograms and the simplicity of the first theoretical seismogram calculated by Lamb. This complexity of the observed seismogram has made seismology a vital branch of earth sciences. If Lamb was able to explain the observed seismogram then, we would not be celebrating the centennial of Lamb's problem.抄録全体を表示PDF形式でダウンロード (424K) -
坪井 誠司, Dimitri KOMATITSCH, Chen JI, Jeroen TROMP2005 年 57 巻 3 号 p. 321-329
発行日: 2005/02/20
公開日: 2010/03/11
ジャーナル フリーWe use a Spectral-Element Method implemented on the Earth Simulator in Japan, to simulate broadband seismic waves generated by various earthquakes. The spectral-element method is based on a weak formulation of the equations of motion and has both the flexibility of a finite-element method and the accuracy of a pseudospectral method (Komatitsch and Tromp, 2002a, b). The method has been developed on a large PC cluster and optimized on the Earth Simulator. We perform a numerical simulation of seismic wave propagation for fully three-dimensional Earth model, which incorporate 3D variations in compressional wave velocity, shear-wave velocity and density, attenuation, anisotropy, ellipticity, topography and bathymetry, and crustal thickness. We use model S20 RTS of the mantle (Ritsema et al., 1999), model CRUST2.0 of the crust (Basin et al., 2000), and topography and bathymetry model ETOPO5. The simulation is performed on 4056 processors, which requires 507 out of 640 nodes of the Earth Simulator. We use a mesh with 206 million spectral-elements, for a total of 13.8 billion global integration grid points (i. e., almost 36.6 billion degrees of freedom). This translates into an approximate grid spacing of 2.0km along the Earth's surface. On this number of nodes, a simulation of 15 minutes of wave propagation accurate at periods of 3.5 seconds and longer requires about 6 hours of CPU time. We show examples of simulation for several large earthquakes and discuss its future application in seismological studies.抄録全体を表示PDF形式でダウンロード (8292K) -
中村 武史, 竹中 博士2005 年 57 巻 3 号 p. 331-342
発行日: 2005/02/20
公開日: 2010/03/11
ジャーナル フリーWe investigate influences of anisotropy on the seismogram around fault zone. We consider a simple model of fault zone, which is a homogeneous transversely isotropic (TI) medium between two isotropic half-spaces. The TI model has symmetric properties around a single axis, and the propagation velocity depends on the incident angle against the axis. In this model the TI layer and the isotropic half-spaces correspond to the fault zone including the alignment of cracks and the surrounding host rock, respectively. We express the TI layer with crack density ε by Hudson's (1980, 1981) crack model, and consider three cases for the TI layer as ε=0.02, 0.06 and 0.1. We calculate the synthetic seismograms inside or outside the fault zone for a strike-slip source by the propagator matrix method. The response of the wavefield for each case shows that the travel times and the S-phases are altered by the presence of anisotropy. These variations mainly depend on the strength of anisotropy and the offset from the fault. Around the fault, especially on the component normal to the strike of the fault, the seismogram is significantly affected by the anisotropic S-wave structure. These results indicate the possibility that observing anisotropic distortions of the seismogram can be a clue to detect the fault zone.抄録全体を表示PDF形式でダウンロード (1735K) -
田中 宏樹, 竹中 博士2005 年 57 巻 3 号 p. 343-354
発行日: 2005/02/20
公開日: 2010/03/11
ジャーナル フリーPlane-wave responses of vertically heterogeneous structure models (1-D media) are often computed in seismology. For horizontally layered media, they can be calculated by semi-analytical methods such as the propagator matrix method. However, for gradient velocity or randomly heterogeneous structures, we have to use numerical methods such as the finite-difference method. Conventional codes for the 2-D or 3-D finite-difference method require huge computer memory and long computation time even for calculating plane-wave responses of 1-D media. In this study we propose an efficient procedure to calculate plane-wave responses of arbitrary 1-D media using the finite-difference method in the time domain (FDTD). We first derive an elastodynamic equation of plane-wave incidence problem for vertically heterogeneous media by applying the Snell's law to 3-D elastodynamic equation. We then discretize the velocity-stress formulation of the derived elastodynamic equation using a staggered-grid finite-difference scheme of fourth-order accurate in space and second-order accurate in time. We also investigate the implementation of the stress-free surface condition for the scheme, and perform a stability check of the total scheme through actual computations. We computed plane-wave responses of a structure model with a velocity gradient using the derived finite-difference method. We focused on the PS-converted phase and found a “offset” phase appearing between the PS-converted phases generated at the top and bottom boundaries of the velocity-gradient layer on the surface responses of the structure model with the velocity gradient due to a P-wave incidence. This phase can be emphasized by calculating the receiver function from the radial and vertical waveforms. In this study we also investigate the “offset” phase attributed to the velocity gradient by numerical computations using the derived finite-difference method.抄録全体を表示PDF形式でダウンロード (1291K) -
岡元 太郎, 竹中 博士2005 年 57 巻 3 号 p. 355-364
発行日: 2005/02/20
公開日: 2010/03/11
ジャーナル フリーWe must consider the seismo-acoustic problem at the irregular fluid-solid boundary in various acoustical and seismological subjects: e. g., the ocean-acoustics problem, the analysis of the data from ocean-bottom seismometers, the analysis of the scattered waves from the irregular core-mantle boundary, and the strong motion prediction at sites close to the ocean. In this paper, we study why and how the fluid-solid or the acoustic-seismic boundary conditions are satisfied in the velocity-stress staggered grid finite-difference method (FDM) which has been widely used for seismo-acoustics problems. A simple analysis of the 2D finite-difference equations together with the comparisons between finite-difference seismograms and the discrete wavenumber method (DWM) seismograms show that the conditions are satisfied provided that (1) the boundary is placed through the shear stress (τxz) grid points (i. e., the rigidity of the shear stress grid points on the boundary must be set to zero), (2) the 2nd-order centered equation with averaged density is applied to the normal velocity component of the grid points right on the boundary, and (3) inadequate higher order finite-difference equations (e. g. the standard 4th-order centered equation) is not applied to grid points neighboring the boundary so that the values with discontinuity at the boundary are not included in the finitedifference equations for grid points off from the boundary. The accuracy of the spatial finitedifference equation at the boundary is O(h) with h being the space increment despite its apparent form of “2nd-order” equation. Violations to these conditions deteriorate the results of computation, especially for the interface wave such as Stoneley wave. In FDM inclined interface is approximated by stairsteps made from the grid. We find that these stairsteps can generate scattered waves with amplitudes up to 20% of the Stoneley wave. We need about 60 grid points per wavelength of the Stoneley wave at the dominant frequency in order to suppress such scattered wave. In the case of basin-like fluid layer with free surface and with “shoreline” we need large number of grid points (about 120 grid points) per wavelength at the dominant frequency in order to model the secondary Rayleigh wave generated (scattered) at the basin edges.抄録全体を表示PDF形式でダウンロード (1403K) -
藤井 雄士郎, 竹中 博士, 宮町 宏樹2005 年 57 巻 3 号 p. 365-375
発行日: 2005/02/20
公開日: 2010/03/11
ジャーナル フリーRecently, the dramatic development of computer resources and computational techniques have allowed us to calculate the seismic wavefield in three-dimensionally heterogeneous structure model (3-D model). The construction of more realistic 3-D model is a key to success for strong-motion prevention or source inversion using Green's functions of the 3-D model.
The purpose of this study is to construct a 3-D model which has an adequate ability for seismic wavefield simulation. We have constructed the 3-D model in northwestern Kagoshima region of Kyushu island, Japan using all kinds of geophysical and geological data which were well investigated in the study area. The 3-D model consists of six layers with width of 80km, length of 80km and depth of 40km. We estimated a depth for a top of each layer and next assigned P-wave velocity to the top and its gradient for each layer.
Using the constructed 3-D model we then simulate the seismic waveforms for two small earthquakes (M4.6, M3.6) by the finite-difference method to assess the ability of the 3-D model for seismic motion simulation with frequency range between 0.1 and 1Hz. These test events are aftershocks of the 1997 Northwestern Kagoshima, Japan, earthquakes, which were well recorded at our stations located on near-field hard rock sites and K-NET (NIED) stations.
As the results, it can then be seen that the synthetic waveforms agree with the observed waveforms in these P and S phases. The synthetics calculated using the 3-D model explain the later phases of the observed waveforms much better than the ones obtained by using a conventional 1-D model, which suggests that our 3-D model is suitable for seismic wavefield simulations in this frequency range. The constructed 3-D model can be applied to the strong motion simulations or source inversions using the 3-D Green's functions for large events occurring in this region, such as the mainshocks of the 1997 Northwestern Kagoshima earthquakes (M6.5, M6.3).抄録全体を表示PDF形式でダウンロード (8807K) -
内田 浩二, 亀 伸樹2005 年 57 巻 3 号 p. 377-385
発行日: 2005/02/20
公開日: 2010/03/11
ジャーナル フリーWe simulate dynamic mode II rupture that finally forms a branched fault trace, where our main interest lies in the resultant seismic wave radiation due to the dynamic branching process. For this purpose we adopt the elastodynamic boundary integral equation method, which enables us to work with non-planar fault geometry. We consider a medium under biaxial compressional load in which Coulomb friction acts on rupture surfaces and apply a critical shear stress criterion in determining the direction and the extension of rupture tips. In our simulation, dynamic branching first occurs due to local off-plane stressing around the fast propagating tips and each branch increases its bending angle. The stress to be released on such branch becomes negative under biaxial compression, the growth of each branch is thus arrested spontaneously. In order to find phases associated with the dynamic branching process, we synthesize waveforms of the branching model and compare them with those radiated from a planar fault model whose growth is arrested without branching. When the observation point is very near the branching point, we can successfully identify a distinct branching phase in a component for which no wave radiation is expected from the planar model. Otherwise we can not find significant effect of the dynamic branching process on the seismic wave radiation: this is because the moment release rate on the branching part of the fault is negligible compared with that on the entire fault.抄録全体を表示PDF形式でダウンロード (1272K) -
原田 隆典, 王 宏沢2005 年 57 巻 3 号 p. 387-392
発行日: 2005/02/20
公開日: 2010/03/11
ジャーナル フリーPDF形式でダウンロード (737K) -
幾何学的波線から有限幅波線への発展吉澤 和範2005 年 57 巻 3 号 p. 393-408
発行日: 2005/02/20
公開日: 2010/03/11
ジャーナル フリーA variety of methods of surface wave inversion, which enables us to investigate detailed images of the upper mantle on a regional scale, are reviewed. The study of surface wave tomography beginning in the 1980's has brought us with a significant jump in our understanding of the Earth's interior, particularly the upper mantle. Most of the studies of surface wave tomography have been based upon a geometrical ray theory, working with either dispersion curves or waveforms of surface waves. Such a simple representation of surface wave propagation has allowed us to treat a greater number of data sets, which are indispensable for obtaining high resolution tomography models. However, the ray theory, which is relying upon the high-frequency approximation, is no longer valid when the scale-length of heterogeneity is comparable to the wavelength of waves to be considered. The effects of finite frequency are particularly important for the higher-frequency surface waves, which mainly sample the crust and uppermost mantle where very strong lateral heterogeneity is likely to exist. Recent development of the 3-D sensitivity kernels allows us to treat the effects of finite frequency in the tomographic inversions. The use of such finite frequency theory will further advance the methods of surface wave tomography.抄録全体を表示PDF形式でダウンロード (6091K)
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