巻号一覧
56 巻 (2003 - 2004)
- 4 号 p. 327-
- 3 号 p. 231-
- 2 号 p. 111-
- 1 号 p. 1-
前身誌
56 巻, 2 号
選択された号の論文の9件中1～9を表示しています
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山田 伸之, 山中 浩明2003 年 56 巻 2 号 p. 111-123
発行日: 2003/09/16
公開日: 2010/03/11
ジャーナル フリーEffects of 3D subsurface structural model must be accurately included in estimation of strong ground motion for earthquakes. In this article we examined performance of two 3D models of the Kanto basin, Japan in a finite difference ground motion simulations. The 3D contour maps proposed by Suzuki (1999) and Yamanaka and Yamada (2002) were digitized and used in simulations of ground motions for two moderate earthquakes, which occurred with an intermediate-depth under the Uraga channel [EQ1 (MJMA5.9)] and with a shallow depth near Izu-Oshima island [EQ2 (MJMA6.5)], for aiming at understanding effect of differences in the models on simulated motion. These two events were chosen to know the differences such effect during the events at the difference location in Kanto basin.
The results of the EQ1 ground motion simulation showed a good agreement between observed and synthetic velocities that were mainly characterized by an impulsive S-wave onset. Although, the synthetic waveforms could qualitatively explain some characteristics of the observed motions, which were a long duration and a predominant long-period component, the observed motion in the EQ2 were not fully reconstructed by simulation. We concluded that this difficulty was caused by the uncertainty subsurface structure which existing propagation path for the EQ2 simulation. A quantitative comparison with between the synthetic motions in the two models was tried by using difference of envelope function. The area with major differences of the synthetic was found around center of Tokyo. The model by Suzuki (1999) showed better result than the model by Yamanaka and Yamada (2002) in the area. However, at the other area showed opposite tendency. These differences for the 3D synthetic motions clearly indicated a performance of the two models. These features can be used for constructing a new 3D underground structure model.抄録全体を表示PDF形式でダウンロード (7758K) -
(2) 不均質な横ずれ断層への応用と改良式宮武 隆, 吉見 雅行, 寺坂 美紀2003 年 56 巻 2 号 p. 125-139
発行日: 2003/09/16
公開日: 2010/03/11
ジャーナル フリーNakamura and Miyatake (2000) had proposed an approximate expression of slip rate time function on the basis of the dynamic rupture model simulation on narrow fault. In the present paper, it is applied to strong ground motion simulation from strike slip fault in order to study the validity of the expression by comparison with dynamic rupture models in several cases. The following three fault rupture processes are considered in the present paper, and the waveforms generated from the approximate slip rate time function for these cases are compared with corresponding dynamic rupture models.
(a) almost unilateral rupture on 20km×10km single strike slip fault with uniform 10MPa stress drop distrubution. (b) two asperity models, i. e., b-1) single 8km×6km single asperity located near the center of a 20km×10km strike slip fault and b-2), three asperites located on the same fault. Stress drop is 10MPa in asperity part, and 0MPa in another part. These cases correspond to the heterogeneous fault. (c) 10km×10km fault on which rupture starts from the deepest corner of the fault, and propagate circularly. Since this situation is very different from that assumed in Nakamura and Miyatake (2000), the approximate function might not be valid. In all the cases, the uppermost depth is assumed to be 1km, and the speed of rupture propagation is assumed to be 0.8 times shear wave velocity.
In case A, approximate slip rates and the generated seismic waves gave the good agreement with those calculated from dynamic faulting model, although at the initial part of the rupture some discrepancy exists. In case B, the agreement is also good, except the initial part of the rupture. It is possible to improve the agreement by adjusting the rise time and peak slip rate using simple correction function.
In case C, it is impossible to apply uniform slip rate function. Slip rates time functions from dynamic model for case C are different from point to point on the fault plane. So rise time and peak slip rate distributions need to be adjusted using the above function.抄録全体を表示PDF形式でダウンロード (1610K) -
池田 倫治, 大野 一郎, 大野 裕記, 岡田 篤正2003 年 56 巻 2 号 p. 141-155
発行日: 2003/09/16
公開日: 2010/03/11
ジャーナル フリーThe Median Tectonic Line active fault system is an about 360-km-long right-lateral strike-slip fault system composed of several active segments. Several researchers have presented segmentation models on the basis of fault geometry and faulting history. However, very few studies so far been made to understand the genesis of segment boundary with a consideration of the 3-D fault geometry system of a long strike-slip fault system. We conducted precise gravity survey and seismic reflection survey to assess 3-D fault geometry and segmentation of the Median Tectonic Line active fault system. A prominent low gravity anomaly in the southwestern Matsuyama Plain is recognized. This low gravity anomaly region reflects a half -graben structure of the basement. Thick sediments filling the graben inferred from the seismic reflection survey across the low gravity anomaly region. The Iyo, Shigenobu, Kitakata, and Kawakami faults surround the low gravity anomaly region. The Iyo and Kawakami faults in these faults array depict an extensional right overstep. The Shigenobu and Kitakata faults within the overstep area may have faulted mainly with vertical displacement in the southern part, as reactivation of existing fault that occurred during Late Cretaceous to Paleogene. This fact indicates that these faults are secondary fault accompanied by the formation of pull-apart basin. We, presently, lack of data that connect activity of these active faults with formation process of the low gravity anomaly region, even though above mentioned facts demonstrate that these active faults have resulted faulting and constructed extensional right overstep structure controlled by the basement structure which have formed about Pliocene. On the basis of the above results, the Median Tectonic Line active fault system of the northwestern Shikoku is divided into two segments, the Iyo segment and the Kawakami segment, recognizing pull-apart basin as the segment boundary, since this pull-apart basin region in a long strike-slip fault causes heterogeneity of faulting.抄録全体を表示PDF形式でダウンロード (10866K) -
佐藤 凡子, 矢部 康男, 山本 清彦, 伊藤 久男2003 年 56 巻 2 号 p. 157-169
発行日: 2003/09/16
公開日: 2010/03/11
ジャーナル フリーThe Nojima fault in Awaji Island, Hyogo prefecture, Japan was ruptured during the 1995 Hyogo-ken Nanbu earthquake (MJMA=7.3). Drillings were performed at sites close to the fault about one year after the earthquake and rock core samples were retrieved from the boreholes. We measured In-situ stresses by Deformation Rate Analysis (DRA) using oriented core samples of three sites. The stresses measured by DRA are expected to reflect the stress state before the earthquake, because the method is based on the rock property of long-term memory of stresses. Two of the borehole sites locate along the fault segment where surface break is observed. The other is near the southern end of the estimated buried fault, which is at a distance of about 3km extended southwest from the surface break. The stresses have been determined at depths between 310m and 415m for the two sites on the surface break segment and at depths from 351m to 720m for the site on the buried fault segment. The measured stresses show that all the sites are in the strike-slip regime. The maximum horizontal stress lies in NW-SE direction at all the depths except for shallower part in the buried fault segment. This direction is almost perpendicular to the fault plane that is nearly vertical. It is also obtained that the r-value defined as the maximum shear stress divided by the normal stress on the maximum shear plane is relatively small within the zone of about 100m distance from the estimated fault trace. The zone is called the damaged zone. These results suggest that the observed stress orientation is the consequence of the small shear stress of the damaged zone. Our data and the long-term crustal strain of the area adjacent to the fault suggested that the Nojima fault is considered to be weak, even before the earthquake.抄録全体を表示PDF形式でダウンロード (1752K) -
向井 厚志, 藤森 邦夫2003 年 56 巻 2 号 p. 171-179
発行日: 2003/09/16
公開日: 2010/03/11
ジャーナル フリーWater injection experiments were performed in 1997 and 2000 at the 1800m-deep borehole near the fracture zone of the 1995 Hyogoken-Nanbu earthquake. During these experiments, contraction of strain changes was observed with three-component strainmeters at a bottom of 800m-deep borehole, 70m southwest of the 1800m-deep borehole. We estimated hydraulic properties of the fracture zone nearby the Nojima fault using the observational data of strain in order to investigate a healing of the fault at the postseismic stage. Strain changes due to water injection depend on hydraulic parameters, such as hydraulic diffusivity, hydraulic conductivity and channel of pore fluid flow. Calculated strain changes due to water injection agreed with the observational data, when we assumed that pore fluid flowed on a vertical plane perpendicular to the Nojima fault. The orientation of the minimum principal stress is parallel to the fault in the fracture zone. It is considered that pore fluid was diffused through the fractures perpendicular to the fault, because pore pressure could easily open fractures in the orientation of the minimum principal stress. Hydraulic diffusivities in 1997 and 2000 were determined to be 1.1±0.1 and 0.5±0.1m2/s, respectively. The reduced permeability suggests that the fractures nearby the fault have been healing after the 1995 Hyogo-ken Nanbu earthquake. Hydraulic conductivity was determined to be 1.4-4.3×10-6m/s. This value is about three orders of magnitude larger than the hydraulic conductivities of general rocks estimated in other studies. It is considered that the large hydraulic conductivity was attributed to the following facts; (1) water was injected in the shallow crust at the depth of 540m, (2) the water injection experiments were performed in the fracture zone nearby the fault, (3) a healing of fractures was not completed.抄録全体を表示PDF形式でダウンロード (1277K) -
阿部 邦昭2003 年 56 巻 2 号 p. 181-187
発行日: 2003/09/16
公開日: 2010/03/11
ジャーナル フリーDuring a seismic activity in Izu Islands from June 26 to August 31, 2000, there was a moderate earthquake MJMA 6.4 followed by a small tsunami on July 1. The tsunami was observed at tide stations in Kouzu-shima and Miyake-jima Islands. Assuming a vertical displacement due to a static fault with the seismic focal mechanism and moment, we numerically computed the tsunami to explain the observed waveforms. The location and the size were varied in a possible finite range and the best values were estimated on the basis of correlation coefficient of amplitude between model and observation. From the maximum value of 0.65 it is concluded that the fault was located at sea in the east of Kouzu-shima Island with length of 11km and depth of 1km. The central position at 34.222°N and 139.235°E corresponds to a north margin of the earthquake swarm. The fault length is comparable with one of the swarm area. The subsidence area, formed at southeast part of the fault, covers the most of the swarm area. This subsidence does not conflict with a magma motion toward the northwest direction.抄録全体を表示PDF形式でダウンロード (835K) -
山科 健一郎2003 年 56 巻 2 号 p. 189-198
発行日: 2003/09/16
公開日: 2010/03/11
ジャーナル フリーAn attempt was made to predict seismic activity in the Izu Islands region, Japan, between 15 July and 23 October 2000. In spite of a simple assumption that the occurrence of a pair of earthquakes (i. e. signal pair) with similar magnitudes may signal an impending major earthquake, 13 out of 25 imminent predictions were successful. Among these, 4-hour alarms using one of the empirical rules (i. e. hypothesis A: when earthquakes with similar magnitudes were observed at intervals of less than 2 hours, a larger event would occur within about 4 hours) were presented 11 times, resulting in 5 successful predictions. Such a success rate is expected only to have a probability of 1%, if the seismic activity did not change before and after the time of prediction. Other empirical rules were also useful, at least to some extent, although the significance was not necessarily clear because of a lack of sufficient experience. In addition, based on the present method, 4 out of 5 major earthquakes with a magnitude of 6 could be predicted, including those during the test period from 27 June to 14 July. Considering the total length of the alarm period, there was only a probability of about 1% if the occurrence of events with a magnitude of 6 was random and independent of the present rules.抄録全体を表示PDF形式でダウンロード (1622K) -
水準測量データの再検討と関連データの総合的解釈に基づく新たなモデル鷺谷 威2003 年 56 巻 2 号 p. 199-211
発行日: 2003/09/16
公開日: 2010/03/11
ジャーナル フリーThe 1918 Omachi Earthquake occurred near Omachi city, along the northern Itoigawa-Shizuoka Tectonic Line Fault Zone. Leveling data associated with the earthquake were re-examined and corrected for artificial alteration. Corrected leveling data revealed about 200mm of uplift in the central Omachi and a slight subsidence in the southern part. However, the leveling data are not enough to resolve a fault model of the Omachi earthquake by themselves. Therefore a new fault model is developed based on the leveling, triangulation, and structural data. Estimated fault plane is 4.4km×10.0km wide, shallowly inclined to ENE at the base of the Matsumoto Basin. Estimated fault slip amounts to 1.2m and the seismic moment is estimated to be 1.8×1018Nm (Mw=6.1). The fault model is consistent with the observed surface rupture and horizontal crustal strain, which have been hardly explained by previous models. The 1918 Omachi earthquake is considered to have released WNW-ESE compressional stress by rupturing a shallow part of the crust. Although the source fault is discriminated from the East Matsumoto Basin Fault, these faults are interpreted to be the members of the same fault system. An integrated understanding of the whole fault system as well as inelastic deformation in the surrounding area is necessary to refine estimates on earthquake probabilities and strong ground motions.抄録全体を表示PDF形式でダウンロード (4173K) -
―三陸沖における近年の進展を中心に―飯尾 能久, 松澤 暢, 吉田 真吾, 加藤 照之, 平田 直2003 年 56 巻 2 号 p. 213-229
発行日: 2003/09/16
公開日: 2010/03/11
ジャーナル フリーWe review recent studies on spatial distribution of asperities, and space-time variations of aseismic slips deduced from analyses of strong ground motions, displacement rates, continuous crustal deformations, and repeated microearthquakes in the Sanriku-oki region along the Japan trench. These various analyses suggest a possible scenario about occurrence of large earthquakes; asperities, which are defined as areas of large slips at earthquakes, repeatedly break when the stresses at asperities are loaded and reach to their strengths by aseismic slips occurring in the surrounding regions. If this scenario is the case and we estimate the strength, extent of asperities, and space-time variations of aseismic slips around the asperities, we will be able to forecast occurrence of large earthquakes to some extent. Moreover, we will be able to simulate the whole subduction process including seismic cycles along a subducting plate boundary if we find a conclusive constitutive law of frictional slip and succeed in estimating detailed distribution of the frictional parameters on the plate boundary. Although loading and generation mechanisms of intraplate earthquakes might not be the same as the interplate ones, the slow slips along the fault surface must play an important role for their generation.抄録全体を表示PDF形式でダウンロード (3008K)
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