1995年神户大地震对之后15年地震学研究的启示

1995年1月17日凌晨发生在大阪湾西侧的M6．9级兵库县南部地震已经过去了15年。此次地震从淡路岛东北端附近开始,同时向西北和东南破裂。在东南方向,右旋断裂从震中延伸约20km到达地球表面。在西北方向,破裂延伸约25km直达神户市下方。断裂没有到达神户地表,但破裂的方向和速度对波场产生了强大的前向方向性效应,     

芦山MS7.0地震前的电磁异常信号

对南北地震带18个地电场及电磁扰动观测台站进行了跟踪与研究, 2013年4月20日四川芦山M_S7.0地震前只有其中5个台站观测到了异常电磁信号. 研究表明, 地震前的这些异常信号时空分布非常不均匀. 在时间上表现出震前电磁异常信号在出现的台站上不是同步的, 且随着时间的推移有向震中方向迁移的趋势; 在空间上体现在成都以北的台站都没有记录到震前电磁异常信号, 主要出现在成都及其以南的一些台站. 芦山M_S7.0地震前地震电磁信号出现的最早时间是震前21天, 其次是震前16天; 临震时间则为震前5天、 4天和1天. 通过接地极注入地下的大电流以及以往的震例分析, 认为仙女山台是又一个比较突出的电磁信号观测的敏感点.      

Changes in paleoseismic studies of active faults in Japan since the 1995 Kobe earthquake

This paper discusses recent progress of onshore active faults studies in Japan, especially since the 1995 destructive Kobe earthquake, when the number of trenching studies, which are essential for the reconstruction of onshore paleoearthquakes, has rapidly increased. The timing and repeat interval of paleoearthquakes are here reviewed for the Miura Peninsula, south of Tokyo and the Awaji Island and Kobe-Osaka area, in central Japan, where trenching have been carried out very intensively in the last few years.     

Estimation of strong motion distribution in the 1995 Kobe earthquake based on building damage data

The 1995 Kobe earthquake caused unprecedented damage to buildings and civil infrastructures in the city of Kobe and its surrounding areas. In order to evaluate the structural damage in this area due to the earthquake, it is important to estimate the distribution of earthquake ground motion. However, since the number of strong ground motion records is not enough in the heavily damaged areas, it is necessary to estimate the distribution using other data sources. In this paper, the fragility curves for low‐rise residential buildings were constructed using the recorded motions and the building damage data from the intensive field survey by the AIJ and CPIJ group. The fragility curves obtained were then employed to estimate the strong motion distribution in the district level for Kobe and the surrounding areas during the earthquake. The results may be useful to investigate the various damages caused by the earthquake. Copyright © 2001 John Wiley & Sons, Ltd.     

Nonlinearity in the response of soils in the 1995 Kobe earthquake in vertical components of records

Strong motion records provided by seismic vertical arrays allow estimation of stress–strain relations in soils at depths from the surface to the location of the deepest device. As an example, time-dependent nonlinear soil behavior was estimated in vertical components of records obtained in the epicentral area of the 1995 Kobe earthquake. Degradation of the rigidity of soils in the strong motion was observed. The constructed nonlinear models of the soil behavior were used for estimating the nonlinear parts in the ground response by the nonlinear system identification technique. Nonlinear parts in the ground response were found to be as high as 50% at 2 km from the fault and 10% at 6–15 km from the fault plane. Odd types of nonlinearity, such as cubic, the fifth, seventh, etc. order, were found to be typical for soils, whereas, nonlinearities of even types are usually weak, but increase in liquefied soils.     

Localized changes in geomagnetic total intensity values prior to the 1995 Hyogo-ken Nanbu (Kobe) earthquake

Changes in total geomagnetic field intensity, of 2–3 nT, were observed prior to the 1995 Hyogo-ken Nanbu (Kobe) earthquake at the Amagase (AMG) site, located approximately 70 km from the epicenter. We examined whether the observed variations are local signals arising from the Earth's crust, or global variations that are unlikely to originate from the crust. To remove global-scale variations in total geomagnetic intensity data, we employed a regional geomagnetic field model. Using data recorded at five reference sites in Japan, we estimated global-scale variations in total geomagnetic intensity, and removed them from the observed total geomagnetic intensity at the AMG site. The reminder still showed variations during the period prior to the Kobe earthquake. In addition, these pre-seismic variations include two of the largest shifts recorded during the entire observation period at the AMG site, raising the possibility that these variations were indeed related to the earthquake. These variations cannot be interpreted as signals arising from the area close to the seismic source because of the large distance between the epicenter and the site. Therefore, our results raise the possibility that the physical state of the Earth's crust shows marked changes over a wide region in the lead-up to a seismic event.     

汶川地震强余震的电磁同震效应

汶川5.12大地震发生后,在武都汉王地震台及其附近地区布置了2个大地电磁测深点,进行了两期余震序列的地震电磁效应连续监测,共观测到Ms4.0及以上的余震事件35次.通过与汉王强震台的地震记录数据中15个强震记录比较发现,与地震波相对应的地震同震电磁信号存在于所有的电场和磁场记录分量中,它们与地震波到达同步.地震破裂发生时的电磁辐射信号在记录数据中相比地震波达到观测点的电磁信号幅度要小得多,无法从观测数据识别出来.通过分析对比地震位移和同震电磁信号的关系,提出了地震电磁同震信号产生的地震波驱动机理,认为观测系统在地球基本磁场中随地面运动产生了地震同震电磁信号,并从观测的地震振动位移和电磁信号进行了验证.     

Characteristic of the vertical seismic waves associated with the 1995 Hyogo‐ken Nanbu (Kobe), Japan earthquake estimated from the failure of the Daikai Underground Station

The dynamic behaviour of underground structures built by cut‐and‐cover methods is discussed. A simple model analysis shows that a column supporting the overburden at midspan (central column) can resonate upon incidence of an elastic wave of a specific frequency. The analytical results indicate that not only the size and material properties of the column, but also the static load acting on the column (overburden) is a decisive factor that influences the resonant frequency. Based on the results obtained by the analysis, the mechanism of the failure at the Daikai Underground Station in Kobe caused by the 1995 Hyogo‐ken Nanbu, Japan, earthquake is investigated. It is shown that the wave‐induced damage to underground structures can concentrate on the sections with specific overburden, and from the induced damage, it is possible to estimate the frequency characteristics of the associated seismic waves. Copyright © 2000 John Wiley & Sons, Ltd.     

Simulation analyses of buildings damaged in the 1995 Kobe,Japan, earthquake considering soil–structure interaction

A series of studies was conducted on three buildings of steel reinforced concrete structures with RC shear walls damaged in the 1995 Hyogo-ken Nanbu earthquake. These buildings are located in an area where structural damage centred around. Two of these buildings suffered severe damage, while the third was not structurally damaged. Our studies deal with site inspections, including micro-tremor measurement of buildings, the evaluation of input motions, and the response analyses considering soil–structure interaction. The results of simulation analyses of the two severely damaged buildings correspond to their actual damage state. From the response analyses of the one slender building with no structural damage, it was concluded that uplifting is the main reason it did not suffer any structural damage. Through these studies, the importance of soil–structure interaction and effective input motion is fully understood. Copyright © 1999 John Wiley & Sons, Ltd.     

Imaging the source area of the 1995 southern Hyogo (Kobe) earthquake (M7.3) using double-difference tomography

To understand the generation process of inland earthquake, we determined the seismic velocity structure in and around the source area of the 1995 southern Hyogo (Kobe) earthquake (M7.3) in SW Japan. We adopted the double-difference (DD) tomography method [Zhang, H. and C. Thurber. Double-Difference Tomography: the method and its application to the Hayward Fault,California. Bull Seism Soc Am 93 (2003) 1875–1889.]. We inverted arrival times recorded by a dense temporary seismic network for aftershocks and seismic networks routinely operated by Japanese Universities. Obtained results are summarized as follows: (1) Low-velocity zones of a few kilometers' width are distributed along the fault or along the aftershock alignment, suggesting that the fault of the 1995 earthquake is located primarily in a low-velocity zone. (2) Amount of velocity decrease within this low-velocity zone varies along the strike of the fault. Most of large slip areas (asperities) seem to correspond to higher velocity areas relative to the surroundings on the fault, rather than to lower velocity areas.     

Influence of the ground profile on the reduction of earthquake motion at the filled man-made islands in the Kobe harbor during the 1995 Hyogoken Nambu earthquake

During the 1995 Hyogoken Nambu earthquake in Kobe, the ground motion at the filled man-made islands in the Kobe harbor was not as severe as that at the mainland. The building damage was also less compared to that on the mainland. It was found by comparative study of earthquake records that the magnitude of acceleration response on the ground surface decreases at the islands as opposed to the mainland. One dimensional effective stress analysis is adopted in this study. Input data has been generated from test results, e.g. the SPT N-value by standard penetration test and shear wave velocity V_s by PS logging. Results obtained by the analyses showed good agreement with the observed records, which is an indication of the suitability of the adopted analysis procedure. From this study, the followings are concluded. By the increase of SPT N-value of the filled layers, liquefaction near ground surface is restrained and damage modes such as ejection of water and soil can be prevented. Since the ground profile at the islands is that considerably soft filled layer and marine clay layers, etc. are present and the thickness of the surface layer is large, the initial natural period of the ground is above 1 s and the natural period is elongated further under the earthquake excitation, which is deemed to be the principal reason for the reduction of the earthquake motion at the ground surface.     

日本阪神地震建筑损坏特点与震后抗震对策

引言 日本是多地震国家,长期的防震、抗震以及震后救灾等为日本在防震抗震方面积累了大量的宝贵经验。他们具有一套良好的防震抗灾体系,大大减少了震灾带来的损失。这一体系由政府、学会、建筑单位及居民共同参与形成,是从政策制度到施工、监察的完整体系。这一体系是以能积极发现问题,有效解决问题为主的防御型抗地震体系。     

Straightforward methods to detect non-linear response of the soil. Application to the recordings of the Kobe earthquake (Japan, 1995)

Simple straightforward methods are applied to testtheir ability to detect the non-linear response of thesoil. Recordings of the main shock and aftershocks ofthe 1995, Hyogo-ken Nanbu (Kobe) earthquake are used.Non-linear effects are investigated using twodifferent techniques, on a collection of data for 12sites situated on different geological structures inthe Kobe and Osaka areas. The first method used is theso-called receiver functions technique (Langston,1979), which consists of computing the spectral ratiobetween horizontal and vertical components of motion.This ratio has been shown to reveal the fundamentalfrequency of a site (Lachet and Bard, 1994; Lachet etal., 1996; Theodulidis et al., 1995, 1996). For eachsite, recordings of the main shock and a set ofaftershocks are considered. The variation of thisspectral ratio for different values of the maximumacceleration recorded at a site is investigated. Bothvariations of the amplitude of the H/V ratio (due tonon-linear behavior, on the horizontal components inparticular) and of the frequency position of theamplified band-width are observed. The secondtechnique used in this study is related to thevariation of the high frequency content of therecordings during the main-shock and its aftershocks.The high frequency spectral decay of the motion,characterized by parameter, is assumed to berelated mainly to the near-surface attenuation. Itshould then increase with increasing peak velocity, incase of non-linearity. The value of kappa iscalculated for the 12 sites in the Kobe area, fordifferent types of soil conditions, and againdifferent values of peak ground acceleration.Variations of kappa are then related to non-linearbehavior of the soil during the Kobe earthquake.     

Revisiting the Basin-edge Effect at Kobe During the 1995 Hyogo-Ken Nanbu Earthquake

The basin edge effect, i.e., the interference of the direct S wave with the surface wave diffracted off the basin edge has been invoked by many authors to explain the damage distribution during the January 17, 1995 Hyogo-Ken Nanbu (Kobe) earthquake. Here we present the results of numerical experiments obtained with the spectral element method in 2-D geometry. Our results confirm that the amplification of horizontal motion close to the basin edge can be twice as large as the one measured in the center of the basin. This additional amplification is shown to depend strongly on the edge geometry and on frequency, due to physical dispersion of diffracted surface waves. In particular we obtain maximal amplification around 3 Hz, at frequencies critical for buildings.     

Nonlinear response of surface soil and NTT building due to soil–structure interaction during the 1995 Hyogo-ken Nanbu (Kobe) earthquake

The 1995 Hyogo-ken Nanbu (Kobe) earthquake brought about enormous damage to structures in the Hanshin and Awaji areas. In this paper the importance of investigating the relationship between ground motion and structural damage is pointed out.

Strong seismic motion was observed at the NTT (Nippon Telegraph and Telephone) Building during this earthquake. The structural damage to this building was relatively slight. In order to evaluate the relationship between ground motion and structural damage, it is necessary to assess the effects of the soil–structure interaction. In this study, the seismic response of the building and of the surface soil were evaluated by means of a nonlinear soil–structure interaction analysis using FEM.

It was found that, the nonlinearity of surface soil near the building had a great effect on the soil–structure interaction, especially the rocking of the building.     

阪神地震对大城市防震减灾工作的启示

１９９５年１月１７日，日本关西兵库县南部发生７．２级地震（阪神地震），神户一带遭受巨大损失，人员伤亡惨重，这次地震是自唐山地震之后近２０年来，世界上最引人注目的一次地震事件。本文首先从地震预报的角度，简要地介绍了阪神地震前观测到的中短期前兆异常，包括震群活动、地震空区，水中氯离子浓度增加以及低频电磁辐射等，分析了阪神地区的地震危险性，指出在中期预测圈定的强震危险区，加强短临预测的跟踪，是最佳地震对策，也是起到减轻地震灾害，取得实际御防效果的基础．同时提出推进工程抗震研究，发展城市抗震工程学，提高城市综合防御地震危害的必要性。最后，文中对近期我国东部震情作了简要分析，并对强化监测，努力捕捉短临前兆信息，提出了相应的建议。     

Morphological characteristics of the earthquake surface ruptures on Awaji Island, associated with the 1995 Southern Hyogo Prefecture Earthquake

Abstract The earthquake surface ruptures on the northern side of Awaji Island accompanying the 1995 Southern Hyogo Prefecture Earthquake in Japan consist of three earthquake surface rupture zones called the Nojima, Matsuho, and Kusumoto Earthquake Surface Rupture Zones. The Nojima Earthquake Surface Rupture Zone is - 18 km long and was formed from Awaji-cho at the northern end of Awaji Island to Ichinomiya-cho. It occurred along the pre-existing Nojima geological fault in the northern segment and as a new fault in the southern segment. The northern segment of the Nojima Earthquake Surface Rupture Zone is composed of some subparallel shear faults showing a right-step en echelon form and many extensional cracks showing a left-step en echelon form. The southern segment consists of some discontinuous surface ruptures which are concentrated in a narrow zone a few tens of meters in width. This surface rupture zone shows a general trend striking north 30°-60° east, and dipping 75°-85° east. The deformational topographies and striations on the fault plane generated during the co-seismic displacement show that the Nojima Earthquake Surface Rupture Zone is a right-lateral strike-slip fault with some reverse component. Displacements measured at many of the outcrops are generally 100-200 em horizontally and 50-100 em vertically in the northern segment and a few em to 20 em both horizontally and vertically in the southern segment. The largest displacements are 180 em horizontally, 130 em vertically, and 215 em in netslip measured at the Hirabayashi fault scarp. The Matsuho Earthquake Surface Rupture Zone striking north 40°-60° west was also found along the coastline trending northwest-southeast in Awaji-cho for ～1 km at the northern end of Awaji Island. The Kusumoto Earthquake Surface Rupture Zone occurred along the pre-existing Kusumoto geological fault for ～ 1.5 km near the northeastern coastline, generally striking north 35°-60° east, dipping 60°-70° west. From the morphological and geomorphological characteristics, the Nojima Earthquake Surface Rupture Zone can be divided into four segments which form a right-step en echelon formation. The geological and geomorphological evidence and the aftershock epicenter distributions show clearly that the distributions and geometry of these four segments are controlled by the pre-existing geological structures.     

Possible thermal brightness temperature anomalies associated with the Lushan(China) M_s7.0 earthquake on 20 April 2013

Lushan MS7.0 earthquake occurred in Lushan county, Ya’an city, Sichuan province of China, on 20 April 2013, and caused 196 deaths, 23 people of missing and more than 12 thousand of people injured. In order to analyze the possible seismic brightness temperature anomalies which may be associated with Lushan earthquake, daily brightness temperature data for the period from 1 June 2011 to 31 May2013 and the geographical extent of 25°E–35°N latitude and 98°E–108°E longitude are collected from Chinese geostationary meteorological satellite FY-2E. Continuous wavelet transform method which has good resolution both in time and frequency domains is used to analyze power spectrum of brightness temperature data. The results show that the relative wavelet power spectrum(RWPS) anomalies appeared since 24 January 2013 and still lasted on 19 April. Anomalies firstly appeared at the middle part of Longmenshan fault zone and gradually spread toward the southwestern part of Longmenshan fault. Anomalies also appeared along the Xianshuihe fault since about 1 March. Eventually, anomalies gathered at the intersection zone of Longmenshan and Xianshuihe faults. The anomalous areas and RWPS amplitude increased since the appearance of anomalies and reached maximum in late March. Anomalies attenuated with the earthquake approaching. And eventually the earthquake occurred at the southeastern edge of anomalous areas. Lushan earthquake was the only obvious geological event within the anomalous area during the time period, so theanomalous changes of RWPS are possibly associated with the earthquake.