Seismological evidence for nonlinear elastic ground behavior during large earthquakes

Amplification of earthquake-induced seismic waves by soft superficial deposits often causes significant damages in the urban areas. In predicting this effect for large future earthquakes, the linear elastic response of soils is customarily assumed. To check this assumption, we have analyzed surface and downhole acceleration data from the SMART1 and SMART2 strong motion arrays in Taiwan, covering peak accelerations of up to 0·3 g. First, frequency-dependent amplification induced by the alluvial deposits at the SMART1 array was estimated using spectral ratio technique, where the records at rock site were taken as a reference motion. Statistically validated reduction in soil amplification in the strong motion relative to the weak motion in the frequency range between approximately 1 and 9 Hz was detected. Secondly, relative site responses between the Pleistocene and recent sedimentary deposits at the SMART2 array were studied. Relative amplification was shown to be clearly dependent on the excitation level. Thirdly, we compared experimentally recorded uphole/downhole spectral ratios on weak and strong ground motion with the theoretical response yielded by the geotechnical code DESRA2 which assumes hysteretic constitutive relationship of soil. Major symptoms of nonlinear ground behavior predicted by the model were found in the observed data. Back-calculation of the shear wave velocities to the depth of 47 m shows nearly 50% decrease in the strongest quakes, also accounted for by the nonlinear soil behavior.     

Seismological evidence for the convergence of crustal stress orientation before large earthquakes

Earthquakes are caused by the failure of faults,driven by tectonic stress build-up in the Earth’s crust.To study the earthquake preparation process and assess regional earthquake potentials,it is vitally important to understand the crustal stress evolution process and identify its change in pattern associated with the seismogenic process.In this study we investigate the focal mechanism orientations of earthquakes in southern California from 1982 to 1999,basing on a focal mechanism catalog from Hauksson.We f...     

2003年云南大姚两次强震破裂区重叠程度的研究

地震破裂后,破裂区应变能是否已经充分释放.能否在破裂面内再次发生震级接近的地震,是双主震研究的重要问题.通过对2003年云南大姚两个地震序列的双差定位,发现余震区扩展,但两余震区无论在深度上还是在水平位置上基本不重叠.又通过相对定位的方法,发现7月21日主震位于10月16日主震的北西西方向.此外,还通过P波初动极性分析...     

Seismological and engineering characteristics of strong motion data from 24 and 26 September 2019 Marmara Sea earthquakes

Bulletin of Earthquake Engineering - An MW 4.5 earthquake took place on September 24, 2019 in the Marmara Sea. Two days after, on September 26, 2019, Marmara region was rattled by an MW5.7...     

Study of anomaly patterns of the ground tilt before two strong earthquakes

The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,15, 123–127, 1993.     

中等地震活动增强作为强震标志的研究

通过对(1990~1996年)发生在我国大陆及边境地区24次6级以上地震的分析研究表明,80%的强震主震前在震源区及附近有中等地震活动增强的过程。异常表现出共同的基本特征,即长期平静→增强活动→震前平静→发震。震前平静至发震大约经历几个月到2年时间(平均10个月左右),可以作为中短期地震预报判据。用岩石蠕变声发射实验来解释这一现象,可把中等地震活动增强看作是稳态蠕变阶段后期发生的广义前震。     

Foreshocks of strong earthquakes

The specific enhancement of ultra-low-frequency (ULF) electromagnetic oscillations a few hours prior to the strong earthquakes, which was previously mentioned in the literature, motivated us to search for the distinctive features of the mechanical (foreshock) activity of the Earth’s crust in the epicentral zones of the future earthquakes. Activation of the foreshocks three hours before the main shock is revealed, which is roughly similar to the enhancement of the specific electromagnetic ULF emission. It is hypothesized that the round-the-world seismic echo signals from the earthquakes, which form the peak of energy release 2 h 50 min before the main events, act as the triggers of the main shocks due to the cumulative action of the surface waves converging to the epicenter. It is established that the frequency of the fluctuations in the foreshock activity decreases at the final stages of the preparation of the main shocks, which probably testifies to the so-called mode softening at the approach of the failure point according to the catastrophe theory.     

Mechanisms of slip nucleation during earthquakes

Slip nucleation during earthquakes is apparently analogous to rupture nucleation within an intact rock sample subjected to triaxial loading. The observations indicate that both these nucleation processes initiate within a relatively small volume and in both the slip propagates unstably along a quasi-planar surface. In both processes a single, pre-existing, shear fracture cannot nucleate the large-scale slip, and in both a ‘process zone' that includes several interacting fractures in a small volume are required to initiate the unstable slip. Both processes require rupture of intact rocks, generate complex fracture geometry, and are associated with intense energy-release rate during slip. Recent observations and analyses are used to correlate rupture nucleation in laboratory tests with nucleation events of large earthquakes. It is proposed that earthquake nucleation occurs by the interaction among multiple fractures within a small volume that develops into unstable yielding of the healed fault zone.     

新疆两次强震前后地震视应力变化

选取了喀什数字地震台记录的喀什及周边地区地震的宽频带波形资料,分析处理了2001～2005年Ms 3.5以上地震164次,通过数字地震波谱计算了标量地震矩和地震辐射能量,从而进行了视应力的计算.给出了巴楚-伽师Ms 6.8和乌什Ms 6.3强震前后视应力的时序变化情况,发现其视应力均表现出震前低应力阶段-主震前的逐渐上升-震后逐渐恢复的动态变化过程,即地震的孕震-发震-震后调整的阶段性变化特征.这一结果可为喀什及周边地区今后进行地震预测预报研究提供参考依据,对该区开展数字地震方面的研究也有所帮助.     

用小波法研究台湾省两次7.6级大震前后仑坪台的地磁变化

用小波法分析了台湾省两次Ms 7．6大震(1986年花莲大震和1999年集集大震)前后仑坪台的地磁变化。从小波分析得出的“日变幅差”变化看,两次大地震前一个月内都有异常变化,花莲地震前出现负异常,集集地震前出现正异常。     

强震速报震级的比较研究

本文对比了成都速报、“中心”速报、地球所临报和 ISC报告所测定的震级 ,初步讨论了出现差异的原因     

Magnetic perturbations before the strong earthquakes

The search for the magnetic precursors of earthquakes by the method of synchronous detection is described. The data of the Guam Observatory (13.6°N, 144.9°E) located in a seismically active region and the USGS/NEIC catalogue of the National Earthquake Information Center of the U.S. Geological Survey for the period from 1991 to 2009 are used. Earthquakes with magnitudes M ≥ 7 in the range of longitudes from 100° to 170°E are analyzed. The intervals of accumulation cover 40 hours before and 40 hours after an earthquake. The preliminary result reveals an enhancement of fluctuations in the Z-component within 12 hours before the earthquake.     

Intracontinental basins and strong earthquakes

Introduction A M=8 earthquake occurred in the intracontinental Linfen basin on September 17, 1303. This is the Hongtong M=8 earthquake in Shanxi Province. It is the first recorded M = 8 earthquake since the Chinese historical seismic records had started. The earthquake killed 200 000 people (LI, 1960; XIE, CAI, 1983). The year 2003 is the 700th anniversary of the earthquake occurrence. In memory of the people died in the earthquake, for further improving earthquake prevention and disas…     

廊固凹陷深部剪切破裂构造的地震学证据

基于区域地震台网观测数据,采用近震波形反演方法,确定2018年2月12日河北永清M4.3地震的最佳双力偶源震源机制解为：节面Ⅰ走向297°,倾角58°,滑动角-32°;节面Ⅱ走向45°,倾角63°,滑动角-144°;是一个略带正断分量的右旋走滑地震.结合近震转换波测定主震的震源深度在19 km附近.地震序列的双差定位结果显示：永清地震序列震中呈北东向窄带展布,表明此次地震主要向北东向破裂;深度集中分布在17~19 km,整体形态近于铅直,显示发震断裂具有走向北东、倾向南东、倾角陡立的特征,与节面Ⅱ的性质比较吻合,推测节面Ⅱ为发震断层面.将发震断层面参数与震源区附近断裂性质进行对比分析,形成了关于廊固凹陷附近区域地震构造的一些认识：（1）推测永清地震的发震构造不是地壳浅部发育的先存正断裂,而是震源区下方一条地壳尺度的深断裂,该深断裂为新生断裂,具有右旋走滑正断性质,倾角陡峭、近于直立、宽度较大,向上与夏垫断裂相通.（2）综合震源区附近多条深地震反射剖面探测结果,推测永清地震的发震断裂与新夏垫断裂同属一条断裂,称为：新夏垫深断裂.该断裂从夏垫向西南方向延伸至文安,并可能与霸县-束鹿-邯郸断裂带相联系,总长度超过150 km.（3）基于2006年文安M5.1地震与2018年永清M4.3地震在震源机制上的相似性及震源位置上的关联性,结合区域构造条件,认为两次地震的发震构造均为新夏垫深断裂.（4）根据研究区几次显著地震的震源深度分布特征,参考区域断层构造、电性结构和流变学模型,推测活化克拉通块体新生断裂的脆韧性转换界面深度在15 km附近.

     

Statistical search for non-random features of the seismicity of strong earthquakes

The space-time-magnitude relationship among worldwide earthquakes of magnitude M ? 7 has been investigated with a view toward discerning the statistical reliability of possibilities of epicenter migration, gaps in seismic activity, and techniques of identifying aftershocks and foreshocks. The statistical procedure involves the calculation of the second-order moment of the sequences. The statistical validity of the results was obtained by modeling the interaction of discrete seismic events by a multidimensional branching stochastic point process.The results are that epicenters of large earthquakes migrate with velocities between 300 and 2,000 km/year with a significance level greater than 99.5%; the maximum likelihood estimate of the velocity of migration of large aftershocks of large earthquakes is about 1,400 km/year; foreshocks of large earthquakes have a maximum likelihood estimate of velocity of migration of about 2,800 km/year. A gap in seismic activity occurs before large earthquakes; with somewhat less confidence, a gap also occurs after large earthquakes, after the aftershock sequence has ended. The number of immediate foreshocks is 35–40% of the number of aftershocks. Aftershocks are comparatively weaker than foreshocks when compared with the main shock. There are hints, that are not fully statistically confirmed, of interaction among large earthquakes in different depth ranges.     

Wave disturbances induced by crustal earthquakes: Case study of two strong earthquakes in the Caucasian-Anatolian sector of the Alpine Mediterranean mobile belt

The display conditions of strong earthquakes in the Caucasian-Anatolian sector of the Mediterranean mobile belt are analyzed with allowance for the instrumental observations by titlmeters and magnetic variometers at the North Caucasian geophysical observatory of Schmidt Institute of Physics of the Earth of the Russian Academy of Sciences (NCGO IPE RAS) (Elbrus volcanic region) and at the magnetic stations operated by the Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation (IZMIRAN). It is demonstrated that the geophysical information obtained during the preparatory stages of the earthquakes on March 8, 2010 and January 19, 2011 in Turkey and the North Caucasus, respectively, reflect the fine structure of the anomalous wave processes of the preparation and evolution of the crustal earthquakes. The results provide a general idea of the geomagnetic activity and some characteristic features of the induced anomalous disturbances in the Caucasian-Anatolian sector of the Alpine Mediterranean mobile belt, which precede and accompany strong seismic events.     

Research on strong earthquake type division and forecast method for subsequent strong earthquakes

IntroductionThe earthquake sequences can be divided as the type of isolation (single shock), mainaftershocks and swarm of earthquake (multi-shocks) (Institute of Geophysics of CAS, 1976; Li,1981; Wu, et al, 1990). Based on the research of earthquake sequence, combined with modernobservation technique and the wide band strong (main) shock records of CDSN, this paper willdetermine the properties of hypocenter and seek the internal relation between the type of earthquake sequence and the hypo…     

强震类型划分和后续强震预测方法探索

地震能量、振幅与周期的关系,由震源性质决定,因而其相关程度可作为强震类型划分的指标．初步将强震划分为主余震、双主震和强震群型3类．同类型强震在震源性质定量指标上有相似性,且有一定重复性．它们可为后续强震预测提供依据．从1988～1997 年50多例CDSN宽频带（BPZ波）强震记录中,分析得出的后续强震预测参考指标,对分型预测法应用有参考价值．