Identifying the occurrence time of an impending major earthquake: a review

The procedure through which the occurrence time of an impending major earthquake can be determined is reviewed in the light of the recent advances. This can be achieved by analyzing in natural time the seismicity in the candidate area. To apply this general procedure, we need two important elements: first, to know when we should start the analysis, i.e., set the natural time equal to zero. This is the time at which the system enters the critical stage. Second a reliable estimation of the candidate epicentral area. If geoelectrical measurements are taken and the VAN method (after the initials of the three researchers Varotsos, Alexopoulos and Nomicos) is applied, both these elements become available upon the recording of a precursory Seismic Electric Signals (SES) activity, because its initiation marks the time when the system enters the critical stage, and in addition the SES data enable the determination of the epicentral area of the impending mainshock. On the other hand, if geoelectrical data are lacking, we make use of the following two recent findings by means of natural time analysis: First, the fluctuations of the order parameter of seismicity in a large area exhibit a minimum a few months before a major earthquake almost simultaneously with the initiation of an SES activity. Second, a spatiotemporal study of this minimum unveils an estimate of the epicentral area of the impending major earthquake. Two examples are given that refer to the strongest earthquakes that occurred in Greece and Japan during the last 3 decades, i.e., the M _W6.9 earthquake in southwestern Greece on 14 February 2008 and the M _W9.0 Tohoku earthquake in Japan on 11 March 2011.     

2014年鲁甸MS6.5地震及其强余震序列重定位

本文采用云南测震台网的观测报告数据,利用双差定位方法对2014年鲁甸M_S6.5地震及其强余震序列进行了重定位,获得了3 658个地震事件的震源参数。重定位后地震序列的震中分布显示,余震分布存在两个优势方向,分别为近EW向和SES向,呈共轭型分布,近EW向条带展布长度为30 km,SES 向条带展布长度为20 km;震源深度的分布显示,地震序列总体表现为主震附近震源较深,沿近EW向和SES向逐渐变浅,地震序列的震源深度主要分布在4—20 km范围内。截至2017年2月28日,鲁甸M_S6.5地震震源区共发生（同一天发生的一组地震算一次）M_S≥4.5强余震4次。重定位后的鲁甸4次强余震序列震中分布存在差异:2014年9月10日和10月27日两次强余震序列的展布特征与主震相同,而2016年和2017年另外两次强余震的后续余震仅分布在强余震的周边,与主震序列明显不同。综合重定位后余震序列分布、震源区地质调查资料以及前人研究认为,鲁甸地震的4次强余震序列是区域应力场和主震引发的震源区应力场共同作用的结果,2014年9月10日和10月27日的两次强余震序列主要受主震引发的震源区应力场的影响;而2016年和2017年两次强余震序列则主要受区域应力场的影响。      

A probabilistic framework for quantification of aftershock ground‐motion hazard in California: Methodology and parametric study

This paper presents a proposed method of aftershock probabilistic seismic hazard analysis (APSHA) similar to conventional ‘mainshock’ PSHA in that it estimates the likelihoods of ground motion intensity (in terms of peak ground accelerations, spectral accelerations or other ground motion intensity measures) due to aftershocks following a mainshock occurrence. This proposed methodology differs from the conventional mainshock PSHA in that mainshock occurrence rates remain constant for a conventional (homogeneous Poisson) earthquake occurrence model, whereas aftershock occurrence rates decrease with increased elapsed time from the initial occurrence of the mainshock. In addition, the aftershock ground motion hazard at a site depends on the magnitude and location of the causative mainshock, and the location of aftershocks is limited to an aftershock zone, which is also dependent on the location and magnitude of the initial mainshock. APSHA is useful for post‐earthquake safety evaluation where there is a need to quantify the rates of occurrence of ground motions caused by aftershocks following the initial rupture. This knowledge will permit, for example, more informed decisions to be made for building tagging and entry of damaged buildings for rescue, repair or normal occupancy. Copyright © 2008 John Wiley & Sons, Ltd.     

芦山7.0级地震前地震活动的临界点特征

直接检测大地震之前空间相关长度的增长是一种从观测的地震活动中检测临界点特征的独立的方法.本文研究了2013年4月20日四川芦山7.0级地震前5年震中周围地区中等地震活动的空间相关长度变化.结果表明,从地震前近两年开始,中等地震以上地震的空间相关长度出现明显增长,变化趋势可以用幂次率进行很好的拟合.用该地区没有7级以上强震发生的时段的地震目录采用相同方法进行了计算,没有发现明显的幂次率增长现象,说明强震前中等地震以上地震的空间相关长度的幂次率增长是地震孕育过程中临界点物理特征的表现.     

2014年于田MS7.3地震对后续余震和远场小震活动的动态应力触发

本文采用离散波数法,计算了2014年于田M_S7.3地震的断层破裂在近场和远场产生的库仑破裂应力变化,并结合地震活动特征,讨论了M_S7.3地震对后续余震活动和远场区域小震活动的动态应力触发作用.结果表明, ① M_S7.3地震产生的库仑破裂应力变化对其西南侧主体余震区的地震活动起到了抑制作用,这可能是本次M_S7.3地震序列余震活动水平不高的主要原因;距主震约30 km的北东方向余震区后续地震活动受到了主震产生的动态和静态应力变化的共同触发作用,动态应力变化峰值为2.78 MPa,静态应力变化为0.80 MPa,这与该区余震较为活跃相一致;距主震约45 km的北部余震区受到动态应力触发作用,应力变化峰值为0.72 MPa. ② M_S7.3地震产生的动态库仑应力变化空间分布呈非对称性,其中北东方向、北部余震分布与动态应力变化正值区存在相关性,从应力变化的角度解释了M_S7.3地震的后续余震空间活动特征. ③ M_S7.3地震在沙雅、伽师地区的远场接收点产生的动态应力变化峰值分别为0.09 MPa、0.1 MPa,对两个区域的小震活动具有动态触发作用.     

Detailed Distribution of Accelerating Foreshocks before a M 5.1 Earthquake in Japan

—The M 5.1 event (May 23, 1993) which occurred in one of the most active swarm areas of Japan was preceded by foreshock activity. We obtained precise hypocenters of the foreshock-mainshock-aftershock sequences with a temporary seismic network installed just above the source region twenty days before the mainshock. The foreshocks are very unique in their accelerating activity; the acceleration in the number of foreshocks enabled us to estimate the time of the mainshock with time-to-failure analysis proposed by . Although substantial snow remained in the swarm area, we quickly installed the network because the time-to-failure analysis disclosed that the mainshock was impending. The temporary network provided detailed information on both the temporal and spatial distribution of the foreshock-mainshock-aftershock sequences. Foreshocks started fifty days before the mainshock and were distributed linearly at the base of the seismogenic layer with a length of 5 km and horizontal and vertical widths of about 1 km. The temporal change of the number of foreshocks is approximated by a power law, and the time of the mainshock can be estimated by extrapolating plots of the inverse of the daily number of events. An area of seismic quiescence appeared 40 hours before the mainshock and propagated with a rate of 20 m/hour. The mainshock occurred 2 km westward from the primary foreshock area. It was located at the base of the aftershock region. This process can be interpreted as source nucleation; preslip on the fault prior to the mainshock.     

Aftershocks of the San Marcos earthquake of April 25, 1989 (M S =6.9) and some implications for the Acapulco-San Marcos,Mexico, seismic potential

Aftershock activity following the April 25, 1989 (M _S =6.9) earthquake near San Marcos, Guerrero, Mexico, was monitored by a temporary network installed twelve hours after the mainshock and remaining in operation for one week. Of the 350 events recorded by this temporary array, 103 were selected for further analysis in order to determine spatial characteristics of the aftershock activity. An aftershock area of approximately 780 km² is delimited by the best quality locations. The area of highest aftershock density lies inside an area delimited by the aftershocks of the latest large event in the region in 1957 (M _S =7.5) and it partially overlaps the zone of maximum intensity of the earlier 1907 (M _S =7.7) shock. Aftershocks also appear to cluster close to the mainshock hypocenter. This clustering agrees with the zone of maximum slip during the mainshock, as previously determined from strong motion records. A low angle Benioff zone is defined by the aftershock hypocenters with a slight tendency for the slab to follow a subhorizontal trajectory after a 110 km distance from the trench axis, a feature which has been observed in the neighboring Guerrero Gap. A composite focal mechanism for events close to the mainshock which also coincides with the zone of largest aftershock density, indicates a thrust fault similar to the mainshock fault plane solution.The San Marcos event took place in an area which could be considered as a mature seismic gap. Due to the manner in which strain release has been observed to previously occur, the occurrence of a major event, overlapping both the neighboring Guerrero Gap and the San Marcos Gap segments of the Mexican thrust, cannot be overlooked.     

2014年新疆于田MS7.3地震序列重定位及其发震构造初步研究

2014年2月12日在新疆于田县发生了M_S7.3地震,主震前一天在震区发生了M_S5.4前震,震后余震活动频繁,由于震区台站十分稀疏和不均匀、地壳速度结构复杂,台网常规定位结果精度有限,很难从中获得序列的空间分布特征和活动趋势的正确认识.本文首先利用位于震区附近的于田地震台5年记录的远震波形数据,采用接收函数方法研究了震区附近的地壳结构,建立了震源区的地壳速度模型.在此基础上,联合震相到时和方位角对2014年于田M_S7.3地震序列(从2014年02月11日-2014年04月30日,共计577次地震)进行了重新绝对定位.结果显示,(1) 重定位后的前震和主震震中位置明显向地表破裂带及其附近的阿尔金分支断裂(南肖尔库勒断裂和阿什库勒-肖尔库勒断裂)靠近,两者相距5.4 km,主震位置为36.076°N、82.576°E,震源深度为22 km, 前震位置为36.055°N、82.522°E,震源深度为19 km;(2) 本文重定位结果显示,余震序列沿NEE-SWW展布,优势分布长度约73 km、宽度约16 km,平均震源深度为14.8 km,其中77%的余震分布在地表破裂带的西南端,这部分余震中少数沿阿什库勒-肖尔库勒断裂分布,绝大多数沿北东东向的南肖尔库勒断裂分布,位于地表破裂带东北端的余震沿阿什库勒-肖尔库勒断裂分布,但发生在地表破裂带的余震极少;重定位后,位于地表破裂带西南侧的震中分布由台网目录的近南北向变为北东向,与地表破裂带、南肖尔库勒断裂和阿什库勒-肖尔库勒断裂走向一致;(3) 沿重定位剖面的地震分布,可推断位于地表破裂带西南段的南肖尔库勒断裂与位于北东段的阿什库勒-肖尔库勒断裂倾向反向,南肖尔库勒断裂的倾向为SE,阿什库勒-肖尔库勒断裂的倾向为NW,这与本次地震野外考察得到的断裂性质一致.综合重定位结果、地表破裂带分布、震源机制解、南肖尔库勒断裂和阿什库勒-肖尔库勒断裂的性质认为,2014年于田M_S7.3地震的发震构造为阿尔金断裂西南尾段的两条分支断裂——南肖尔库勒断裂和阿什库勒-肖尔库勒断裂.     

1999年海城-岫岩地震序列的精准定位

1999年11月29日,北京时间12点10分39秒(04:10:39 UTC)在我国辽宁海城-岫岩地区发生了一次Ms5.4地震.这次地震是继1975年2月4日海城Ms7.3地震之后该地区发生的震级最大的一次地震.区域性的辽宁数字地震台网记录了这一序列的前震、主震与 余震.本文运用该台网的记录资料,用双差法对海城-岫岩地...     

2020年3月20日西藏定日MS 5.9地震总结

2020年3月20日西藏定日M_S 5.9地震发生在申扎—定结断裂带分支断裂附近,震源机制解反演结果显示,本次地震为近NS走向断层产生正断层型错动的结果,与区域构造特征一致;地震序列跟踪发现,主震与最大余震的震级差为2.8,主震释放能量占全部序列的99.99%,属于孤立型地震序列。系统梳理地震发生前出现的地震活动和地球物理观测等异常,结果如下：震中附近震前存在年尺度和月尺度的热红外长波辐射异常;对震中附近异常进行回溯,发现震前存在地震发生率和地震危险度D值异常;震中300 km范围内无地球物理观测手段。通过对本次地震进行总结分析,以期为地震监测能力较低的定日地区积累宝贵的震例资料。     

中国大陆中强地震余震序列的部分统计特征

依据1970年以来记录相对完整的294次50级以上地震序列资料,研究中国大陆中强地震余震序列统计特征,探讨序列类型、最大余震震级、强余震活动持续时间等与主震震级及主震断层性质之间的关系.中国大陆孤立型、主余型及多震型地震余震序列分别约占23%、59%及18%.其中走滑型、具有倾滑分量的走滑型、具有走滑分量的倾滑型及逆冲型分别占48%、24%、17%及11%. 余震序列1年内最大余震震级与主震震级正相关,但主震震级较低时相对离散,孤立型序列离散程度较高,主余型及多震型序列线性相关性较好.绝大多数序列最大余震均发生在震后200天内,少数具有晚期强余震的序列主要属主余型序列,孤立型及多震型序列通常没有晚期强余震发生.68%的序列1年内最大余震发生在震后10天内,77%发生在震后30天之内,95%发生在震后120天之内.序列最大余震发生时间及5、6级强余震活动持续时间与序列类型及主震震级大小有关,多震型序列最大余震发生最快,孤立型次之、主余型最长.若仅就主余型序列而言,当主震震级较高时最大余震与主震间时间间隔相对较短,主震震级较低时最大余震与主震间时间间隔相对较长.     

1976年唐山MS7.8地震余震序列持续时间及对地震危险性分析的意义

1976年7月28日唐山MS7.8大地震对唐山及其周边地区造成了重大的人员伤亡和财产损失. 主震之后约15小时滦县又发生了MS7.1地震; 同年11月15日宁河也发生了MS6.9地震. 唐山MS7.8主震后的余震一直持续至今,使该区域至今保持了与主震前相比具有较高的地震活动性．如何估计余震的持续时间,并进一步将余震从主震目录中去除,一直是地震学中所关注的问题．该文通过对数线性回归和理论计算,从不同角度求取并讨论了1976年唐山MS7.8大地震的余震持续时间．结果表明,由对数线性回归计算得到的余震持续时间约为80 a．而基于Dieterich的余震触发理论所得到的余震持续时间则与区域剪应力变化率有关．区域剪应力变化率可有几种不同方法求得: ① 根据剪应力变化率和静态应力降Delta;tau;e及地震回复周期tr之间的关系求取应力变化率,该方法所得到的余震持续时间约为70——100 a;② Ziv和Rubin对Dieterich的方法进行了修正,给出了通过远场加载速率和断层宽度求取应力变化率, 该方法得到的余震持续时间约为80 a;③ 由背景场地震活动性求取远场剪应力速率, 可以得到该地区二维分布式的余震持续时间,此方法得到的研究区域内余震持续时间为130——160 a．综上,唐山地区余震持续时间约为70——140 a,据此, 该地区现今所发生的地震仍为MS7.8唐山地震所触发的余震．      

2020年4月1日四川石渠5.6级地震总结

系统梳理2020年4月1日四川石渠5.6级地震震前地震活动异常特征、地球物理观测异常以及区域构造情况,结果如下:①地震活动:石渠5.6级地震震中区域属于历史弱震区,震前出现前兆震群,也是其前震活动;②地球物理观测:出现8项异常,其中形变5项,电磁3项,且电磁异常对于该地震具有预测意义;③综合方法:地震发生前,震中附近存...     

利用DEMETER卫星数据分析强震前后的电离层异常

基于法国DEMETER卫星观测的离子温度(T_i)、 VLF电磁场单频点频谱数据探索了2008年5月12日汶川M_S8.0、 2010年1月12日海地M_S7.3和2010年2月27日智利M_S8.8等3次强震前后与地震有关的电离层异常现象. 结果发现, 汶川地震前3天(5月9日)震中北偏西方向离子温度明显升高, 震前4天(5月8日)VLF磁场低于200 Hz的频段范围频谱在震中2°以内有明显突升; 智利地震前9天(2月18日)震中北东方向离子温度有剧烈扰动, 震前4天(2月23日)VLF磁场100—160 Hz频段范围内出现突升; 海地地震震前没有观测到明显的异常现象, 但地震发生当天(震后约4—5小时)的T_i, 40—160 Hz频段电场频谱, 以及120—480 Hz磁场频谱均有明显突升, 应为地震发生后能量释放所引起. 分析认为, 不同地震由于发震机制等各种情况的不同, 其地震前后的表现也各不相同. 虽然目前没有直接的证据表明本文研究的异常变化是由地震的孕育和发生引起的, 但在数据处理中已尽可能排除了太阳、 地磁等因素的影响, 并且研究结果与前人的研究经验吻合, 因此本文发现的异常可能与地震发生的关系较大.      

Building service life economic loss assessment under sequential seismic events

Aftershocks have been shown to exacerbate earthquake‐induced financial losses by causing further damage to structural and nonstructural components in buildings that have already been affected by a mainshock event and increasing the duration of disrupted functionality. Whereas seismic loss assessment under isolated events has been addressed thoroughly in previous studies, comparatively less has been accomplished in the area of loss assessment under sequences of mainshock‐aftershock ground motions. The main objective of the current study is to formulate a comprehensive framework for quantifying financial losses under sequential seismic events. The proposed framework is capable of accounting for the uncertainties in the state of structure due to accumulation of earthquake‐induced damage, the time‐dependent nature of seismic hazard in the post‐mainshock environment, and the uncertainties in the occurrence of mainshock and aftershock events. Application of the proposed framework to a 4‐story reinforced concrete moment frame shows that consideration of aftershocks could increase lifecycle earthquake‐induced losses by up to 30% compared with mainshock‐only assessments.     

Variation of stress field in the source region around a strong shock：an example

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2014年2月12日新疆于田MW7.0地震源区静态库仑应力变化和地震活动率

2014年2月12日新疆于田M_W7.0地震源区位于巴颜喀拉块体与西昆仑块体的连接部位,东西向拉张构造发育,距离2008年3月21日于田M_W7.1地震震中位置约100 km.根据有限断层地震破裂过程模型,计算了2008年新疆于田地震产生的静态库仑应力变化.此次地震的断层面呈北偏东方向,在断层两端出现3个应力加载区,2014年于田主震位于破裂前端的库仑应力加载区.这一结果表明,2008年于田地震可能对2014年地震事件起到了触发作用.2008和2014年新疆于田地震产生的静态库仑应力变化与余震事件的空间分布具有明显的相关性,大多数余震位于应力加载区,发生在卸载区的余震较少.静态库仑应力变化与余震序列吻合较好.2014年3月21日新疆于田地震之后40天,在北西方向发生M_W5.2强余震,其震中位置的应力增量达到0.63×10⁵Pa.通过比较,发现静态库仑应力变化和地震活动率之间具有较好的相关性.地震活动率较高的区域与静态库仑应力加载区相对应,如康西瓦断裂东段、贡嘎错断裂中段和东北段等区域.     

2010年青海玉树7.1级地震发生的构造背景及其动力学分析

为了解2010年4月14日青海玉树7.1级地震活动背景和构造背景,探讨其发震的动力学过程,综合历史和现代地震资料,通过对玉树地震发震前的青藏高原东部地震活动特征分析,探讨了地震活动的大尺度时-空图像与玉树地震孕育发生的关系,研究其发震构造、震源机制解、余震分布和余震序列特征,从而综合探讨玉树地震发生的动力学背景和过程.结果表明,玉树地震的发生不是一个偶然事件,而是地震活动和构造运动的必然结果,是地震孕育和地球动力学过程的必然表现.     

汶川8.0级地震序列及相关问题讨论

汶川8.0级地震序列具有明显的分段特性,较强余震分布于茂县、绵竹以南及平武以北。主破裂过程在中南段以逆冲为主,序列逐渐衰减,呈主余型的序列衰减特征;北段是汶川地震破裂过程的终止区域,主破裂过程在该区域以走滑为主,形成多震型的序列特征,也成为汶川序列较大余震的主体活动区域。序列较强余震活动明显受引潮力调制,大多数较强余震发生在固体潮大、小潮时段,并且16时前后是较强余震的优势发震时段。序列衰减系数p值随时间增加而逐渐增大,最终基本稳定在1附近变化。结合以往的研究,对序列类型及最大强余震震级、强余震活动持续时间等进行了初步讨论。初步的统计结果还显示,8级左右强震序列中主震与最大余震之间的震级差正比于主震破裂尺度,这意味着当震级大体接近时,较大的破裂尺度与较为充分的能量释放相对应     

Seismic risk assessment considering cumulative damage due to aftershocks

Calculating the limit state (LS) exceedance probability for a structure considering the main seismic event and the triggered aftershocks (AS) is complicated both by the time‐dependent rate of aftershock occurrence and also by the cumulative damage caused by the sequence of events. Taking advantage of a methodology developed previously by the authors for post‐mainshock (MS) risk assessment, the LS probability due to a sequence of mainshock and the triggered aftershocks is calculated for a given aftershock forecasting time window. The proposed formulation takes into account both the time‐dependent rate of aftershock occurrence and also the damage accumulation due to the triggered aftershocks. It is demonstrated that an existing reinforced concrete moment‐resisting frame with infills subjected to the main event and the triggered sequence exceeds the near‐collapse LS. On the other hand, the structure does not reach the onset of near‐collapse LS when the effect of triggered aftershocks is not considered. It is shown, based on simplifying assumptions, that the derived formulation yields asymptotically to the same Poisson‐type functional form used when the cumulative damage is not being considered. This leads to a range of approximate solutions by substituting the fragilities calculated for intact, MS‐damaged, and MS‐plus‐one‐AS‐damaged structures in the asymptotic simplified formulation. The latter two approximate solutions provide good agreement with the derived formulation. Even when the fragility of intact structure is employed, the approximate solution (considering only the time‐dependent rate of aftershock occurrence) leads to higher risk estimates compared with those obtained based on only the mainshock. Copyright © 2016 John Wiley & Sons, Ltd.