Contribution of tsunami data analysis to constrain the seismic source: the case of the 1693 eastern Sicily earthquake

The January 11, 1693 eastern Sicilyearthquake is comparable only with the December 28,1908 Messina Straits event in the Italian seismichistory as regards magnitude (M _L 7), level ofdestruction and number of victims. The shock generateda strong tsunami, which hit the entire eastern coastof Sicily and particularly the town of Augusta. Theproblem of which fault was responsible for theearthquake is still open. Several hypotheses have beenformulated in the literature on the basis of differentkinds of geological, macroseismic and tectonic data,but a general agreement has not been reached yet. Animportant contribution to the discussion may come fromthe analysis of the tsunami data. In two previouspapers (Piatanesi et al., 1996; Piatanesi and Tinti,1998), the hydro-dynamical study of the tsunami basedon finite-element (FE) numerical simulations wascarried out taking into consideration mostlytheoretical faults, i.e. faults selected for theirsignificance in tsunami generation irrespective of thecorresponding geological evidence. This paper has tobe considered the continuation of the mentioned works.We have studied new sources proposed in theliterature, consistent with macroseismic data and/orgeological observations, and simulated thecorresponding tsunamis. We also built a new FE gridintroducing significant improvements in the coastlinerepresentation, and developed and applied a newalgorithm in order to account for the effect of thesea-bottom topography on the tsunami initialcondition. Some of the examined faults are located inthe Scordia-Lentini (SL) graben region and interceptthe coastline, others are placed in correspondencewith the Hyblaean-Malta (HM) escarpment and runparallel or sub-parallel to the coast. The conclusionof our work is that none of these faults respectsfully the available observations on tsunami, and thatthe faults exhibiting the best fit are those placed inthe SL region. It is worth stressing that our resultsare important for the assessment of seismic andtsunami hazard/risk in eastern Sicily, keeping also inmind that the 1693 earthquake has been selected as thereference event for an earthquake-scenario study inCatania and in south-eastern Sicily, called `CataniaProject', funded by CNR/GNDT (Consiglio Nazionaledelle Ricerche/Gruppo Nazionale Difesa dai Terremoti).     

Consistency of dynamic site response at Port Island

System identification (SI) methods are used to determine empirical Green's functions (EGF) for soil intervals at the Port Island Site in Kobe, Japan and in shake table model tests performed by the Port and Harbor Research Institute (PHRI) to emulate the site during the 17 January 1995 Hyogo‐ken Nanbu earthquake. The model form for the EGFs is a parametric auto‐regressive moving average (ARMA) model mapping the ground motions recorded at the base of a soil interval to the top of that interval, hence capturing the effect of the soil on the through‐passing wave. The consistency of site response at Port Island before, during, and after the mainshock is examined by application of small motion foreshock EGFs to incoming ground motions over these time intervals. The prediction errors (or misfits) for the foreshocks, the mainshock, and the aftershocks, are assessed to determine the extent of altered soil response as a result of liquefaction of the ground during the mainshock. In addition, the consistency of soil response between field and model test is verified by application of EGFs calculated from the shake table test to the 17 January input data. The prediction error is then used to assess the consistency of behaviour between the two cases. By using EGFs developed for small‐amplitude foreshock ground motions, ground motions were predicted for all intervals of the vertical array except those that liquefied with small error. Analysis of the post‐liquefied ground conditions implies that the site response gradually returns to a pre‐earthquake state. Site behaviour is found to be consistent between foreshocks and the mainshock for the native ground (below 16 m in the field) with a normalized mean square error (NMSE) of 0.080 and a peak ground acceleration (PGA) of 0.5g. When the soil actually liquefies (change of state), recursive models are needed to track the variable soil behaviour for the remainder of the shaking. The recursive models are shown to demonstrate consistency between the shake table tests and the field with a NMSE of 0.102 for the 16 m to surface interval that liquefied. The aftershock ground response was not modelled well with the foreshock EGF immediately after the mainshock (NMSE ranging from 0.37 to 0.92). One month after the mainshock, the prediction error from the foreshock modeled was back to the foreshock error level. Copyright © 2001 John Wiley Sons, Ltd.     

Static stress changes and fault interaction during the 1997 Umbria-Marche earthquake sequence

We study the static stress changes caused by moderatemagnitude earthquakes that occurred in Umbria-Marcheduring a seismic sequence which started on September3, 1997, with a M_L 4.7 foreshock and consisted ofeight earthquakes whose magnitudes range between 5.0and 6.0. The earthquakes occurred on normal faultsstriking in the Apennine direction and dipping at lowangles towards the SW. The goal is to verify if stresschanges induced by each mainshock can explain theoccurrence of subsequent events. Our results show thatthe foreshock slightly increased the Coulomb stress onthe first mainshock fault plane. The distribution ofseismicity that followed the foreshock is clustered inthe area of Coulomb stress increase comprised betweenthe two faults which ruptured in opposite directionsduring the two largest shocks of September 26. Thelocations and the geometry of the three largestearthquakes agree well with the pattern of Coulombstress changes suggesting elastic interaction betweenthese faults. However, we were not able to model thewhole sequence of M_L 5.0 events in terms ofCoulomb stress changes. The difficulties are due tothe similarity of fault plane solutions for eventslocated very close to each other and in the hangingwall of the mainshock rupture planes. Our results showthat normal stress changes agree better with thespatial pattern of the whole sequence of moderatemagnitude events. If previous ruptures unclamp thefault planes of subsequent earthquakes, fluid flow canplay a dominant role in promoting earthquakes duringthe seismic sequence.     

前兆地震活动图象及其在地震预报中的作用(英文)

本文根据长、中和短期预报研究对主要地震活动图象加以归类和简述。结果表明:(1) 基于地震活动图象分析的中、短期预报,在预报时间上可能达到1、2年,甚至几个月的量级。但要进一步缩短预报的时间尺度十分困难,除非有明显的前震序列活动。(2) 地震活动图象分析对于辨认未来主震(尤其是强主震)的位置是有效的。(3) 强震和中强震前的图象特点是有区别的;据此,可以大概估计未来主震的震级。然而,图象的空间尺度与主震震级无明显的相关性。(4) 信号震、前兆震群和前震(包括早期前震)总是伴随着异常的地震活动时空图象。     

前震研究进展综述

介绍了当前国内外前震研究领域的一些进展。 对比了不同前震定义条件下前震序列的共性特征, 对几种主要的前震机理及主要的前震识别方法进行了简要综述, 对其特点及存在的问题进行了评述和讨论。 前震指主震之前在主震断层面上、 紧邻主震破裂起始点发生的小地震。 主震发生之前的一系列前震活动构成前震序列。 在不同的前震定义条件下, 具有“直接前震”的震例比例从10%至40%不等。 理论上前震可用级联应力触发或预滑动模型进行解释。 前震空间上主要集中分布在距离主震10~75 km范围内, 但其时间分布形式复杂, 大多发生在主震前1~2天, 部分前震序列的地震活动率显示明显的加速特征, 但许多前震序列单独来看却往往显示主-余型序列的衰减特征。 序列地震震源机制一致、 序列b值偏低是前震序列最突出的特征。 前震的发生与主震破裂形式和构造环境似有一定关系, 在有限的前震震例中, 逆冲型主震似乎具有相对较多的前震。 部分震例的研究结果显示, 随主震的临近, 前震震源深度有逐渐下迁的特点。 到目前为止, 震前很难判定一次地震或一个地震序列是否为前震或前震序列, 所使用的前震识别主要有基于统计类比的方法、 基于震源机制一致及衍生的相关方法以及基于对地震成核过程精细检测的方法。 从现有不多的震例研究结果来看, 尽管地震时空丛集和震源机制高度一致是前震序列的最显著特征, 但却不是判定前震序列的充分条件。 由于成核的破裂扩展速度和滑动位移有随时间较快增大的趋势, 因而基于对地震成核过程精细检测的方法有望在前震识别中发挥更为重要的作用, 但需更多震例进行验证。     

Time Distribution of Immediate Foreshocks Obtained by a Stacking Method

—We apply a stacking method to investigate the time distribution of foreshock activity immediately before a mainshock. The foreshocks are searched for events with M≥ 3.0 within a distance of 50 km and two days from each mainshock with M≥ 5.0, in the JMA catalog from 1977 through 1997/9/30. About 33% of M≥ 5.0 earthquakes are preceded by foreshocks, and 50–70% in some areas. The relative location and time of three types of representative foreshocks, that is, the largest one, the nearest one to the mainshock in distance, and the nearest one in time, are stacked in reference to each mainshock. The statistical test for stacked time distribution of foreshocks within 30km from and two days before mainshocks shows that the inverse power-law type of a probability density time function is a significantly better fit than the exponential one for all three types of representative foreshocks. Two explanations possibly interpret the results. One is that foreshocks occur as a result of a stress change in the region, and the other one is that a foreshock is the cause of a stress change in the region and it triggers a mainshock. The second explanation is compatible with the relationship between a mainshock and aftershocks, when an aftershock happens to become larger than the mainshock. However the values of exponent of the power law obtained for stacked foreshocks are significantly smaller than those for similarly stacked aftershocks. Therefore the foreshock–mainshock relation should not be explained as a normal aftershock activity. Probably an increase of stress during foreshock activity results in apparently smaller values of the exponent, if the second explanation is the case.     

1975年2月4日海城地震(M=7.3)的前震系列/

我们研究了1975年2月4日海城地震(M=7.3)前震的定位及其辐射图象.用六个区域台的到时,相对于一次参考地震,将前震及主震进行了定位.这组前震开始相互很接近,然后随时间及其分布有一定的延伸.最大前震前,前震均位于直径约两公里的小体积内,而在最大前震后,其活动则向北西和南东方向扩展,形成六公里长的在北西方向上伸展的分布带.初动及 P 波 S 波振幅比表明,在前震系列中有两种不同的断裂机制.我们推测这两种辐射图象可能和前震处于分布带的不同部位有关.可能主震震源不处于前震震源所决定的断层上,而是位于这组前震南面6公里、且较这组前震浅几公里处.我们认为,在垂直于主震破裂面及前震分布带走向的方向上,前震和主震相距这么大的距离,可能是由于主震时产生滑动的断层是以雁行排列的.分析了在前震期间断层上滑动所引起的应力变化,认为由前震引起的主震断层上剪应力的增加是很小的.因之由前震直接触发主震的可能性不大.      

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.     

前震研究进展综述

简要介绍了当前国内外前震研究领域的一些进展。对比了不同前震定义条件下前震序列的共性特征,对几种主要的前震机理及主要的前震识别方法进行了简要综述,对其特点及存在的问题进行了评述和讨论。前震指主震之前在主震断层面上、紧邻主震破裂起始点发生的小地震。主震发生之前的一系列前震活动构成前震序列。在不同的前震定义条件下,具有“直接前震”的震例比例从10%至40%不等。理论上前震可用级联应力触发或预滑动模型进行解释。前震空间上主要集中分布在距离主震10~75km范围内,但其时间分布形式非常复杂,大多发生在主震前1~2天,部分前震序列的地震活动率显示明显的加速特征,但许多前震序列单独来看却往往显示主-余型序列的衰减特征。序列地震震源机制一致、序列b值偏低是前震序列最突出的特征。前震的发生与主震破裂形式和构造环境似有一定关系,在有限的前震震例中,逆冲型主震似乎具有相对更多的前震。部分震例的研究结果显示,随主震的临近,前震震源深度有逐渐下迁的特点。到目前为止,震前很难判定一次地震或一个地震序列是否前震或前震序列,所使用的前震识别主要有基于统计类比的方法、基于震源机制一致及衍生的相关方法以及基于对地震成核过程精细检测的方法。从现有不多的震例研究结果来看,尽管地震时空丛集和震源机制高度一致是前震序列的最显著特征,但却不是判定前震序列的充分条件。由于成核的破裂扩展速度和滑动位移有随时间较快增大的趋势,因而基于对地震成核过程精细检测的方法有望在前震识别中发挥更为重要的作用,但需更多震例进行验证。     

Acoustic and vibration precursors from shear-slip characteristics of simulated zigzag-type gouge fault

Earthquake precursors are vitally important for warning and risk evaluation, but shedding light on them is a complex task for seismologists and geologists. We attempted to illuminate the precursory rules of fault failure by traditional biaxial-direction shear tests for a zigzag-type gouge with different size and arrangement of simulated faults manufactured from polymethyl methacrylate materials. Shear stress, acoustic emissions, and vibration signals were recorded and comparatively analyzed. Two important findings were obtained: (1) the linearly increasing trend of shear stress peaks generated by foreshocks may indicate the foreshock effect before a larger earthquake triggered by natural fault gouge failure; and (2) four kinds of earthquake precursors were discovered, and earthquake intensity and precursors may be dependent on the time of macro-fracture formation and the quantity of micro-fractures initiated before mainshock. The study contributes to interpreting earthquake shear-slip characteristics and may even help provide warnings of failure and instability.     

Temporal evolution of the focal mechanism consistency of the 2021 Yangbi MS 6.4 earthquake sequence in Yunnan

Using the Cut And Paste (CAP) method, we invert the focal mechanism of 38 moderate earthquakes (M_S ≥ 3.0) recorded by Yunnan seismic network and analyze the corresponding focal mechanism consistency based on the minimum spatial rotation angle. Our results indicate that the M_S 6.4 mainshock is induced by a lateral strike slip fault (with a rake angle of ～ ?165°) and a little normal-faulting component event along a nearly vertical plane (dipping angle～ 79° and strike ～138°). Combining our results with high resolution catalog, we argue that the seismogenic fault of this earthquake sequence is a secondary fault western to the major Weixi-Qiaohou-Weishan fault. The focal mechanism evolution can be divided into three periods. During the first period, the foreshock sequence, the focal mechanism consistency is the highest (KA<36°); during the second period which is shortly after the mainshock, the focal mechanism shows strong variation with KA ranging from 8° to 110°; during the third period, the seismicity becomes weak and the focal mechanism of the earthquakes becomes more consistent than the second period (18°<KA<73°). We suggest that the KA, to some extent, represents the coherence between local tectonic stress regime and the stress state of each individual earthquake. Furthermore, high focal mechanism consistency and high linearity of seismic distribution may serve as indicators for the identification of foreshock sequence.     

水库地震主震发生时间的预测

水利工程对水库地震的抗震设防要求事先对水为地震进行预测。预测内容包括蓄水后诱发地震的危险性（可能性、）最大震级、最大震的地占紧及不库地震对场址地震动参数的影响。除主震时间的预测外，本文第一作者都曾提出过初步的预测方法。本文认为，水库地震前震波速比异常、非线性灰色模型以及前震系列回归等方法，是预测主震时间的有前途的方法。     

包头西6.4级震前地震活动研究

1995年内蒙古临河地区出现中等地震增强活动，之后，阴山地震带西部、中部、东部全部演化出小震异常有序分布图象，主震前震中区无前震活动，阴山带震情窗口反应不充分，强震孕育、发展过程显示出特殊性。     

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.     

广西北流与靖西地区地震的应力触发现象分析

2019年广西相继发生北流5．2级和靖西5．2级地震,两次地震间隔仅一个多月,为广西陆区有地震记载以来之少见现象.分别计算广西北流4．2级前震对其5．2级主震、靖西5．2级主震对其 4．3级最大余震,以及北流5．2级地震对靖西5．2级地震的库仑破裂应力变化作用大小.结果显示:北流4．2级前震对5．2级主震可能存在明显的动态与静态应力触发作用;靖西5．2级主震对4．3 级最大余震地震可能存在动态应力触发作用和静态库仑应力抑制作用,靖西4．3级余震的发生可能是构造应力场和其5．2级主震动态应力触发共同作用的结果.由于震级较小且距离较远,北流 5．2级地震对靖西5．2级地震的库仑应力触发作用不明显.     

Statistical Variability and Tokunaga Branching of Aftershock Sequences Utilizing BASS Model Simulations

Aftershock statistics provide a wealth of data that can be used to better understand earthquake physics. Aftershocks satisfy scale-invariant Gutenberg–Richter (GR) frequency–magnitude statistics. They also satisfy Omori’s law for power-law seismicity rate decay and Båth’s law for maximum-magnitude scaling. The branching aftershock sequence (BASS) model, which is the scale-invariant limit of the epidemic-type aftershock sequence model (ETAS), uses these scaling laws to generate synthetic aftershock sequences. One objective of this paper is to show that the branching process in these models satisfies Tokunaga branching statistics. Tokunaga branching statistics were originally developed for drainage networks and have been subsequently shown to be valid in many other applications associated with complex phenomena. Specifically, these are characteristic of a universality class in statistical physics associated with diffusion-limited aggregation. We first present a deterministic version of the BASS model and show that it satisfies the Tokunaga side-branching statistics. We then show that a fully stochastic BASS simulation gives similar results. We also study foreshock statistics using our BASS simulations. We show that the frequency–magnitude statistics in BASS simulations scale as the exponential of the magnitude difference between the mainshock and the foreshock, inverse GR scaling. We also show that the rate of foreshock occurrence in BASS simulations decays inversely with the time difference between foreshock and mainshock, an inverse Omori scaling. Both inverse scaling laws have been previously introduced empirically to explain observed foreshock statistics. Observations have demonstrated both of these scaling relations to be valid, consistent with our simulations. ETAS simulations, in general, do not generate Båth’s law and do not generate inverse GR scaling.     

The earthquake of 6 September 2002 and the seismic history of Palermo (Northern Sicily,Italy): Implications for the seismic hazard assessment of the city

On September 6, 2002, a M_L = 5.6 earthquake, occurring some tens of kilometres offshore from the Northern Sicilian coast (Southern Tyrrhenian Sea), slightly damaged the city of Palermo and surroundings (degree 6 in the European Macroseismic Scale 1998). The macroseismic investigation of the shock and a detailed study of effects of the main earthquakes which affected Palermo in the past have been performed in order to evaluate the seismic response of the city. Moreover, the comparison of the recent event, which is instrumentally constrained, with historical earthquakes allows us to infer new insights on the seismogenic sources of the area, that seem located offshore in the Tyrrhenian sea.In the last 500 years, Palermo has never been completely destroyed but has suffered effects estimated between intensities 6 and 8 EMS-98 many times (1693, 1726, 1751, 1823, 1940, 1968, 2002). The damage scenarios of the analysed events have shown that damage distribution is strongly conditioned by soil response in the different parts of the city and by a high building vulnerability, mainly in the historical centre and in the south-eastern zone of the modern city. As a matter of fact, Palermo has always suffered greater effects than those reported for other nearby localities. The hazard assessment obtained using observed site intensities has shown that the probability of occurrence for intensity 8 (the strongest intensity observed in Palermo) exceeds 99% for 550 years, while the estimated mean return period is 152 ± 40 years. These results, in connection with building vulnerability due to the urban expansion before the introduction of seismic code, suggest that the city is exposed to a relatively high seismic risk.This paper has not been submitted elsewhere in identical or similar form, nor will it be during the first 3 months after its submission to Journal of Seismology.     

强余震的灾害评估

极震区的烈度分布是由主震和大余震产生的。最重的地震灾区为余震区，其范围可由震级与地震区烈度的统计关系求得。在震级与震中烈度关系中，初期余震的烈度略高于主震，晚期强余震的烈度比主震的低。不同烈度对建筑物的损害程度可通过烈度与损失率曲线进行评估。强余震和后续强震的人口伤亡比主震轻。     

地震断裂系统的分形和在地震预报中的应用

本文基于地震断层具有自相似结构,从分形概念出发,依据地震资料提出了适合于前震和余震的分形模型,在此基础上讨论了前震和余震的b值特征,提出了地震矩不均衡度的概念。本文还探讨了利用分数维和地震矩不均衡度进行地震预报的可能性。研究发现,正常情况下,地震矩不均衡度呈现低值,大震发生前出现高值异常,震后降低;在空间上,震前2—4年其异常范围大,震前一年异常区逐渐缩小,大震发生在异常集中区内或其边缘。     

Computer estimation of intensities: The 1985 Berhida,Hungary, earthquake

In this paper a computerized method for the intensity estimation from macroseismic observations is presented. The successive steps of the MSK intensity scale (2° to 10°) receives weights according to the observations marked on the earthquake questionnaires. These weights are summed for the individual scale steps. The distribution of these summed weights and their relation to an experimentally chosen function of the criterion make it possible to estimate the appropriate intensity value and its reliability. By computer evaluation of earthquake reports an objective comparison of estimated intensities given by different seismologists can be made, provided that uniform earthquake questionnaires are used. The application of this method will result in more homogeneous intensity data.