Morishita播散指数在地震空间分布丛集分析中的应用

本文以汾渭地震丛集分布研究为例探讨了Ｍｏｒｉｓｈｉｔａ播散指数在地震空间分布分析中的应用，并与地震空间中度Ｃ值统计方法进行了比较。结果表明，这种方法无需对资料中特定分布作任何假定，能够直接表述地震的空间分布集中程度，且计算方法简单，可靠。震例分析，，５左右中强地震前几年，地震形成小的丛集团块，地震活动以小震为主，前一年左右地震成团块结构，尺度约为几十公里，但并不伴随明显的震群活动，这种不同时期的地     

大同—阳高6.1级地震活动背景

本文从较大时空范围研究了１９８９年大同－阳高６．１级地震的地震活动性背景，认为大同－阳高地震不是一次孤立的地震事件，是大同盆地历史６级以上地震活动的继续和必然。在时间进程中它们受华北地震区和山西地震带强震活动周期的制约，空间上与北三省交汇区中强地震成丛活动密切相关。大同－阳高６．１级、５．８级地震以及此期间的侯马４．９级、析州５．１级地震是山西地震带中强地震即将活跃的一个迹象，也是华北区域应力场增     

Delineation of Potential Seismic Source Regions of Moderately Strong Earthquakes Without Seismogenic Tectonics

In this paper,approaches are developed to delineate the potential seismic source regions of moderately strong earthquakes that do not have clear seismotectonic settings.Based on comprehensive analysis of regional tectonic backgrounds and seismicity,the data,such as isoseisrnals,spatial distribution of after shocks,regional tectonic stress field,and focal mechanisms,are employed for the delineation of the potential seismic source regions.The reliability of such potential seismic source regions is also discussed.     

东亚地震活动的时空分布及其与区域应力场的关系

来自板块之间相对运动的构造力可以传递到大陆地壳,从而形成了大陆内部的地震应力场.在某一地区,包括小地震在内的地震活动性的分布可以反映该地区的应力场的变化.根据这一观点,本文根据大量的震源机制解的结果以及最近500年的地震活动资料,详细地研究了东亚地区内几个地区的区域应力场的特征.其结果表明,来自太平洋板块相对欧亚板块的俯冲所形成的构造力,控制了从华北地区到南北地震带北段的应力场.本文根据小震的地震活动变化的特征,讨论了日本一部分地区由地震活动性的变化所反映的区域应力场的变化.中国西部以及印度-澳大利亚和欧亚大陆板块边界地区,最近大约100年地震活动性的同步变化表明,来自印度-澳大利亚板块和欧亚板块碰撞所产生的构造力,传递到了中国西部.印度-澳大利亚板块和欧亚大陆板块边界,以及中国西部的地震活动,现在依然处在地震活动高潮期.      

A model for eruptive mechanisms of Mt. Etna from the study of seismicity from 1978 to 1983

The papers deals with the seismic activity occurred on Mt. Etna from 1978 to 1983, and special emphasis is given to the seismicity linked to eruptive phenomena that took place during that period.Location of epicentres and hypocentres of all earthquakes occurred during the considered years and in association with each eruption has shown to be a useful tool to investigate relationships between seismicity and characteristics of various eruptions.A preliminary model is proposed to explain seismo-eruptive mechanisms controlling the uprise of magma and subsequent eruptions of Mt. Etna. The complexity of phenomena observed in the Etnean area could be interpreted as the result of the combined effect of regional stress field and local changes of it due to the volcano structural inhomogeneities. Thus, the earthquakes occurring in the studied area may cause either partial intrusion of magma at various depth, or final opening of surface fractures and subsequent output of lava.     

Evolution of Stress Fields and Faulting in Seismic Zones

—Measurements indicate that stress magnitudes in the crust are normally limited by the frictional equilibrium on pre-existing, optimally oriented faults. Fault zones where these limitations are frequently reached are referred to as seismic zones. Fault zones in the crust concentrate stresses because their material properties are different from those of the host rock. Most fault zones are spatially relatively stable structures, however the associated seismicity in these zones is quite variable in space and time. Here we propose that this variability is attributable to stress-concentration zones that migrate and expand through the fault zone. We suggest that following a large earthquake and the associated stress relaxation, shear stresses of a magnitude sufficient to produce earthquakes occur only in those small parts of the seismic zone that, because of material properties and boundary conditions, encourage concentration of shear stress. During the earthquake cycle, the conditions for seismogenic fault slip migrate from these stress-concentration regions throughout the entire seismic zone. Thus, while the stress-concentration regions continue to produce small slips and small earthquakes throughout the seismic cycle, the conditions for slip and earthquakes are gradually reached in larger parts of, and eventually the whole, seismogenic layer of the seismic zone. Prior to the propagation of an earthquake fracture that gives rise to a large earthquake, the stress conditions in the zone along the whole potential rupture plane must be essentially similar. This follows because if they were not, then, on entering crustal parts where the state of stress was unfavourable to this type of faulting, the fault propagation would be arrested. The proposed necessary homogenisation of the stress field in a seismic zone as a precursor to large earthquakes implies that by monitoring the state of stress in a seismic zone, its large earthquakes may possibly be forecasted. We test the model on data from Iceland and demonstrate that it broadly explains the historical, as well as the current, patterns of seismogenic faulting in the South Iceland Seismic Zone.     

Spatial Separation of Large Earthquakes, Aftershocks, and Background Seismicity: Analysis of Interseismic and Coseismic Seismicity Patterns in Southern California

We associate waveform-relocated background seismicity and aftershocks with the 3-D shapes of late Quaternary fault zones in southern California. Major earthquakes that can slip more than several meters, aftershocks, and near-fault background seismicity mostly rupture different surfaces within these fault zones. Major earthquakes rupture along the mapped traces of the late Quaternary faults, called the principal slip zones (PSZs). Aftershocks occur either on or in the immediate vicinity of the PSZs, typically within zones that are ??2-km wide. In contrast, the near-fault background seismicity is mostly accommodated on a secondary heterogeneous network of small slip surfaces, and forms spatially decaying distributions extending out to distances of ??10?km from the PSZs. We call the regions where the enhanced rate of background seismicity occurs, the seismic damage zones. One possible explanation for the presence of the seismic damage zones and associated seismicity is that the damage develops as faults accommodate bends and geometrical irregularities in the PSZs. The seismic damage zones mature and reach their finite width early in the history of a fault, during the first few kilometers of cumulative offset. Alternatively, the similarity in width of seismic damage zones suggests that most fault zones are of almost equal strength, although the amount of cumulative offset varies widely. It may also depend on the strength of the fault zone, the time since the last major earthquake as well as other parameters. In addition, the seismic productivity appears to be influenced by the crustal structure and heat flow, with more extensive fault networks in regions of thin crust and high heat flow.     

Preliminary analysis of earthquake probability based on the synthetic seismic catalog

The analysis of seismic hazards relies on the statistical analysis of historical seismic data and the instrumental seismic catalog to obtain the regional earthquake recurrence interval and earthquake probability. The accuracy of analysis thus depends strongly on the completeness of the seismic data used. However, available seismic catalogs are too short or incomplete for the reliable analysis of the statistical characteristics of earthquakes. If a long-term synthetic seismic catalog can be generated using a physics-based numerical simulation, and the simulation results match the crustal deformation, seismicity, and other observations,then such a synthetic catalog helps us to further understand the characteristics of seismic activity and analyze the regional seismic hazard. In this paper, taking the northeastern Tibetan Plateau as a case study, we establish a three-dimensional visco-elastoplastic finite-element model to simulate earthquake cycles and the spatiotemporal evolution of earthquakes on the model fault system and obtain a seismic catalog on a time scale of tens of thousands of years. On the basis that the model satisfies the regional geodynamics of the northeastern Tibetan Plateau, we analyze seismicity on the northeastern Tibetan Plateau using the simulated synthetic earthquake catalog. The characteristics of earthquake recurrence at different locations and different magnitudes, and the long-term average probability of earthquake occurrence within the fault system on the northeastern Tibetan plateau are studied. The results are a reference for regional seismic hazard assessment and provide a basis for the physics-based numerical prediction of earthquakes.     

地壳介质非均匀性对华北地区强震活动的影响

华北地区是我国地震灾害最频繁的地区之一,该区域的强震空间分布可能与地壳介质的不均匀性有关.本文建立了华北地区岩石圈三维Maxwell模型,并尝试采用剪切波分裂参数为新的约束条件,对区域地壳运动场和应力应变场进行了模拟,探讨地壳介质不均匀性与区域地震活动的关系.研究结果表明:外部动力源控制了华北地区整体的地壳运动方式,地...     

长宁地区复杂地震活动的力学成因分析

长宁地区地震活动复杂,发生在长宁背斜轴部的长宁Ms6.0地震及其后续中强余震的震源机制与相距仅十几公里的南部向斜区内的中强震具有显著差异;这种差异仅仅是由于发震断层构造不同所造成的,还是孕育地震的应力场本身也存在局部变化?为了回答这一问题,本文在通过双差层析成像反演修正小震定位和波速结构的基础上,利用小震综合震源机制解...     

The not so simple relationship between seismicity and oil production in the Permian Basin,west Texas

We have relocated seismicity occurring in the Permian Basin of west Texas between 1975 and 1979 using three-dimensional velocity models constructed from well log information and compared the locations to detailed geological and geophysical models of specific oil fields. The seismicity appears to be related to a variety of causes including migration of naturally occurring overpressured fluids, tectonic activity, reservoir production, and enhanced recovery operations. Many earthquakes may represent a combination of these effects. Although the Permian Basin is the site of numerous oil and gas fields, only a limited number of fields appears to be associated with seismicity. We find that there are similarities in the structural setting in the fields associated with seismicity. Furthermore, fields within the Delaware Basin that are associated with seismicity are located in regions of high vertical and lateral fluid pressure gradients related to an overpressured zone within the Delaware Basin.     

Seismicity of the Indian Peninsular Shield from Regional Earthquake Data

--In this comprehensive study of seismicity and seismotectonics of the peninsular Indian shield region, seismic data of regional earthquakes spanning two decades (1978-1997), obtained at Gauribidanur seismic array (India) and integrated where necessary with data from other seismological stations in the region, have been analyzed in detail. With a slow rate of stress accumulation, the shield is found to have low to moderate seismicity that takes into account a couple of earthquakes of magnitude slightly larger than 6. The frequency-magnitude analysis of the data set gives a b value of 1.18. The spatio-temporal pattern of occurrences of the earthquakes combined with their magnitude and seismic energy distribution is consistent with the view that the peninsular seismicity is low to moderate and episodic in nature. Regions of moderate seismicity and its low-grade counterpart constituted by microearthquakes (magnitude less than 3), appear correlated to the areas traversed by known geologic faults and subfaults, shear zones, and other such tectonic features. Microearthquakes represent about two-thirds of the total regional seismic events during the past two decades.     

Exploration tree approach to estimate historical earthquakes Mw and depth,test cases from the French past seismicity

The estimation of the seismological parameters of historical earthquakes is a key step when performing seismic hazard assessment in moderate seismicity regions as France. We propose an original method to assess magnitude and depth of historical earthquakes using intensity data points. A flowchart based on an exploration tree (ET) approach allows to apply a consistent methodology to all the different configurations of the earthquake macroseismic field and to explore the inherent uncertainties. The method is applied to French test case historical earthquakes, using the SisFrance (BRGM, IRSN, EDF) macroseismic database and the intensity prediction equations (IPEs) calibrated in the companion paper (Baumont et al. Bull Earthq Eng, 2017). A weighted least square scheme allowing for the joint inversion of magnitude and depth is applied to earthquakes that exhibit a decay of intensity with distance. Two cases are distinguished: (1) a “Complete ET” is applied to earthquakes located within the metropolitan territory, while (2) a “Simplified ET” is applied to both, offshore and cross border events, lacking information at short distances but disposing of reliable data at large ones. Finally, a priori-depth-based magnitude computation is applied to ancient or poorly documented events, only described by single/sporadic intensity data or few macroseismic testimonies. Specific processing of “felt” testimonies allows exploiting this complementary information for poorly described earthquakes. Uncertainties associated to magnitude and depth estimates result from both, full propagation of uncertainties related to the original macroseismic information and the epistemic uncertainty related to the IPEs selection procedure.     

基于特大地震发生率的川滇地区地震危险性预测新模型

川滇地区是我国地震危险性较高的地区之一.本文基于对特大强震的风险性考虑,使用全球地震模型OpenQuake软件,建立了川滇地区地震危险性预测新模型.首先根据构造特征划分多个震源分区,并整理出这些震源分区内断层活动特征与滑动速率;基于震源分区和断层模型,使用GPS应变率转换成的锥形古登堡-里克特关系作为整个区域的地震积累率,并允许超过历史最大震级的特大地震的出现,结合活动断层滑动速率所积累的地震发生率,给出震源分区内断层地震源和背景地震源的地震发生率的比率分配关系;在活动断层分段上,保留了大型断裂或其主要部分,没有根据小的阶区来对断层进行详细分段,以便分配特大地震发生率;并使用地震率平滑方法分配背景地震发生率.最后在OpenQuake中加入地震动预测方程,计算出了川滇地区的PGA分布图,为区域地震危险性提供科学依据.     

摩擦状态-速率依从的区域地震触发模型研究

设计一个具有摩擦状态-速率依从机制模拟大地震后区域触发地震的模型, 该模型由一定数量表示地震成核断层的函数组成。 大震对区域静应力场的影响由应力阶跃载荷表现, 利用摩擦状态-速率依从的一维自由度弹簧滑块模型结果计算地震发生时间提前。 结果表明, 施加幅度为地震应力降几十分之一至百分之一的应力阶跃, 可以引起地震发生速率的明显变化, 该变化通过统计检验, 与大地震后的区域触发地震活动相似。     

The precursory phenomena before the Karymskii seismovolcanic crisis in parameters of seismicity in a wide range of energy

We report extensive anomalies identified in seismicity parameters at different energy levels which were observed during the precursory process of the Karymskii seismovolcanic crisis of January 1–2, 1996. The seismicity of different energies includes earthquakes contained in the Kamchatka regional catalog and seismic noise (amplitudes of 10^?9–10^?12 m, frequencies of a few tens of hertz), which is a manifestation of the seismic process in the lowest energy range. The parameters of background seismicity are considered in retrospect using techniques for analyzing the dynamics of the seismic process: RTL and the Z function. Microseismicity is examined using these authors’ own method based on monitoring the response of high frequency seismic noise to tidal excitation     

Time-dependence of electromagnetic transfer functions and their association with tectonic activity

Laboratory experiments and theoretical considerations have suggested that anomalous dilatant regions can develop in the earth's crust during the period of strain accumulation prior to an earthquake. For moderate and major earthquakes such anomalous regions could be tens or even hundreds of kilometers in extent and should be detectable at the surface with appropriate survey or sounding techniques. Since electrical resistivity is one of the rock properties likely to be strongly modified in a dilatant zone, magnetotelluric impedance and geomagnetic transfer functions might be expected to show time-dependent precursory effects if monitored over a period of time above the focal region of an impending earthquake. Such experiments have been conducted in Japan and in other parts of the world and several examples of resitivity changes in the crust prior to earthquake occurrence have been reported. These results and their association with local seismicity are reviewed in this paper. The available evidence indicates that transfer functions and impedance can display significant time-dependent response to changing crustal conditions in some regions. However, the correspondence between these effects and earthquake occurrence is usually not very clear.Contribution from the Earth Physics Branch No. 858.     

Why does near ridge extensional seismicity occur primarily in the Indian Ocean?

The possibility that thermoelastic stresses due to plate cooling contribute significantly to the stress field and seismicity in young oceanic lithosphere has been a subject of considerable recent interest. This effect is suggested by three key observations: a decrease in seismicity with lithospheric age, the fact that focal mechanisms show extension perpendicular to the spreading direction, and a depth stratification of mechanism types. A difficulty with this idea is that although thermoelastic stresses should be comparable in different regions, the intraplate seismicity seems to occur in local concentrations. In particular, the ridge-parallel extensional seismicity occurs preferentially in the Central Indian Ocean region.We explore the possibility that much of the data favoring thermoelastic stresses can be interpreted in terms of stresses resulting from individual plate geometry and local boundary effects. In particular, the dramatic concentration of extensional seismicity in the Central Indian Ocean region is consistent with finite element results for the intraplate stress incorporating the effects of the Himalayan collision and the various subduction zones. The ridge parallel extensional stresses show a decrease with age similar to that of the seismicity. As earthquakes in this area provide a major portion of the data for both ridge-parallel extension and depth stratification, these effects may be due more to the regional stress. We thus propose that thermoelastic stresses provide a low level “background” in all plates, but that the dominant effect is that of individual plate geometry and local boundary effects.     

Mantle convection pattern and subcrustal stress field under Asia

Gravitational field models derived from satellite tracking and surface gravity data have been used to derive the forces in the earth's mantle under Asia. Based on studies of tectonic forces from these models, a subcrustal stress field under China has been obtained. The stresses are due to mantle convection. According to the stress patterns, the east and west China blocks and five seismic zones are identified. The tensional stresses exerted by the upwelling mantle convection flows under the crust of Tibet seem to be related to the Tibetan uplift. The compressional orogenic region from the southern tip of Lake Baikal, through Tien Shan, Hindu Kush and the Himalayas to northern Burma appears to be connected with the downwelling mantle convection flows. It is found that the directions of the subcrustal stresses under China are disposed perpendicularly to the major fault systems and seismic belts. The results of stress calculations show that the crust of north China should be in compression and that stresses within it should be sufficient to form the Shansi Graben and Linfen Basin Systems and fracture the lithosphere. This gives a possible explanation of why strong earthquakes occurred in north China which is an isolated narrow region of highest seismicity far from plate boundaries. The tensional stress fields, caused by the upwelling mantle convection flows, are found to be regions of structural kinship characterized by major concentrations of mineral and metal deposits in China.     

Crustal inhomogeneities and seismicity near the margins of the North Atlantic Ocean

Some very pronounced crustal inhomogeneities have been found in the North Sea and in Greenland, in regions which were adjacent to each other before the opening of the North Atlantic Ocean by sea-floor spreading. Some of these regions of inhomogeneities are zones of relatively high seismicity, while others are aseismic. In the North Sea, the Viking Graben has earthquakes, while its southern continuation, the Central Graben, is almost aseismic. In eastern Greenland, there are only few earthquakes near the Mesozoic graben, while there is a prominent earthquake zone at the coast line in north-eastern Greenland. It is concluded that, superposed on the apparent similarities in the two regions, some differences in stress conditions or crustal weakness patterns must exist.