Earthquake activity related to seismic cycles in a model for a heterogeneous strike-slip fault

We investigate the evolution of seismicity within large earthquake cycles in a model of a discrete strike-slip fault in elastic solid. The model dynamics is governed by realistic boundary conditions consisting of constant velocity motion of regions around the fault, static/kinetic friction and dislocation creep along the fault, and 3D elastic stress transfer. The fault consists of brittle parts which fail during earthquakes and undergo small creep deformation between events, and aseismic creep cells which are characterized by high ongoing creep motion. This mixture of brittle and creep cells is found to generate realistic aftershock sequences which follow the modified Omori law and scale with the mainshock size. Furthermore, we find that the distribution of interevent times of the simulated earthquakes is in good agreement with observations. The temporal occurrence, however, is magnitude-dependent; in particular, the small events are clustered in time, whereas the largest earthquakes occur quasiperiodically. Averaging the seismicity before several large earthquakes, we observe an increase of activity and a broadening scaling range of magnitudes when the time of the next mainshock is approached. These results are characteristics of a critical point behavior. The presence of critical point dynamics is further supported by the evolution of the stress field in the model, which is compatible with the observation of accelerating moment release in natural fault systems.     

Tectonic Stress State Changes Before and After the Wenchuan Ms 8.0 Earthquake in the Eastern Margin of the Tibetan Plateau

Crustal tectonic activities are essentially the consequences of the accumulation and release of in situ stress. Therefore, studying the stress state near active faults is important for understanding crustal dynamics and earthquake occurrences. In this paper, using in situ stress measurement results obtained by hydraulic fracturing in the vicinity of the Longmenshan fault zone before and after the Wenchuan Ms 8.0 earthquake and finite element modeling, the variation of stress state before and after the Wenchuan Ms 8.0 earthquake is investigated. The results show that the shear stress, which is proportional to the difference between principal stresses, increases with depth and distance from the active fault in the calm period or after the earthquakes, and tends to approach to the regional stress level outside the zone influenced by the fault. This distribution appears to gradually reverse with time and the change of fault properties such as frictional strength. With an increase in friction coefficient, low stress areas are reduced and areas with increased stress accumulation are more obvious near the fault. In sections of the fault with high frictional strengths, in situ stress clearly increases in the fault. Stress accumulates more rapidly in the fault zone relative to the surrounding areas, eventually leading to a stress field that peaks at the fault zone. Such a reversal in the stress field between the fault zone and surrounding areas in the magnitude of the stress field is a potential indicator for the occurrence of strong earthquakes.     

从断层中心论向块体中心论转变--论活动块体在地震活动中的作用

讨论活动块体在地震活动中的作用。板缘地震在空间上呈线性分布,而中国大陆地震在空间上呈片状分布。大陆强震往往涉及两个以上方向断层的活动,且强震往往沿块体边界迁移或在其两侧断层上对迁。一些强震前后由中小地震震源机制解反映的Ｐ轴方向往往发生近９０°的转向,这种现象难以用区域应力方向变化来解释,但可从块体活动角度出发,用块体两个边界断层的先后错动来解释。地震前异常的远程效应、震后烈度异常分布图像以及地震序列特点等也显示了活动块体的作用。不同地区由地震活动性推测的块体活动方式有所差别,其原因可能和区域主压应力轴与块体两个边界断层走向的夹角不同有关。据此认为,在分析中国地震活动时要把视角从以活动断层为中心转变为以活动块体为中心     

云南彝良907地震次生地质灾害特征分析

2012年9月7日11时19分40秒,云南省彝良县发生M5.7级地震,12时16分29秒,再次发生M5.6级地震,两次地震诱发了大量的崩塌、滑坡、泥石流等次生地质灾害。通过现场调查,分析了本次地震次生地质灾害的主要特征:震后次生地质灾害数量明显增加、由地震直接诱发的地质灾害分布明显受震中控制;Ⅷ度区、Ⅶ度区的地震地质灾害点密度分别是Ⅵ度区地震地质灾害点密度的11倍与4.5倍;地质灾害的分布表现出很明显的上/下盘效应,上盘地质灾害密度是下盘地质灾害密度的2.6～3.9倍;震后新增次生地质灾害点的分布受岩土体性质控制明显;震后次生地质灾害降雨叠加效应与链生效应较为明显,具有震级小、灾情大、灾害多、规模小的特点。     

Multi-resolution clustering analysis and 3-D visualization of multitudinous synthetic earthquakes

We use modern and novel techniques to study the problems associated with detection and analysis of multitudinous seismic events, which form the background for isolated great earthquakes. This new approach involves multivariate analysis of low and large magnitude events recorded in space over a couple of centuries in time. We propose here the deployment of the clustering scheme both for extracting small local structures and large-scale trends in synthetic data obtained from four numerically simulated models with: uniform properties (U), a Parkfield-type asperity (A), fractal brittle properties (F), and multi-size-heterogeneity fault zone (M). The mutual nearest neighbor (mnn) clustering scheme allows for extraction of multi-resolutional seismic anomalies in both the spatio-temporal and multi-dimensional feature space. We demonstrate that the large earthquakes are correlated with a certain pathway of smaller events. Visualization of the anomalies by using a recently introduced visualization package Amira reveals clearly the spatio-temporal relationships between clusters of small, medium and large earthquakes, indicating significant stress relaxation across the entire fault region. We demonstrate that this mnn scheme can extract distinct clusters of the smallest events, which precede and follow a singularly large magnitude earthquake. These clusters form larger spatio-temporal structures comprising a series of large earthquakes. The link between the large and medium magnitude events is not so clearly understood. Short-ranged correlations are dominated by strong spatio-temporal anomalies, thus reflecting the global seismic properties of the entire fault zone.Electronic Supplementary Material Supplementary material is available for this article if you access the article at . A link in the frame on the left on that page takes you directly to the supplementary material.     

辽宁海城及其邻区地震b值空间分布特征

震级-频度分布（FMD）是地震学研究中最重要的经验公式之一,相关系数b是构造学和地震危险性评估的重要因子,具有表征前震和余震的特性。辽宁省地震多发生在金州断裂附近,自1975年海城7.3级地震发生后,与金州断裂交汇的海城河—大洋河断裂开启活跃模式,其东南端岫岩附近在1999年又发生5.6级地震。近年来盖州附近地震活动也在增强。因此,本文利用b值空间分布特征对海城及其邻区的应力分布特点进行研究。震源定位准确与否直接影响b值计算,双差定位后的数据与常规目录相比具有更高的精度,但是完整性有一定下降。本文收集了中国地震台网1981—2005年的辽宁省地震目录,并进行双差定位,比较分析了常规目录数据和双差数据的b值分布差异,认为在地震密集区,双差定位后的数据可以被用来获得更准确的b值。对主要研究区进行网格划分,使用双差数据,得到b值的水平和垂直分布特征。结果表明：b值为0.6~1.8,随深度增加而降低;岫岩和盖州震区具有较低的b值,意味着具有较高的地震危险性;浑河震区与海城河—大洋河断裂东南方向具有较高的b值,说明该区域未来发生大地震的概率很低;与金州断裂交汇区域的b值在1.0附近,说明该地区应力暂时处于稳定状态,未来具有较低的地震危险性。     

中国东北地区地震空间分布与主要断裂带、深部构造及应力场关系

中国东北地区在北东东向应力场控制下, 地震有其特殊性。地震空间分布和深度统计结果表明, 东北地区地震主要受岩石圈断裂(开原-赤峰断裂带) 控制, 以44°N为界, 南北具有明显差异。北部地震发生的数量少于南部, 震源深度主要集中在4～6 km和8～12 km两个深度范围内; 而南部主要集中在8～12 km和28～30 km两个深度范围内。发震深度分析显示, 开原-赤峰、郯庐断裂带、大兴安岭断裂带下可能存在地温较低的区域, 使地震可以在深部孕育。地震分布的Vp和Vs剖面资料的研究暗示, 断裂与速度变化带、断裂与断裂的相交区域是地震易发生区域。     

Relationship between Crustal 3D Density Structure and the Earthquakes in the Longmenshan Range and Adjacent Areas

This paper presents the 3D density structure of crust in the Longmenshan range and adjacent areas,with constraints from seismic and density data.The density structure of crust shows that the immense boundary plane of density distribution in relation to the Longmeshan fault belt is extended downward to～80 km deep.This density boundary plane dips towards the northwest and crosses the Moho.With the proximity to the Longmenshan fault belt,it has a larger magnitude of undulation in the upper and middle crust ...     

Catastrophic earthquakes in Iceland

A short review on the geographical distribution of earthquakes, and some hints at special features of catastrophic earthquakes in Iceland. It is suggested that large areas of destruction in several historic earthquakes are related to displacements on large fault areas rather than to high acceleration.     

太原盆地应力场演化与强震关系的数值模拟研究

近年来,太原盆地内尽管没有强震发生,小震却异常活跃,而研究太原盆地构造应力场的分布和演化规律的工作比较少.采用有限元数值方法,综合考虑区域地质构造差异、主要活动断裂带、地形起伏和岩石圈分层结构,引入深部速度结构,建立包含太原盆地在内的山西地区三维粘弹性模型.以GPS观测值和最大主压应力方向测量值为约束条件,重建研究区现今构造背景应力场.在此基础上,依次模拟了自公元512年以来太原盆地6级以上和山西地震带7级以上历史强震序列.计算结果显示,太原盆地内的3次6级以上历史地震均落在应力场增加值大于0.01 MPa的区域,盆地近年来小震空间分布与现今应力增加值大于0.01 MPa的区域有较好的对应关系.研究表明:山西地震带内历史强震序列和长期构造加载对太原盆地内3次历史强震均有促进作用,太原盆地的地震活动性明显受区域当前应力水平的控制.      

Relationship between Crustal 3D Density Structure and the Earthquakes in the Longmenshan Range and Adjacent Areas

Abstract: This paper presents the 3D density structure of crust in the Longmenshan range and adjacent areas, with constraints from seismic and density data. The density structure of crust shows that the immense boundary plane of density distribution in relation to the Longmeshan fault belt is extended downward to ~80 km deep. This density boundary plane dips towards the northwest and crosses the Moho. With the proximity to the Longmenshan fault belt, it has a larger magnitude of undulation in the upper and middle crust levels. Density changes abruptly across Longmeshan fault belt. Seismic data show that most of the earthquakes in the Longmenshan area after the 2008 Ms8.0 Wenchuan Earthquake occurred within the upper to middle crust. These earthquakes are clearly distributed in the uplifted region of the basement. A few of them occurs in the transitional zone between the uplifted and subsided areas. But most of the earthquakes distributes in transitional zone from subsided to uplifted areas in the upper and middle crust where relatively large density changes occurr The 3D density structure of crust in the Longmenshan and adjacent areas can thus help us to understand the pattern of overthrusting from the standpoint of deep crust and where the earthquakes occurred.     

A way to synchronize models with seismic faults for earthquake forecasting: Insights from a simple stochastic model

Numerical models are starting to be used for determining the future behaviour of seismic faults and fault networks. Their final goal would be to forecast future large earthquakes. In order to use them for this task, it is necessary to synchronize each model with the current status of the actual fault or fault network it simulates (just as, for example, meteorologists synchronize their models with the atmosphere by incorporating current atmospheric data in them). However, lithospheric dynamics is largely unobservable: important parameters cannot (or can rarely) be measured in Nature. Earthquakes, though, provide indirect but measurable clues of the stress and strain status in the lithosphere, which should be helpful for the synchronization of the models.The rupture area is one of the measurable parameters of earthquakes. Here we explore how it can be used to at least synchronize fault models between themselves and forecast synthetic earthquakes. Our purpose here is to forecast synthetic earthquakes in a simple but stochastic (random) fault model. By imposing the rupture area of the synthetic earthquakes of this model on other models, the latter become partially synchronized with the first one. We use these partially synchronized models to successfully forecast most of the largest earthquakes generated by the first model. This forecasting strategy outperforms others that only take into account the earthquake series. Our results suggest that probably a good way to synchronize more detailed models with real faults is to force them to reproduce the sequence of previous earthquake ruptures on the faults. This hypothesis could be tested in the future with more detailed models and actual seismic data.     

郯庐断裂带安徽段现代地震活动及应力场特征

分析了郯庐断裂带安徽段地震活动特征,利用近震直达波最大振幅比方法反演了99次中-小地震震源机制,并作了统计和聚类分析。现代地震活动整体上继承了历史地震分布格局,未显示增强趋势;断裂带及邻区应力场P轴和T轴优势方位大致呈近东西向和近南北向分布,现今承受的作用力以近水平或斜向为主,但也存在部分其它方向的应力场和近垂直方向的作用力。震源断层的破裂类型,带内以逆冲(或正断),或近逆冲(或正断)型为主。带附近较明显地呈现出走滑型或近走滑型破裂的优势;北、南亚段应力场有明显差异,即北压南张,其分界大约在北纬32°附近,北西西走向的桥头集—东关断裂是两者间的构造变换带。     

Seismic triggering effect of tidal stress

The tidal stress field of a layered, spherically symmetric earth due to the sun and moon is used to investigate its relationship to the occurrence of earthquakes. For 70 severe earthquakes which occurred in or near China during the last 24 years, we computed the tidal stress in spherical polar coordinates at the focus at the time of occurrence. The normal and tangential stress components along the slip direction on the earthquake fault plane were then found by a coordinate transformation. Coulomb's criterion of shear fracture is used to determine whether these stress components will trigger the earthquakes or not. The results show that of the 70 earthquakes, 43 have been subjected to a triggering effect and for the 18 events that occurred in North China, 14 have been triggered. It may be concluded that for shallow strike-slip earthquakes there is a significant triggering effect, while for oblique-slip and dip-slip earthquakes the tidal stress has only a small triggering effect. From 72 events which occurred in other parts of the world similar conclusions are obtained. Finally, for the purpose of predicting earthquakes the time interval during which the tidal stress may trigger an earthquake on faults in North China is investigated.     

Weak tidal correlation of NW-Bohemia/Vogtland earthquake swarms

We analyze the possible effect of solid Earth tidal stresses upon a vertical strike-slip fault in NW-Bohemia/Vogtland, central Europe, typical by occurrence of swarm earthquakes. The horizontal components of solid Earth tidal stresses were found strongly to prevail and to reach the level of 2 kPa. We examined tidal triggering as influence of tidal stresses to launching the swarm activity in relative absence of other stress disturbances. The onset times of 46 swarms of mostly M_L < 3 earthquakes that occurred in the period 1991–2005 displayed an increased occurrence near the fortnightly maximum of tidal extensive normal stress. The statistical test however did not prove a statistically significant correlation indicating a triggering effect of fault extension due to tidal loading. We also examined tidal effects to the already running seismic activity of the prominent 2000 swarm by comparing the tidal stress distribution in the investigated period with the distribution of tidal stresses in the occurrence times of each earthquake. The results show that these distributions are almost similar, which indicates that individual earthquakes occur independent of tidal stresses. The unclear tidal correlation of the swarm seismicity may be interpreted by small amplitudes and rates of tidal stress changes compared to the amplitudes and rates of coseismic stress perturbations and of pressure bursts of deep generated fluids.     

Seismotectonics and seismicity of the Silakhor region, Iran

This paper deals with seismotectonic and seismicity of the Silakhor region that shows high seismic activity in western Iran. Silakhor is a vast plain with several villages and cities of Dorud and Borujerd and a small town of Chalanchulan that were destroyed and/or damaged many times by large earthquakes. This paper addresses the historical and instrumental earthquakes and their causative faults, seismotectonic provinces and seismotectonic zones of the region. Available seismic data were normalized by means of time normalization technique that resulted in the magnitude-frequency relation for the Silakhor area and estimation of the return period of earthquakes with different magnitudes. Some active faults in this region include the Dorud fault, the main Zagros thrust, the Galehhatam fault, the Sahneh fault and others. Among them, the Dorud fault is an earthquake fault and is the cause for most of the large and intermediate earthquakes in the region. The return period of large earthquakes with magnitudes greater than 7.0 (Ms) is very low, however, the occurrence of destructive earthquakes is greater in the region than in the neighboring provinces. The study proves the high seismicity of this zone and it is required to develop an accurate national plan for future building and reinforcement of the existing buildings in this region.     

Seismicity changes associated with reservoir loading

Changes in seismic activity have been related to the filling of large reservoirs in over thirty cases. These changes range from variations in the level of micro-earthquake activity detectable only with instruments of high sensitivity to destructive earthquakes with magnitudes greater than 6. On the other hand, the filling of many other large reservoirs has not been accompanied by increased seismicity.

A number of factors may contribute to the generation or absence of post-impounding seismicity. Increased vertical stress due to the load of the reservoir and decreased effective stress due to increased pore pressure can modify the stress regime in the reservoir region. Whether or not these stress changes are sufficient to generate earthquake activity will depend on a complex interaction of the induced stress with the state of pre-existing stress near the reservoir, and on the geologic and hydrologic conditions at the site. The combined effect of increased vertical load and increased pore pressure will have the greatest tendency to increase activity in regions where the maximum compressive stress is vertical (normal faulting). In regions where the minimum compressive stress is vertical (thrust faulting) increased stress due to a vertical load should have a minimum effect. For all of the larger reservoir-induced earthquakes the stress system determined from fault plane solutions is in agreement with the pre-existing stress field in the region of the reservoir. These earthquakes are all of strike-slip or normal type, there being no reported cases of large induced earthquakes with thrusting mechanisms.

The potential for major changes in seismicity may be highest in regions of moderate strain accumulation (low to moderate natural seismicity). In areas of high strain accumulation and high levels of natural seismicity, the stress changes induced by the reservoir will be small compared to natural variations. In aseismic areas, with low strain accumulation, the reservoir-induced stresses may be insufficient to raise the stress level to a state of failure.     

Tectonic and seismic conditions for changes in spring discharge along the Garni right lateral strike slip fault (Armenian Upland)

Abstract

This study presents the relationship between local seismicity and springs discharge, in which the microseismicity modifies the state of stress of the aquifer matrix and these modifications of the local stress control the aquifer behaviour from an example of a basaltic reservoir crossed by a seismic fault.

Along the right-lateral Garni seismic wrench fault, the distribution in space and time of more than 500 micro-earthquakes has been studied providing a pattern of stress distribution. Together with this tectonic survey, the hydrographs of springs issuing from an aquifer located in basaltic lava flows and crossed by the fault have been gauged for 4 years. According to its tectonic, geologic and hydrodynamic properties, the reservoir has been divided in to several parts. The hydrological behaviour of the reservoir and of each of its parts has been modelled, based on the rainfall and hydrodynamic properties of the basaltic reservoir and of nearby reservoirs.

The model allows us to define the ‘normal behaviour’ of the springs, when their discharge is not affected by an earthquake. Anomalies to the normal hydrometric curves are defined, and correlated to small (M < 3.5) earthquakes along the Garni fault. We propose that the circulation of underground water in the area around the fault depends largely on the fracture pattern of the basalts and the aquifer basement, as pointed out from field observations. Changes in spring discharges are explained by variations in the state of stress around the Garni fault, induced by the alternation of elastic strain and stick-slip movement on the fault, and by creep far from the fault. A model of stress distribution is proposed which explains changes in fracture charateristics before and after earthquakes. © Elsevier, Paris     

藏南安岗地堑的史前大地震遗迹、年龄及其地质意义

地表调查发现, 沿近南北向亚东-谷露裂谷中段的安岗地堑存在地震大滑坡、多世代断层崖和断层崩积楔等多种类型的史前大地震遗迹.进一步的观测和年代分析表明: 该区的古地震滑坡体至少存在新、老两期, 其中规模最大的"尼续大滑坡体"应该是最新一次大地震所形成.该区T₁到T₆各阶地的形成时代从新到老分别为7.7~2.1 ka、11.0~10.5 ka、17.6~12.1 ka、25.7~22.9 ka、58.4~70.6 ka和130~150 ka, 它们沿主边界正断层的平均垂直断距依次为2.8 m、6.1~7.9 m、10.3~12.5 m、16.6~19.0 m、28.0 m和76.0 m.其中T₁和T₂阶地上的断崖剖面揭示, 最近两次大地震发生在距今约5.8±1.0 ka和2.4±0.2 ka.综合分析认为: 安岗地堑的大地震活动具有较明显的丛集性特征, 并且在距今约23~26 ka以来一直处于大地震活跃期, 期间的断层垂直活动速率为0.8~1.3 mm/a, 大地震的原地复发间隔大致为3.3~3.6 ka, 特征地震的矩震级为7.0~7.2, 推算整个尼木地堑群的大震复发间隔最短可能只有约1.0~1.2 ka.研究结果指示, 藏南裂谷的大地震活动性明显比藏北的近南北向正断层更显著.      

Depth variation of seismic source scaling relations: implications for earthquake hazard in southeastern Australia

Advances in earthquake data acquisition and processing techniques have allowed for improved quantification of source parameters for local Australian earthquakes. Until recently, only hypocentral locations and local magnitudes (M_L) had been determined routinely, with little attention given to the inversion of additional source parameters. The present study uses these new source data (e.g. seismic moment, stress drop, source dimensions) to further extend our understanding of seismicity and the continental stress regime of the Australian landmass and its peripheral regions.

Earthquake activity within Australia is typically low, and the proportion of small to large events (i.e. the b value) is also low. It is observed that average stress drops for southeastern Australian earthquakes appear to increase with seismic moment to relatively high levels, up to approximately 10 MPa for M_L 5.0 earthquakes. This is thought to be indicative of high compressive crustal stress, coupled with strong rocks and fault asperities. Furthermore, the data indicates that shallow focus earthquakes (shallower than 6 km) appear to produce lower than average stress drops than deeper earthquakes (between 6 and 20 km) with similar moment.

Recurrence estimates were obtained for a discrete seismogenic zone in southeastern Australia. Decreasing b values with increasing focal depth for this zone indicate that larger earthquakes (with high stress drops) tend to occur deeper in the crust. This may offer an explanation for the apparent increase of stress drop with hypocentral depth. Consequently, earthquake hazard estimates that assume a uniform Gutenburg–Richter distribution with depth (i.e. constant b value) may be too conservative and therefore slightly overestimate seismic hazard for surface sites in southeastern Australia.