Influence of 2008 Wenchuan earthquake on earthquake occurrence trend of active faults in Sichuan-Yunnan region

The Wenchuan earthquake coseismic deformation field is inferred from the coseismic dislocation data based on a 3-D geometric model of the active faults in Sichuan-Yunnan region. Then the potential dislocation displacement is inverted from the deformation field in the 3-D geometric model. While the faults' slip velocities are inverted from GPS and leveling data, which can be used as the long-term slip vector. After the potential dislocation displacements are projected to long-term slip direction, we have got the influence of Wenchuan earthquake on active faults in Sichuan-Yunnan region. The results show that the northwestern segment of Longmenshan fault, the southern segments of Xianshuihe fault, Anninghe fault, Zemuhe fault, northern and southern segments of Daliangshan fault, Mabian fault got earthquake risks advanced of 305, 19, 12, 9.1 and 18, 51 years respectively in the eastern part of Sichuan and Yunnan. The Lijiang-Xiaojinhe fault, Nujiang fault, Longling-Lancang fault, Nantinghe fault and Zhongdian fault also got earthquake risks advanced in the western part of Sichuan-Yunnan region. Whereas the northwestern segment of Xianshuihe fault and Xiaojiang fault got earthquake risks reduced after the Wenchuan earthquake.     

Characterization of Fault Zones

— There are currently three major competing views on the essential geometrical, mechanical, and mathematical nature of faults. The standard view is that faults are (possibly segmented and heterogeneous) Euclidean zones in a continuum solid. The continuum-Euclidean view is supported by seismic, gravity, and electromagnetic imaging studies; by successful modeling of observed seismic radiation, geodetic data, and changes in seismicity patterns; by detailed field studies of earthquake rupture zones and exhumed faults; and by recent high resolution hypocenter distributions along several faults. The second view focuses on granular aspects of fault structures and deformation fields. The granular view is supported by observations of rock particles in fault zone gouge; by studies of block rotations and the mosaic structure of the lithosphere (which includes the overall geometry of plate tectonics); by concentration of deformation signals along block boundaries; by correlation of seismicity patterns on scales several times larger than those compatible with a continuum framework; and by strongly heterogeneous wave propagation effects on the earth's surface. The third view is that faults are fractal objects with rough surfaces and branching geometry. The fractal view is supported by some statistical analysis of regional hypocenter locations; by long-range correlation of various measurements in geophysical boreholes; by the fact that observed power-law statistics of earthquakes are compatible with an underlying scale-invariant geometrical structure; by geometrical analysis of fault traces at the earth's surface; and by measurements of joint and fault surfaces topography.¶There are several overlaps between expected phenomenology in continuum-Euclidean, granular, and fractal frameworks of crustal deformation. As examples, highly heterogeneous seismic wavefields can be generated by granular media, by fractal structures, and by ground motion amplification around and scattering from an ensemble of Euclidean fault zones. A hierarchical granular structure may have fractal geometry. Power-law statistics of earthquakes can be generated by slip on one or more heterogeneous planar faults, by a fractal collection of faults, and by deformation of granular material. Each of the three frameworks can produce complex spatio-temporal patterns of earthquakes and faults. At present the existing data cannot distinguish unequivocally between the three different views on the nature of fault zones or determine their scale of relevance. However, in each observational category, the highest resolution results associated with mature large-displacement faults are compatible with the standard continuum-Euclidean framework. This can be explained by a positive feedback mechanism associated with strain weakening rheology and localization, which attracts the long-term evolution of faults toward progressive regularization and Euclidean geometry. A negative feedback mechanism associated with strain hardening during initial deformation phases and around persisting geometrical irregularities and conjugate sets of faults generates new fractures and granularity at different scales. We conclude that long-term deformation in the crust, including many aspects of the observed spatio-temporal complexity of earthquakes and faults, may be explained to first order within the continuum-Euclidean framework.     

汶川地震同震形变场对川滇地区主要活动断裂地震发生趋势的影响

以所建立的川滇地区主要活动块体及其周边断裂带的模型和前期利用GPS及水准资料反演所得到的断裂带长期运动速率作为基础,将汶川地震引起的同震错动量加入到三维断裂几何模型中,计算出汶川地震大范围的同震形变场,然后基于该同震形变场和活动断裂三维几何模型反演了各条断裂对该同震形变场的反映,并通过与各断裂带长期运动速率对比,得到了汶川地震对川滇地区各主要活动断裂带发震趋势的影响．结果表明,在汶川地震引起的同震形变场作用下,在川滇交界东部地区,龙门山断裂带南西段地震危险性提前了305a,鲜水河断裂带南东段大致提前了19a,安宁河断裂带和则木河断裂带分别提前了21a和12a,大凉山断裂带北段和南段分别提前了9.1a和18a,马边断裂带的地震危险性则提前了51a;对川滇交界西部的丽江——小金河断裂带南西段、怒江断裂带、龙陵——澜沧断裂带、南汀河断裂带、中甸断裂带等断裂带地震的能量积累也有促进作用;相反在鲜水河断裂带北西段、小江断裂带等历史地震频发的断裂带上,地震危险性具有一定的减速作用．      

地震序列的一个自相似扩展模式

本文引入地震序列的一种自扩展模式,其地震序列的发生由地震断层的不同尺度的闭锁段相继破裂而发生,同时给出了以力学参数和几何参数定标的断层的自相似破裂机制,用以解决地震断层的分形特征及破裂的慢化过程。     

Numerical Simulation of Fault Zone Guided Waves: Accuracy and 3-D Effects

-- Fault zones are thought to consist of regions with reduced seismic velocity. When sources are located in or close to these low-velocity zones, guided seismic head and trapped waves are generated which may be indicative of the structure of fault zones at depth. Observations above several fault zones suggest that they are common features of near fault radiation, yet their interpretation may be highly ambiguous. Analytical methods have been developed to calculate synthetic seismograms for sources in fault zones as well as at the material discontinuities. These solutions can be used for accurate modeling of wave propagation in plane-parallel layered fault zone structures. However, at present it is not clear how modest deviations from such simplified geometries affect the generation efficiency and observations of trapped wave motion. As more complicated models cannot be solved by analytical means, numerical methods must be employed. In this paper we discuss 3-D finite-difference calculations of waves in modestly irregular fault zone structures. We investigate the accuracy of the numerical solutions for sources at material interfaces and discuss some dominant effects of 3-D structures. We also show that simple mathematical operations on 2-D solutions generated with line sources allow accurate modeling of 3-D wave propagation produced by point sources. The discussed simulations indicate that structural discontinuities of the fault zone (e.g., fault offsets) larger than the fault zone width affect significantly the trapping efficiency, while vertical properly gradients, fault zone narrowing with depth, small-scale structures, and moderate geometrical variations do not. The results also show that sources located with appropriate orientations outside and below a shallow fault zone layer can produce considerable guided wave energy in the overlying fault zone layer.     

Statistical physics of fault patterns self-organized by repeated earthquakes

This work presents at attempt to model brittle ruptures and slips in a continental plate and its spontaneous organization by repeated earthquakes in terms of coarse-grained properties of the mechanical plate. A statistical physics model, which simulates anti-plane shear deformation of a thin plate with inhomogeneous elastic properties, is thus analyzed theoretically and numerically in order to study the spatio-temporal evolution of rupture patterns in response to a constant applied strain rate at its borders, mimicking the effect of neighboring plates. Rupture occurs when the local stress reaches a threshold value. Broken elements are instantaneously healed and retain the original material properties, enabling the occurrence of recurrent earthquakes. Extending previous works (Cowie et al., 1993;Miltenberger et al., 1993), we present a study of the most startling feature of this model which is that ruptures become strongly correlated in space and time leading to the spontaneous development of multifractal structures and gradually accumulate large displacements. The formation of the structures and the temporal variation of rupture activity is due to a complex interplay between the random structure, long-range elastic interactions and the threshold nature of rupture physics. The spontaneous formation of fractal fault structures by repeated earthquakes is mirrored at short times by the spatio-temporal chaotic dynamics of earthquakes, well-described by a Gutenberg-Richter power law. We also show that the fault structures can be understood as pure geometrical objects, namely minimal manifolds, which in two dimensions correspond to the random directed polymer (RDP) problem. This mapping allows us to use the results of many studies on the RDP in the field of statistical physics, where it is an exact result that the minimal random manifolds in 2D systems are self-affine with a roughness exponent 2/3. We also present results pertaining to the influence of the degree of stress release per earthquake on the competition between faults. Our results provide a rigorous framework from which to initiate rationalization of many, reported fractal fault studies.     

3-D Simulation of Earthquake Generation Cycles and Evolution of Fault Constitutitve Properties

--The earthquake generation cycle consists of tectonic loading, quasi-static rupture nucleation, dynamic rupture propagation and stop, and subsequent stress redistribution and fault restrengthening. From a macroscopic point of view, the entire process of earthquake generation cycles should be consistently described by a coupled nonlinear system of a slip-response function, a fault constitutive law and a driving force. On the basis of such a general idea, we constructed a realistic 3-D simulation model for earthquake generation cycles at a transcurrent plate boundary by combining the viscoelastic slip-response function derived for a two-layered elastic-viscoelastic structure model, the slip- and time-dependent fault constitutive law that has an inherent mechanism of fault restrengthening, and the steady relative plate motion as a driving force into a single closed system. With this model we numerically simulated the earthquake generation cycles repeated in a seismogenic region on a plate interface, and examined space-time changes in shear stress, slip deficits and fault constitutive properties during one complete cycle in detail. The occurrence of unstable dynamic slip brings about decrease both in fault strength and shear stress to a constant residual level. After the arrest of dynamic slip, the breakdown strength drop j†_p of fault is restored rapidly and the process of stress accumulation resumes in the seismogenic region. On the other hand, the restoration of the critical weakening displacement D_c proceeds gradually with time through the interseismic period. The restoration of D_c can be regarded as the macroscopic manifestation of the microscopic recovery process of fractal fault surface structure. Through numerical simulation with a multi-segmented fault model, we examined the effects of viscoelastic fault-to-fault interaction. The effect of transient viscoelastic stress transfer through the asthenosphere is significant as well as the direct effect of elastic stress transfer, and it possibly explains the time lag of the sequential occurrence of large events along a plate boundary.     

随机性细胞自动机的地震模拟的动力学含义

结合随机的能量输入和确定的能量损耗，本文用随机性细胞自动机(CA)进行了地震事件的数字模拟，并对事件进行了能量分维和时间序列的多重分形分析。初步结果表明，大量事件的能量-频次的统计分布可能遵从最经典的Gutenberg—Richter关系．不同的初始能量分布和不同的能量传递准则都对模型的输出产生影响，b值与模型参数的设置密切相关，地震现象本质上的复杂性可能是随机性与确定性的统一体现。     

The fractal nature of the inhomogeneities in the lithosphere evidenced from seismic wave scattering

In this paper we show evidences of the fractal nature of the 3-D inhomogeneities in the lithosphere from the study of seismic wave scattering and discuss the relation between the fractal dimension of the 3-D inhomogeneities and that of the fault surfaces. Two methods are introduced to measure the inhomogeneity spectrum of a random medium: 1. the coda excitation spectrum method, and 2. the method of measuring the frequency dependence of scattering attenuation. The fractal dimension can be obtained from the inhomogeneity spectrum of the medium. The coda excitation method is applied to the Hindu-Kush data. Based on the observed coda excitation spectra (for frequencies 1–25 Hz) and the past observations on the frequency dependence of scattering attenuation, we infer that the lithospheric inhomogeneities are multiple scaled and can be modeled as a bandlimited fractal random medium (BLFRM) with an outer scale of about 1 km. The fractal dimension of the 3-D inhomogeneities isD ₃=31/2–32/3, which corresponds to a scaling exponent (Hurst number)H=1/2–1/3. The corresponding 1-D inhomogeneity spectra obey the power law with a powerp=2H+1=2–5/3. The intersection between the earth surface and the isostrength surface of the 3-D inhomogeneities will have fractal dimensionD ₁=1.5–1.67. If we consider the earthquake fault surface as developed from the isosurface of the 3-D inhomogeneities and smoothed by the rupture dynamics, the fractal dimension of the fault trace on the surface must be smaller thanD ₁, in agreement with recent measurements of fractal dimension along the San Andreas fault.     

断层活动方式与地震地表变形分布特征研究

基于断层弹性位错理论及断层滑动非均匀模型,用三维有限元方法计算了发震断层逆断、正断和水平走滑三种不同活动方式下的地表变形,探讨了断层不同活动方式下的地震应变与位移的分布规律及震级、断层倾角对地震地表变形分布的影响。研究结果表明,地震地表变形影响因素很多,如地质构造条件、岩性介质特征、断层活动强度、断层产状和区域构造应力场等,但分布形态最终决定于断层活动方式,变形大小则决定于断层活动强度,其它均为局地因素,只影响分布形态的局部扭曲。断层不同活动方式下的地震地表变形分布各有其自身的规律和特点,这些分布特征可作为地震研究及近活动断层建筑工程抗震设计或加固防护参考。     

Structural Properties and Deformation Patterns of Evolving Strike-slip Faults: Numerical Simulations Incorporating Damage Rheology

We present results on evolving geometrical and material properties of large strike-slip fault zones and associated deformation fields, using 3-D numerical simulations in a rheologically-layered model with a seismogenic upper crust governed by a continuum brittle damage framework over a viscoelastic substrate. The damage healing parameters we employ are constrained using results of test models and geophysical observations of healing along active faults. The model simulations exhibit several results that are likely to have general applicability. The fault zones form initially as complex segmented structures and evolve overall with continuing deformation toward contiguous, simpler structures. Along relatively-straight mature segments, the models produce flower structures with depth consisting of a broad damage zone in the top few kilometers of the crust and highly localized damage at depth. The flower structures form during an early evolutionary stage of the fault system (before a total offset of about 0.05 to 0.1 km has accumulated), and persist as continued deformation localizes further along narrow slip zones. The tectonic strain at seismogenic depths is concentrated along the highly damaged cores of the main fault zones, although at shallow depths a small portion of the strain is accommodated over a broader region. This broader domain corresponds to shallow damage (or compliant) zones which have been identified in several seismic and geodetic studies of active faults. The models produce releasing stepovers between fault zone segments that are locations of ongoing interseismic deformation. Material within the fault stepovers remains damaged during the entire earthquake cycle (with significantly reduced rigidity and shear-wave velocity) to depths of 10 to 15 km. These persistent damage zones should be detectable by geophysical imaging studies and could have important implications for earthquake dynamics and seismic hazard.     

多分形断层模型与地震序列研究

本文提出形成地震序列的多分形断层模型并利用分形分维理论讨论该模型及其序列。给出本模型的震级—频度关系、余震序列中强余震预报公式和多分维D_q—q关系式,探讨了利用多分维预报地震的有效性。     

A new fault rupture scenario for the 2003 Mw 6.6 Bam earthquake, SE Iran: Insights from the high-resolution QuickBird imagery and field observations

The December 26, 2003 M_w 6.6 Bam earthquake is one of the most disastrous earthquakes in Iran. QuickBird panchromatic and multispectral satellite imagery with 61 cm and 2.4 m ground resolution, respectively provide new insights into the surface rupturing process associated with this earthquake. The results indicate that this earthquake produced a 2–5 km-wide surface rupture zone with a complex geometric pattern. A 10-km-long surface rupture zone developed along the pre-existing Bam fault trace. Two additional surface rupture zones, each 2–5 km long, are oblique to the pre-existing Bam fault in angles of 20–35°. An analysis of geometric and geomorphic features also shows that movement on the Bam fault is mainly right-lateral motion with some compressional component. This interpretation is consistent with field investigations, analysis of aftershocks as well as teleseismic inversion. Therefore, we suggest that the 2003 Bam earthquake occurred on the Bam fault, and that the surface ruptures oblique to the Bam fault are caused by secondary faulting such as synthetic shears (Reidel shears). Our fault model for the Bam earthquake provides a new tectonic scenario for explaining complex surface deformations associated with the Bam earthquake.     

断层结构非均匀性对地震活动性影响的细胞自动机模拟

基于断层强度分布的非均匀性,构建了由81×81个细胞单元组成的4种不同匀质度及其各自6种不同随机构型共计24个非均匀二维单断层模型样本,并通过设计的细胞自动机模拟程序,在保持其它模拟参数不变的条件下对它们进行了模拟试验.研究结果表明,随着断层结构非匀质度的增加,其宏观变形破坏行为由相对脆性向塑性变化;地震序列类型依次表...     

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。     

2017年精河MS6.6地震的断层参数和破裂过程

利用远震资料、近场强震资料和合成孔径雷达干涉同震形变资料确定了2017年8月9日精河M_S6.6地震的断层面参数及震源破裂细节。为得到可靠的断层几何参数,发展了一套基于InSAR数据滑动分布反演的三维格点搜索流程,对本次地震断层面的走向、倾角和震源深度进行了格点搜索。结果显示,地震断层面走向为95°,倾角为47°,震源深度为14 km。基于搜索得到的断层模型进行破裂过程联合反演的结果显示:精河M_S6.6地震为一次单侧破裂事件,最大滑动量约为0.8 m,滑动区域集中在断层面上震源以西5—15 km,沿倾向15—25 km,破裂主要发生在10 km深度以下区域。断层面上的平均滑动角为106°。整个破裂过程释放的标量地震矩为3.6×1018 N·m,对应矩震级为M_W6.3。破裂过程持续约9 s,期间的破裂速度约为2.1—2.6 km/s。由于地震破裂主要集中在10 km以下,未来可能需要关注该区域0—10 km发生潜在地震的可能性。      

A new fault rupture scenario for the 2003 Mw 6.6 Bam earthquake,SE Iran: Insights from the high-resolution QuickBird imagery and field observations

The December 26, 2003 M_w 6.6 Bam earthquake is one of the most disastrous earthquakes in Iran. QuickBird panchromatic and multispectral satellite imagery with 61 cm and 2.4 m ground resolution, respectively provide new insights into the surface rupturing process associated with this earthquake. The results indicate that this earthquake produced a 2–5 km-wide surface rupture zone with a complex geometric pattern. A 10-km-long surface rupture zone developed along the pre-existing Bam fault trace. Two additional surface rupture zones, each 2–5 km long, are oblique to the pre-existing Bam fault in angles of 20–35°. An analysis of geometric and geomorphic features also shows that movement on the Bam fault is mainly right-lateral motion with some compressional component. This interpretation is consistent with field investigations, analysis of aftershocks as well as teleseismic inversion. Therefore, we suggest that the 2003 Bam earthquake occurred on the Bam fault, and that the surface ruptures oblique to the Bam fault are caused by secondary faulting such as synthetic shears (Reidel shears). Our fault model for the Bam earthquake provides a new tectonic scenario for explaining complex surface deformations associated with the Bam earthquake.     

STUDY ON THE ELECTRICAL STRUCTURE OF THE ANQIU AND JUXIAN ELECTROMAGNETIC STATIONS IN THE TANLU FAULT ZONE

The Yishu fault zone is one of the branch faults of the Tanlu fault zone in its central part. Moderate and strong earthquakes occurred in the Yishu fault zone repeatedly. Due to its complex structure, the Yishu fault zone attracts much attention from earthquake researches. The Anqiu and Juxian electromagnetic stations in Shandong Province locate near the Anqiu-Juxian Fault and Changyi-Dadian Fault, which are branches of the Yishu fault zone, respectively. Geoelectric field and geomagnetic field observation were carried out in these two stations. The Wudi electromagnetic station is in the west of Tanlu fault zone in the Jidong-Bohai block and 230km from Anqiu electromagnetic station. This paper firstly describes the crustal structure near the electromagnetic stations by using magnetotelluric(MT)method. By processing the data carefully, we obtain the MT data in good quality near the stations. The MT data of each electromagnetic station and its nearby area suggests that the electrical structure and geological structure of the station are comparable. This paper applied 1-D and 2-D inversion for MT data and obtained the crustal electrical structure model beneath the Anqiu and Juxian seismic station. The shallow electrical structure from the MT method was compared with the results of symmetrical quadrupole electrical sounding. The model suggests that the electrical structure beneath the Anqiu and Juxian electromagnetic stations is complex and shows the feature of block boundary. The Wudi electromagnetic station is located inside a basin, the crustal structure shows layered feature typical for the stable blocks. Beneath the Anqiu electromagnetic station, there is a 1km-thick relative low resistivity layer in the shallow crust and a high resistivity body beneath it with a depth of 13km. There is a high resistivity structure in the crust beneath the Juxian electromagnetic station. The crustal structures are divided into two different parts by Anqiu-Juxian Fault and Changyi-Dadian Fault, respectively. More conductive layers appear to the west of the two faults. Plenty of fluid possibly exists within the conductive body to the west of Changyi-Dadian Fault, which plays important role in the earthquake generation. There is a relative low resistivity layer in the crust within 1~2km beneath the Wudi electromagnetic station. Beneath the relatively low resistivity layer, a relatively high resistivity layer extends to a depth of around 15km, and the resistivity value decreases with the increase of depth. The electrical resistivity model suggests the seismic activity of the Yishu fault zone around the Anqiu and Juxian electromagnetic stations should be taken into account seriously, and monitoring and research on it need to be strengthened. The results of this paper provide a certain reference value for the crustal structure research to similar stations.     

贺兰山—银川地堑及邻区重力异常特征及构造意义

贺兰山—银川地堑及邻区地质结构复杂,对该区域深浅结构特征的研究具有重要意义.本文采用重力归一化总梯度成像和二维小波多尺度分解方法对研究区内重力异常进行了垂向和横向构造分析.重力归一化总梯度成像结果显示高低转换带的倾角、倾向与地质上的贺兰山东麓断裂、银川断裂和黄河断裂分布吻合较好,贺兰山西麓断裂与贺兰山东麓断裂汇交深度约18 km,银川断裂与黄河断裂汇交深度约25 km;二维小波多尺度分解成像结果表明正谊关断裂、贺兰山西麓断裂、芦花台断裂和银川断裂为上地壳断裂,贺兰山东麓断裂、青铜峡—固原断裂以及黄河断裂为下地壳断裂,且这三大断裂可能分别是阿拉善地块东南边界和鄂尔多斯地块西南边界;1739年平罗M 8.0古地震震中与银川断裂在重力剖面深度约15 km汇交,其垂向高低梯度为强变形带,同时古地震震中位于重力正负异常转换部位的低值区,据此可推断此次古地震的发震构造是银川断裂.这些结论可提高对贺兰山—银川地堑及邻区地质结构的认识,为该区地壳动力学过程及强震的孕震机理研究提供一定的科学依据.