Asperity distribution of the 1964 Great Alaska earthquake and its relation to subsequent seismicity in the region

The 1964 Alaska earthquake was the second largest seismic events in the 20th century. The aim of this work is the use of surface deformation data to determine asperity and slip distributions on the fault plane of the Alaska earthquake: these distributions are calculated by a Monte Carlo method. To this aim, we decompose the fault plane in a large number of small square asperity units with a side of 25 km; this allows us to obtain plane surfaces with an irregular shape. In the first stage, each asperity unit is allowed to slip a constant amount or not to slip at all, providing the geometry of the dislocation surface that best reproduces the observed displacements. To this purpose, a large number of slip distributions have been tried by the use of the Monte Carlo method. The slip amplitude is the same for all the asperities and is equal to the average fault slip inferred from the seismic moment. In the second stage, we evaluate the slip distribution in the dislocation area determined by the Monte Carlo inversion: in this case, we allow unit cells to undergo different values of slip in order to refine the initial dislocation model. The results confirm the previous finding that the slip distribution of the great Alaska earthquake was essentially made of two dislocation areas with a higher slip, the Prince William Sound and the Kodiak asperities. Analysis of the post-1964 seismicity in the rupture region shows a strong correlation between the larger earthquakes (M_w≥6) and the distribution of locked asperities following the 1964 event, which can be considered as an independent test of the validity of the model. We do not find slip values higher than 25 m for any of the patches, and we determine two separate high-slip zones: one correspondent to the Prince William Sound asperity, and one (18 m slip) to the Kodiak asperity. The slip distribution connected with the 1964 shock appears to be consistent with the following seismicity in the region.     

Seismic coupling and uncoupling at subduction zones

Seismic coupling has been used as a qualitative measure of the “interaction” between the two plates at subduction zones. Kanamori (1971) introduced seismic coupling after noting that the characteristic size of earthquakes varies systematically for the northern Pacific subduction zones. A quantitative global comparison of many subduction zones reveals a strong correlation of earthquake size with two other variables: age of the subducting lithosphere and convergence rate. The largest earthquakes occur in zones with young lithosphere and fast convergence rates, while zones with old lithosphere and slow rates are relatively aseismic for large earthquakes. Results from a study of the rupture process of three great earthquakes indicate that maximum earthquake size is directly related to the asperity distribution on the fault plane (asperities are strong regions that resist the motion between the two plates). The zones with the largest earthquakes have very large asperities, while the zones with smaller earthquakes have small scattered asperities. This observation can be translated into a simple model of seismic coupling, where the horizontal compressive stress between the two plates is proportional to the ratio of the summed asperity area to the total area of the contact surface. While the variation in asperity size is used to establish a connection between earthquake size and tectonic stress, it also implies that plate age and rate affect the asperity distribution. Plate age and rate can control asperity distribution directly by use of the horizontal compressive stress associated with the “preferred trajectory” (i.e. the vertical and horizontal velocities of subducting slabs are determined by the plate age and convergence velocity). Indirect influences are many, including oceanic plate topography and the amount of subducted sediments.All subduction zones are apparently uncoupled below a depth of about 40 km, and we propose that the basalt to eclogite phase change in the down-going oceanic crust may be largely responsible. This phase change should start at a depth of 30–35 km, and could at least partially uncouple the plates by superplastic deformation throughout the oceanic crust during the phase change.     

Frictional discharge at fault asperities: Origin of fractal seismo-electromagnetic radiation

To investigate the relation between the rock friction and the fractal electromagnetic radiation before the main-shock of earthquakes, we conducted a friction experiment simulating the motion of an asperity on a fault plane, and observed photon emissions due to electric discharge by dielectric breakdown of ambient gases from friction contacts between rock minerals. This indicates that frictional discharges (plasma generations) could occur locally at microscopic asperities on fault surfaces. From concepts on the fractal size-distribution and temporal evolution of fault asperities, the frictional discharge occurring at asperities on the fault plane can be one of origins of the fractal electromagnetic radiation (Benioff electromagnetic radiation) prior to earthquakes.     

断层面摩擦强度对工作面开采影响数值模拟分析

断层面摩擦强度是评价煤炭开采中应力扰动诱发断层滑动危险性的依据。依托晋城矿区成庄井田,采用理论分析和数值模拟计算方法,分析了断层面摩擦强度对深部地应力的约束机制,研究了成庄井田F13断层及其在不同摩擦强度条件下对回采工作面顶板稳定性、超前支承压力分布和断层滑动的影响规律。研究结果表明:地壳深部最大与最小主应力比值受断层面摩擦强度的限制,当其达到临界方向断层的摩擦强度极限时,断层就会发生滑动;断层破碎带的存在导致初始应力场扰动,形成断层带低应力区及高应力集中区,在回采过程中将直接影响煤层顶板移动变形和采动应力分布;断层面摩擦强度较小时,工作面开采至断层附近顶板下沉量及断层上下盘错动位移较大,支承压力峰值由大变小明显,断层面上剪应力与正应力的比值易达到断层面的摩擦系数,断层滑动的危险性较大。      

基于实验结果讨论断层破裂与强震物理过程的若干问题

基于断层摩擦滑动实验、含凹凸体断层的变形破坏实验、断层撕裂扩展的实验、交叉断层的变形实验等多种实验结果并结合前人的工作 ,讨论了与断层破裂与强震物理过程相关的若干问题。研究表明 ,断层的整体滑动引起其两侧块体弹性应变的释放 ,是强震发生的原因 ,因此构造活动区具有较大尺度、结构连续且简单、介质均匀的断层 (或断层段 )是产生强震必备的构造条件 ,深部新生断层(盲断层 )向上撕裂扩展产生强震 ,尚需“弱层”提供“解耦”条件以便断层发生整体滑动。强震孕育过程中包含着凹凸体的破裂 ,断层面上凹凸体的尺度、强度及数量决定着前震活动的特征、强震动态破裂过程以及前兆现象。由断层分割的块体通过边界断层的交替滑动、以“框动”的方式运动 ,因此块体周边的断层上强震活动具有交替性。     

Near-fault strong motion complexity of the 2000 Tottori earthquake (Japan) from a broadband source asperity model

The October 6/2000 Tottori earthquake that occurred in central Japan was an intermediate size strike-slip event that produced a very large number of near field strong motion recordings. The large amount of recorded data provides a unique opportunity for investigating a source asperity model of the Tottori earthquake that, combined with a hybrid strong motion simulation technique, is able to reproduce the observed broadband frequency near-fault ground motion.

We investigated the optimum source asperity parameters of the Tottori earthquake, by applying a Genetic Algorithm (GA) inversion scheme to optimise the fitting between simulated and observed response spectra and Peak Ground Acceleration (PGA) values. We constrained the initial model of our inversion by using the heterogeneous slip distribution obtained from a kinematic inversion of the source of previous studies. We used all the observed near-fault ground motions (−100 m) from the borehole strong motion network of Japan (KiK-Net), which are little affected by surficial geology (site effects).

The calculation of broadband frequency strong ground motion (0.1–10 Hz) is achieved by applying a hybrid technique that combines a deterministic simulation of the wave propagation for the low frequencies and a semi-stochastic modelling approach for the high frequencies. For the simulation of the high frequencies, we introduce a frequency-dependent radiation pattern model that efficiently removes the dependence of the pattern coefficient on the azimuth and take-off angle as the frequency increases. The good agreement between the observed and simulated broadband ground motions shows that our inversion procedure is successful in estimating the optimum asperity parameters of the Tottori earthquake and provides a good test for the strong ground motion simulation technique.

The ratio of background stress drop to average asperity stress drop from our inversion is nearly 50%, in agreement with the theoretical asperity model of Das and Kostrov [Das, S., Kostrov, B.V., 1986. Fracture of a single asperity on a finite fault: a model for weak earthquakes? Earthquake Source Mechanics, AGU, pp. 91–96.], and an empirical ratio of asperities to rupture area [Seismol. Res. Lett. 70 (1999) 59–80.].

The simulated radiation pattern is very complex for epicentral distances within half the fault length, but it approaches the radiation of a double-couple point source for larger distances.

The rupture velocity and rise time have a significant influence on the Peak Ground Velocity (PGV) distribution around the fault. An increase in rupture velocity produces a similar effect on the ground motion as a reduction in rise time.     

Evolution of a fold-thrust belt deforming a unit with pre-existing linear asperities: Insights from analog models

Heterogeneity, whether geometric or rheologic, in crustal material undergoing compression affects the geometry of the structures produced. This study documents the thrust fault geometries produced when discrete linear asperities are introduced into an analog model, scaled to represent bulk upper crustal properties, and compressed. Varying obliquities of the asperities are used, relative to the imposed compression, and the resultant development of thrust fault traces and branch lines in map view is tracked. Once the model runs are completed, cross-sections are created and analyzed. The models show that asperities confined to the base layer promote the clustering of branch lines in the surface thrusts. Strong clustering in branch lines is also noted where several asperities are in close proximity or cross. Slight reverse-sense reactivation of asperities cut through the sedimentary sequence is noted in cross-section, where the asperity and the subsequent thrust belt interact. The model results are comparable to the situation in the Dinaric Alps, where pre-existing faults to the SW of the NE Adriatic Fault Zone contribute to the clustering of branch lines developed in the surface fold-thrust belt. These results can therefore be used to evaluate the evolution of other basement-involved fold-thrust belts worldwide.     

Estimates of source parameters of <Emphasis Type="Italic">M</Emphasis>4.9 Kharsali earthquake using waveform modelling

This paper presents the computation of time series of the 22 July 2007 M 4.9 Kharsali earthquake. It occurred close to the Main Central Thrust (MCT) where seismic gap exists. The main shock and 17 aftershocks were located by closely spaced eleven seismograph stations in a network that involved VSAT based real-time seismic monitoring. The largest aftershock of M 3.5 and other aftershocks occurred within a small volume of 4 × 4 km horizontal extent and between depths of 10 and 14 km. The values of seismic moment (M _∘) determined using P-wave spectra and Brune’s model based on f ² spectral shape ranges from 10¹⁸ to 10²³ dyne-cm. The initial aftershocks occurred at greater depth compared to the later aftershocks. The time series of ground motion have been computed for recording sites using geometric ray theory and Green’s function approach. The method for computing time series consists in integrating the far-field contributions of Green’s function for a number of distributed point source. The generated waveforms have been compared with the observed ones. It has been inferred that the Kharsali earthquake occurred due to a northerly dipping low angle thrust fault at a depth of 14 km taking strike N279°E, dip 14° and rake 117°. There are two regions on the fault surface which have larger slip amplitudes (asperities) and the rupture which has been considered as circular in nature initiated from the asperity at a greater depth shifting gradually upwards. The two asperities cover only 10% of the total area of the causative fault plane. However, detailed seismic imaging of these two asperities can be corroborated with structural heterogeneities associated with causative fault to understand how seismogenesis is influenced by strong or weak structural barriers in the region.     

近断层强地震动预测中的有限断层震源模型

提出了近断层强地震动预测中建立活断层上设定地震有限断层震源模型的方法和步骤.首先,根据地震地质和地震活动性调查以及地球物理勘探等资料,确定活断层的空间方位和滑动类型; 然后,根据地震定标律确定活断层的宏观震源参数; 第三,将高强体模型与k平方滑动模型相结合,产生断层破裂面上的混合滑动分布.在此基础上,预测了与1994年Northridge地震断层类型、矩震级(M_w6.7)基本一致的设定地震的有限断层震源模型.最后,将预测的有限断层震源模型与基于地震学的、使用动力学拐角频率的地震动随机合成方法相结合,预测了1994年Northridge地震近断层12个基岩台站的加速度时程,并和实际记录进行了对比.结果表明,用上述方法和步骤建立的有限断层震源模型是可行、实用的.      

Selection and quantification of near-fault velocity pulses owing to source directivity

Design ground motions are typically governed by large earthquakes at close distances, but the increasing number of near-field recordings manifest the large variability in near-source ground-motion amplitudes which result in significant differences in the building response. In the near-field, this variability arises mainly from source-directivity effects, which generate strong near-fault velocity pulses. We investigate the effect of source complexity on the generation of velocity pulses using a geometrical approach to quantify directivity at near-fault sites based on kinematic rupture models. We propose selection algorithms that can be implemented as search strategies for directivity-related strong ground motion records which may serve as ground motion selection tools in large databases for structural analysis and liquefaction studies. We find that the existence of directivity pulses is strongly related to slip heterogeneity on the fault plane, i.e. that the location and size of asperities (large slip areas) determine directivity pulse generation. In this context we quantify several pulse properties, testing a variety of approaches, and develop predictive relationships between a number of source parameters and pulse properties. We find strong dependence of pulse period on total area of asperities, as well as on a geometrical directivity parameter. The empirical observations on velocity pulse generation determined by the proposed selection procedure are compared with the predictions using the geometrical directivity model. The results are important for determining the probability of observing a pulse at a site as a function of magnitude, distance, and slip heterogeneity on the fault plane.     

Mechanism of the 2011 Tohoku-oki earthquake (Mw 9.0) and tsunami: Insight from seismic tomography

To clarify the generating mechanism of the 2011 Tohoku-oki earthquake (Mw 9.0) and the induced tsunami, we determined high-resolution tomographic images of the Northeast Japan forearc. Significant lateral variations of seismic velocity are visible in the megathrust zone, and most large interplate thrust earthquakes are found to occur in high-velocity (high-V) areas. These high-V zones may represent high-strength asperities at the plate interface where the subducting Pacific plate and the overriding Okhotsk plate are coupled strongly. A shallow high-V zone with large coseismic slip near the Japan Trench may account for the mainshock asperity of the 2011 Tohoku-oki earthquake. Because it is an isolated asperity surrounded by low-velocity patches, most stress on it was released in a short time and the plate interface became decoupled after the Mw 9.0 earthquake. Thus the overriding Okhotsk plate there was shot out toward the Japan Trench and caused the huge tsunami.     

AN INTERFACE MODEL FOR ANALYSIS OF DEFORMATION BEHAVIOUR OF DISCONTINUITIES

An interface constitutive model is presented accounting for slip and sliding effects and also for dilatancy phenomena. The microslip effects are described by considering spherical asperity interaction with variation of contact area and generation of progressive or reverse slip zones. The incremental constitutive equations are derived with proper memory rules accounting for generation and annihilation of particular slip zones during the process of variable loading. It is further assumed that sliding of spherical contacts occurs along large asperities whose slope varies due to the wear process. The predicted shear and dilatancy curves are shown to provide close quantitative simulation of available experimental data. The strain ratchetting effect for non-symmetric cyclic loading was exhibited using the asperity wear model. The model presented could be applied to simulate rock joints, masonry, or concrete cracked interfaces, under monotonic and cyclic loading.     

Source complexity of the 4 March 2010 Jiashian,Taiwan, Earthquake determined by joint inversion of teleseismic and near field data

The Jiashian earthquake (M_L 6.4) occurred on 4 March 2010. It was the largest inland event in southern Taiwan of 2010. The mainshock location was unexpected since it occurred in an area with relatively low background seismicity. In addition, reports of earthquake focal mechanisms do not fit with any known active fault geometry. In order to understand the origin of this earthquake, especially its rupture process, we perform a joint source inversion by using teleseismic body wave, GPS coseismic displacements and near field ground motion data. In this study, we considered a northwest–southeast trending fault with a northeast dip retrieved from GPS coseismic data and aftershocks distribution. To analyze the detailed slip distribution in space and time, we used near field 3D Green’s functions provided by spectral-element method and a full time–space inversion technique. We find a complex rupture process with several slip patches distributed inside two main asperities. The slip map reveals a mean slip of 12.9 cm for a maximum slip of 27.3 cm leading to a M_w 6.47 for this event. The rupture initiates in the deepest portion of the fault at 20 km depth, and propagated upward up to 2 km depth to form the two asperities. The source time function of this event revealed two pulses corresponding to the two asperities, for a total duration time of about 16 s. Most aftershocks occurred near the upper boundary of the deepest asperity while no aftershocks are located close to the shallowest one. We infer that the locations of these slip patches are related to the surrounding fault systems that may have restricted the rupture propagation during the earthquake.     

Influence of interaction between small asperities on various types of slow earthquakes in a 3-D simulation for a subduction plate boundary

Recently, the occurrence of slow earthquakes such as low-frequency earthquakes and very low-frequency earthquakes have been recognized at depths of about 30 km in southwest Japan and Cascadia. These slow earthquakes occur sometimes in isolation and sometimes break into chain-reaction, producing tremor that migrates at a speed of about 5–15 km/day and suggesting a strong interaction among nearby small asperities. In this study, we formulate a 3-D subduction plate boundary model with two types of small asperities chained along the trench at the depth of 30 km. Our simulation succeeds in representing various types of slow earthquakes including low-frequency earthquakes and rapid slip velocity in the same asperity, and indicates that interaction between asperities may cause the very low-frequency earthquakes. Our simulation also shows chain reaction along trench with propagation speed that can be made consistent with observations by adjusting model parameters, which suggests that the interactions also explain the observed migration of slow earthquakes.     

Reduction of friction on geological faults by weak-phase smearing

Most common crustal rock types display friction coefficients of 0.6 or higher, but some faults must be frictionally weak as they slip when the stress state is unfavourably-oriented (i.e. the resolved shear stress is low for a given normal stress across the fault surface). A role for low-friction minerals and high pore fluid pressures, either separately or in combination, is frequently invoked to explain such slip, but volume fractions of dispersed weak phases often seem to be present in fault gouge in amounts too small to produce significant mechanical weakening. By means of mechanical tests on synthetic fault gouge and microstructural study of run products, we show that the effective area of an embedded weak phase (graphite) on a slip plane can be substantially increased by mechanical smearing, and that the enlarged area of the weak phase on the slip plane follows a linear mixing law. This allows a relatively small volume fraction of the initially dispersed weak phase to have a disproportionately large effect, provided the smearing is concentrated into a narrow planar slip zone or into an interconnected network of them.     

甘肃昌马盆地构造特征与成因

阿尔金断裂是由阿尔金主干断层及其他次级断裂面组成的巨型走滑断裂系，其分布规律符合左行走滑挤压模式。昌马盆地位于阿尔金断裂带东段，它的古应力场与阿尔金断层的运动方向一致。昌马盆地北界是阿尔金主干断层；南界由锯齿状相交的半截子井－野马山断层、野观山断层、石板墩－龚岔口断层、中祁连北缘断层组成，分别是阿尔金主干断层的S面、小型Y面、P面、S面；东界由香毛山西断层、马舌头断层、青石岩断层组成，分别是阿尔金主干断层的X面、S顶、R’面，昌马盆地是由这些断层围限的狭长三角形。因此，昌马盆地是阿尔金断裂带中主干断层与分枝断层联合作用形成的走滑分枝盆地。盆地经历了侏罗纪－白垩纪的拉张作用，接受了巨厚的沉积；晚第三纪到第四纪由于阿尔金断裂左行走滑运动，盆地反转，处于受挤压状态，形成走滑分枝盆地。通过对昌马盆地石油地质特征的分析，认为昌马盆地具有形成油气的有利地质条件，应加强勘探力度。     

A fractal earthquake source with a slip zone generates acceleration time histories with flat spectra

An important problem of the studies of earthquake sources is to clarify the mechanism of formation of radiated source spectra of the ω^?2 (“omega-square”) kind, or equivalently, of flat acceleration spectra. This spectral model is well established empirically and has the status of a classical one in source seismology; however, it lacks adequate theoretical foundation. It is shown that spectra of the ω^?2 kind can be explained by combining the following three concepts regarding source rupture: (1) the fault asperity model of Das-Kostrov; (2) the Andrews’s concept that the field of the stress drop over the fault is a 2D flicker-noise with amplitude spectrum of the 1/k type; and (3) the hypothesis that the distance of propagation of Rayleigh waves from a failing spot on a fault is determined by the width of the slip zone associated with the rupture front.     

Influence of interaction between small asperities on various types of slow earthquakes in a 3-D simulation for a subduction plate boundary

Recently, the occurrence of slow earthquakes such as low-frequency earthquakes and very low-frequency earthquakes have been recognized at depths of about 30 km in southwest Japan and Cascadia. These slow earthquakes occur sometimes in isolation and sometimes break into chain-reaction, producing tremor that migrates at a speed of about 5–15 km/day and suggesting a strong interaction among nearby small asperities. In this study, we formulate a 3-D subduction plate boundary model with two types of small asperities chained along the trench at the depth of 30 km. Our simulation succeeds in representing various types of slow earthquakes including low-frequency earthquakes and rapid slip velocity in the same asperity, and indicates that interaction between asperities may cause the very low-frequency earthquakes. Our simulation also shows chain reaction along trench with propagation speed that can be made consistent with observations by adjusting model parameters, which suggests that the interactions also explain the observed migration of slow earthquakes.     

Stress,strain and fault patterns

If faulting is treated as a stress-controlled phenomenon, the generation of a single fault set, or two sets in conjugate arrangement are inevitably predicted implying plane strain. Alternatively, considering faulting as a strain-controlled process, multiple-set patterns can be predicted. The analysis of multiple-set patterns requires identifying the type of fault pattern from four possibilities: Coulomb, isolated, orthorhombic and complex fault patterns.There are techniques that permit a unique solution of strain tensor for Coulomb and orthorhombic fault patterns. For isolated fault patterns, the principal paleostress directions could be used to approximate the principal strain directions. In this case, we need to assume a homogeneous stress field, independence between faults, and parallelism between shear stress and slip vector on the sliding plane.For complex fault patterns, it is not possible to uniquely determine the total strain tensor without knowledge of all the slip planes. Furthermore, inverting fault-slip data to determine the stress tensor is not correct because the assumptions of the inversion methods are not satisfied. Only a rough approximation is possible assuming that strain produced by major faults represents the total strain tensor.     

塔里木盆地深层走滑断层分段特征及对油气富集的控制:以塔北地区哈拉哈塘油田奥陶系走滑断层为例

塔里木盆地奥陶系走滑断层是发育在克拉通内部稳定区的小滑移距走滑断层,对于深部储层形成与油气富集具有重要的控制作用。基于哈拉哈塘油田4 140 km2三维地震资料,在高精度相干切片提取与地震精细解释基础上,对研究区奥陶系走滑断层进行了分段研究并讨论了分段性对于储层发育与油气富集的控制作用。结果表明：(1)哈拉哈塘油田奥陶系走滑断层整体格局为由北东、北西向断层组成的纯剪机制下形成的共轭走滑断层,单条走滑断层的构造特征符合Riedel剪切模型,主干断层周围主要发育R剪切分支断层。(2)根据走滑断层不同部位构造样式、应力状态的差异,建立了小滑移距走滑断层分段发育模式。走滑断层端部为应力发散区,多表现为马尾状构造,可分为伸展型和挤压型马尾状构造。走滑断层内部由线性段、斜列叠覆段、分支断层段、辫状构造段组合而成。线性段呈线性延伸,剖面上为孤立的高陡直立断层。斜列叠覆段分为拉张型叠覆段和挤压型叠覆段,其类型受控于次级断层旋向与阶步的关系。分支断层段多为斜交压扭样式,羽状断层发育较少。辫状构造段内部断垒与断堑交错发育,划分为张扭段、压扭段。(3)不同段具有不同的储层发育特征。马尾状构造段、斜列叠覆段、辫状构造段储层最为发育,分支断层段储层较为发育,线性段储层相对不发育。(4)综合储层发育位置、油源断裂与分支断层配置关系、局部构造高3方面因素,建立了6类与走滑断层相关的油气富集模式。R剪切分支断层与主干断层夹持部位、压扭段内部、马尾状分支断层高部位是北部潜山顺层岩溶区最为发育的3种油藏富集模式;压扭段内部、张扭段是南部断控岩溶区油气更富集的部位。论文成果认识对于完善克拉通盆地稳定区小滑移距走滑断层分段发育规律具有重要的理论意义,对于受控于走滑断层的岩溶缝洞型油气藏的勘探开发具有一定的生产指导意义。