Thrust sheet motion and earthquake mechanisms

The mechanism of thrust sheet motion and that of earthquake generation are two aspects of the same phenomenon. Studies of earthquake mechanisms show that only a part of the thrust fault, generally less than 10km in length, is in motion at any given instant; this part sweeps the length of the fault, moving at approximately 3 km/s. Since shear fracture propagation is inherently a dynamic phenomenon, it cannot be understood on the basis of the equations of static equilibrium. The previously proposed mechanisms all rest on such an equilibrium, which necessarily implies that the motion of a thrust sheet occurs simultaneously over its whole length; since the latter may exceed 100 km, the motion becomes a mechanical “enigma”. This enigma vanishes once the dynamic nature of the problem is recognized. The proposed mechanism also suggests that solution features on the fault result from the dissolution of barriers, thus explaining the motion of only part of a thrust sheet.     

Tectonic and Kinematic Regime along the Northern Caribbean Plate Boundary: New Insights from Broad-band Modeling of the May 25, 1992, Ms = 6.9 Cabo Cruz, Cuba, Earthquake

—On May 25th, 1992, an M _s = 6.9 earthquake occurred off the southwestern tip of Cuba, along the boundary between the Caribbean and North American plates. This earthquake was the largest to strike southern Cuba since 1917 and the largest ever recorded in that region by global seismic networks. It is therefore a key element for our understanding of the tectonic and kinematic regime along the northern Caribbean plate boundary. In order to test the previously proposed source parameters of the Cabo Cruz earthquake and to better constrain its focal mechanism, we derived a new set of source parameters from unfiltered broad-band teleseismic records. We used a hybrid ray tracing method that allows us to take into account propagation effects of seismic waves in a realistic crustal model around the source. Our solution is consistent with the long-period focal mechanism solution of Virieux et al. (1992). Our solution also models the higher frequency crustal and water layer phases. The primarily strike-slip focal mechanism has a small thrust component. Its shows an east-west trending nodal plane dipping 55° to the north that we interpret as the rupture plane since it corresponds to the geometry of the major active fault in that area. The displacement on this plane is a left-lateral strike-slip combined with a small amount of southward thrust. The result is in good agreement with the active tectonic structures observed along the Oriente fault south of Cuba. The small thrust component demonstrates that, contrary to prior belief, the transpressive regime extends along this whole segment of the Caribbean/North American plate boundary. Together with historical seismicity, it suggests that most of the stress accumulated by the Caribbean/North American plate motion is released seismically along the southern Cuban margin during relatively few but large earthquakes.     

Can Euler deconvolution outline three-dimensional magnetic sources?

Severe limitations of the standard Euler deconvolution to outline source shapes have been pointed out. However, Euler deconvolution has been widely employed on field data to outline interfaces, as faults and thrust zones. We investigate the limitations of the 3D Euler deconvolution–derived estimates of source dip and volume with the use of reduced-to-the-pole synthetic and field anomalies. The synthetic anomalies are generated by two types of source bodies: (1) uniformly magnetized prisms, presenting either smooth or rough interfaces, and (2) bodies presenting smooth delimiting interfaces but strong internal variation of magnetization intensity. The dip of the first type of body might be estimated from the Euler deconvolution solution cluster if the ratio between the depth to the top and vertical extent is relatively high (>1/4). For the second type of body, besides dip, the source volume can be approximately delimited from the solution cluster envelope, regardless of the referred ratio. We apply Euler deconvolution to two field anomalies which are caused by a curved-shape thrust zone and by a banded iron formation. These anomalies are chosen because they share characteristics with the two types of synthetic bodies. For the thrust zone, the obtained Euler deconvolution solutions show spatial distribution allowing to estimate a source dip that is consistent with the surface geology data, even if the above-mentioned ratio is much less than 1/4. Thus, there are other factors, such as a heterogeneous magnetization, which might be controlling the vertical spreading of the Euler deconvolution solutions in the thrust zone. On the other hand, for the iron-ore formation, the solution cluster spreads out occupying a volume, in accordance with the results obtained with the synthetic sources having internal variation of magnetization intensity. As conclusion, although Euler deconvolution–derived solutions cannot offer accurate estimates of source shapes, they might provide a sufficient degree of reliability in the initial estimates of the source dip and volume, which may be useful in a later phase of more accurate modelling.     

USE OF SEISMIC WAVEFORMS AND INSAR DATA FOR DETERMINATION OF THE SEISMOTECTONICS OF THE MAINLING MS6.9 EARTHQUAKE ON NOV.18, 2017

On November 18, 2017, a M_S6.9 earthquake struck Mainling County, Tibet, with a depth of 10km. The earthquake occurred at the eastern Himalaya syntaxis. The Namche Barwan moved northward relative to the Himalayan terrane and was subducted deeply beneath the Lhasa terrane, forming the eastern syntaxis after the collision of the Indian plate and Asian plates. Firstly, this paper uses the far and near field broadband seismic waveform for joint inversion (CAPJoint method)of the earthquake focal mechanism. Two groups of nodal planes are obtained after 1000 times Bootstrap test. The strike, dip and rake of the best solution are calculated to be 302°, 76° and 84° (the nodal plane Ⅰ)and 138°, 27° and 104° (the nodal plane Ⅱ), respectively. This event was captured by interferometric synthetic aperture radar (InSAR)measurements from the Sentinel-1A radar satellite, which provide the opportunity to determine the fault plane, as well as the co-seismic slip distribution, and assess the seismic hazards. The overall trend of the deformation field revealed by InSAR is consistent with the GPS displacement field released by the Gan Wei-Jun's team. Geodesy (InSAR and GPS)observation of the earthquake deformation field shows the northeastern side of the epicenter uplifting and the southwestern side sinking. According to geodetic measurements and the thrust characteristics of fault deformation field, we speculate that the nodal plane Ⅰ is the true rupture plane. Secondly, based on the focal mechanism, we use InSAR data as the constraint to invert for the fine slip distribution on the fault plane. Our best model suggests that the seismogenic fault is a NW-SE striking thrust fault with a high angle. Combined with the slip distribution and aftershocks, we suggest that the earthquake is a high-angle thrust event, which is caused by the NE-dipping thrust beneath the Namche Barwa syntaxis subducted deeply beneath the Lhasa terrane.     

Approximate evaluation of stresses in degraded tunnel linings

This paper uses numerical and analytical methods to examine the static and seismic response of tunnels with intact and degraded segmental concrete tunnel liners. Concrete degradation is simulated using a non-linear finite element (FE) model that accounts for soil-structure interaction and the non-linear stress–strain response of the soil and concrete. The non-linear FE model is used to calculate radial stresses in tunnel linings with local concrete delaminations and that are subject to both static and seismic loads. Then, the FE results are compared with an analytical solution for jointed tunnel linings in order to assess the accuracy of the solution for predicting stresses in degraded liners. The analyses and results presented in this paper illustrate a simple method for estimating and evaluating the effect of concrete degradation on the distribution of thrust and moment in segmental tunnel linings subject to either static or seismic loads.     

Analytical solution of seismic pseudo-static active thrust acting on fascia retaining walls

This paper presents a limit equilibrium method, based on the approach of Mononobe and Okabe, for calculating the active thrust on fascia retaining walls, where common methods cannot be used owing to the narrowness of the backfill which does not permit the development of the thrust wedge in the shape and sizes predicted by these methods. The proposed method examines three distinct failure mechanisms, called Mechanism 1, Mechanism 2 and Mechanism 3, where the thrust wedge is formed by one, two or three blocks, respectively; separated by plane slip surfaces. The seismic forces have been simulated with the pseudo-static method. For all three mechanisms, the active thrust is obtained in closed form: in particular, with a cubic equation for Mechanism 2, and with a system of two equations, one cubic and the other quartic, for Mechanism 3. Mechanisms with more than three blocks cannot have analytical solutions. The study is completed by an examination of some significant cases from which the higher attenuation of the seismic thrust, with respect to the static, emerges as the backfill width reduces.     

2013年12月1日新疆柯坪县5.3级地震及房屋震害特点分析

2013年12月1日新疆柯坪县发生M5.3地震,柯坪县境内震感强烈,极震区烈度为Ⅵ度。综合分析余震分布和震源机制解,确定本次地震的发震构造为柯坪推覆构造最南端的柯坪塔格断褶带。对灾区乡镇进行房屋调查,土木结构房屋破坏最严重,砖木和砖混结构房屋基本无破坏。该地区为盐渍土发育,地下水埋藏较浅,对地震破坏有放大效应,加大了地震的震害损失。     

流-固耦合作用对计算同震静态库仑应力变化的影响

传统方法计算同震静态库仑应力变化一般都是基于Okada的解析解,模型中不考虑流体对固体骨架力学行为的影响.但实际上,流体对固体变形有着非常重要的作用.为此,本文基于孔隙弹性理论,考虑流-固之间的完全耦合效应,针对三种典型的断层错动模型（走滑型、逆冲型以及正断型）,分别计算静态库仑应力变化与传统算法之间的差别.计算结果表明：三种地震模型得到的介质孔隙压变化在空间的分布格局完全不同,走滑型地震产生的孔隙压变化图案在空间中呈正负相间的四象限分布,近场的静态库仑应力明显下降,流-固耦合作用对静态库仑应力变化影响较大.逆冲型和正断层型地震产生的介质孔隙压变化在空间的分布图案类似,但正负区域正好相反,孔隙压在逆冲型地震的震源区域上升,而在正断型地震的震源区域下降.同传统方法计算的库仑应力相比,逆冲型地震产生的介质孔隙压变化使得震源区的应力影区面积减小,这会触发更多的余震;而正断型地震产生的孔隙压变化则正好相反,增大了震源附近的应力影区范围,这样会降低该区域余震发生的概率.可见,介质的流-固耦合作用对计算库仑应力变化的影响不可忽视.因此,在利用库仑应力变化研究地震触发时,应考虑流-固耦合作用,使得库仑模型的预测结果更符合实际.

     

新疆主要逆断层-褶皱构造区基本地震构造特征与潜在震源划分问题

本文介绍了新疆主要逆断层-褶皱构造区的基本特征,并对其潜在震源划分问题进行了初步的讨论。北天山山前推覆构造及乌鲁木齐以南的逆断裂-褶皱构造相对比较简单,由根部断裂、推覆体和前缘逆断裂-褶皱构造所组成;强地震的极震区或地震动的高值区可能位于推覆构造的根部断裂附近,而地震地表破裂和同震地表变形则位于山前逆断层-褶皱带内。南天山的柯坪推覆构造、库车推覆构造、帕米尔东北缘的弧形推覆构造,虽然也由多排逆断裂-褶皱构造带组成,但是其中的规模巨大、发育时间较长的逆断裂-背斜带,往往具备发生强震的条件。强震的极震区分布与地震地表断层位置比较一致,可作为强震的潜在震源。盆地内的新的盲逆断层-褶皱构造也具备发生6.5—7.0 级地震的能力,应作为震级上限为 7.0 级的潜在震源。由于对逆断层-褶皱构造的深浅构造关系及发震模型认识的不足,在潜在震源划分中应考虑这种不确定性。同时在潜在震源区划分中,还应考虑地震构造区的地震活动历史及构造活动性参数。     

Microearthquake observations around Thien Dam in northwest Himalayas

A microearthquake survey has been conducted around Thien Dam (32°26.5′N, 75°43.5′E) during March–April 1983 by operating seven high gain stations. The data recorded at these stations have been supplemented by other seismological stations in the region under the Beas and Salal Projects. Surficial velocity structure has been obtained from waves generated from blasts in the dam area and hypocentres were determined using the program HYPO 71.Microearthquake distribution shows four main zones of active seismicity. Deeper microearthquakes with focal depths greater than 10 km below the dam were attributed to the Satlitta fault. Composite focal mechanism solution shows that the Satlitta fault at this depth is of normal type. Shallow microearthquakes below the dam are due to the Suruin-Mastgarh anticline which is of the thrust type. Microearthquakes located northeast of the dam are apparently associated with a ‘back’ thrust.     

2019年西藏墨脱MS6.3地震震源参数及其构造意义

北京时间2019年4月24日04:15,西藏自治区林芝市墨脱县发生了M_S6.3地震,该地震位于印度板块与欧亚板块俯冲碰撞的东北犄角地区,构造背景十分复杂.本研究基于我们在东喜马拉雅构造结地区架设的宽频带地震台站记录的近震波形数据,结合中国和国际地震台网的波形和到时资料,对该地震的震源位置、震源机制解和破裂过程进行了重新确定.结果显示,此次墨脱6.3级地震发生在（94.56±0.01°E,28.41±0.01°N）,震源深度为地表以下13.3±1.6（或海平面以下11.5±1.6）km.震源机制解走向/倾角/滑移角分别为202°/17°/20°,震源破裂较大的位置主要集中在初始破裂点NNE侧约5 km附近.结合其他地球物理和地质学资料,我们推测该地震位于主喜马拉雅逆冲断裂发生近90°突然偏转的大拐弯地区,桑构造结相对于其西侧南迦巴瓦构造结的西向俯冲和北向推挤是该地震发生的主要构造背景.

     

Source parameters determination for earthquakes in Kushiro,Japan considering source time function

This paper applies a new formulation to do moment tensor inversion for earthquakes in the Kushiro area of Japan. Comparing with conventional moment tensor inversion method, the new one takes the effect of source time function into consideration. For the inversion, best solution is obtained by minimizing the difference between the observed seismograms and the synthetic ones. And the best-fitting focal depth is determined from the variance reduction. The results indicate that half duration of source time function is proportional to the magnitude of earthquakes. Large earthquakes have long half duration, whereas that of moderate-small earthquakes is comparatively shorter. The focal mechanisms of all three earthquakes are of thrust fault type, which is mainly ascribed to the collision of the North American plate with the Eurasia plate in the late Cretaceous or Paleogene.     

荥经-马边-盐津逆冲构造带断裂运动组合及地震分段特征

逆冲构造带的分段性研究是评价该类发震构造地震危险性的基础工作。荥经-马边-盐津逆冲构造带是青藏高原东南边缘重要的NW向强震构造带,该构造带以逆冲错动为主要活动形式,其组合形式与逆冲强度存在南北差异。通过NE向横向断裂的构造地貌分析,发现横向断裂以右旋走滑活动为主,兼有倾滑活动。根据其与纵向断裂的交接关系,将横向断裂概括为横向分割断裂、横向撕裂断裂和横向转换断裂3种类型,讨论了3类横向断裂在逆冲构造带分段中所起的不同作用,进而将荥经-马边-盐津逆冲构造带分为独立的3段,并分析了各段的地震活动特征。研究表明,荥经-马边-盐津逆冲构造带以横向断裂为标志的3分段特点,既体现了段与段之间断裂活动强度、地震破裂强度与步调的差异,又体现了段内地震破裂步调的一致性,表明横向断裂在一定程度上控制了逆冲构造带的破裂分段,只是横向断裂的类型不同,其所起的作用也不同     

阿尔金构造系晚更新世中晚期以来的逆冲活动

在阿尔金构造系中,阿尔金走滑断裂具有逆冲分量。文中将阿尔金构造系的逆冲活动分为西、中、东3段描述。西段从阿依耐克至车尔臣河河口,阿尔金南缘断裂具有逆冲活动迹象,在山前发育了规模较小的逆冲断层,有较新的地貌面被错动;中段从车尔臣河河口至拉配泉一带,在阿尔金山北缘发育大规模的逆冲断层,有较新的地貌面被错动;东段从拉配泉至宽滩山,逆冲断层有2种形式,此段阿尔金北缘断裂有逆冲分量,同时在阿尔金山北缘及山前冲洪积扇上发育逆冲断裂。自晚更新世中晚期以来,中段及东段逆冲速率<2mm/a。中段西部江尕拉萨依地区自16kaBP以来逆冲速率约为0.33mm/a,中部米兰桥一带自32kaBP以来的逆冲速率约为1.42mm/a。东段最大的逆冲速率在近中部的团结乡,自约5.31kaBP以来达到约1.81mm/a,向东西两端有减小的趋势,在西部柳城子自约72.36kaBP以来的逆冲速率为0.57mm/a,而东端的红柳沟自约8.99kaBP以来仅为0.05mm/a。团结乡地区约自19kaBP以来,逆冲活动有增强的趋势     

Depth migration anisotropy analysis in the time domain

In areas of complex geology such as the Canadian Foothills, the effects of anisotropy are apparent in seismic data and estimation of anisotropic parameters for use in seismic imaging is not a trivial task. Here we explore the applicability of common‐focus point (CFP)‐based velocity analysis to estimate anisotropic parameters for the variably tilted shale thrust sheet in the Canadian Foothills model. To avoid the inherent velocity‐depth ambiguity, we assume that the elastic properties of thrust‐sheet with respect to transverse isotropy symmetry axis are homogeneous, the reflector below the thrust‐sheet is flat, and that the anisotropy is weak. In our CFP approach to velocity analysis, for a poorly imaged reflection point, a traveltime residual is obtained as the time difference between the focusing operator for an assumed subsurface velocity model and the corresponding CFP response obtained from the reflection data. We assume that this residual is due to unknown values for anisotropy, and we perform an iterative linear inversion to obtain new model parameters that minimize the residuals. Migration of the data using parameters obtained from our inversion results in a correctly positioned and better focused reflector below the thrust sheet. For traveltime computation we use a brute force mapping scheme that takes into account weakly tilted transverse isotropy media. For inversion, the problem is set up as a generalized Newton's equation where traveltime error (differential time shift) is linearly dependent on the parameter updates. The iterative updates of parameters are obtained by a least‐squares solution of Newton's equations. The significance of this work lies in its applicability to areas where transverse isotropy layers are heterogeneous laterally, and where transverse isotropy layers are overlain by complex structures that preclude a moveout curve fitting.     

The Banda Sea earthquake of 24 November 1983: evidence for intermediate depth thrust faulting in the Benioff zone

On 24 November 1983, a major earthquake occurred at 180 km depth beneath the Banda Sea. In the focal mechanism solution the pressure axis is almost horizontal, (azimuth 191°, plunge 02°) and the tension axis nearly vertical (plunge 88°). A comparison with the foreshock-aftershock pattern suggests that shear failure took place within the north-north-westerly dipping Benioff zone by thrust faulting along a southerly dipping plane. The focal mechanism solution does not conform to the usual pattern of the tension or compression axis being roughly parallel to the dip of the Benioff zone. Consequently the faulting could not have been caused by down-dip tension or compression within a sinking slab.     

西昆仑东南构造样式及其对增生弧造山作用的意义

西昆仑东南甜水海地区表现为由东北往西南以麻扎-康西瓦冲断系、泉水沟冲断系、甜水海冲断系和南南西边缘盲冲断系而限定数个招皱-逆冲构造带或者褶皱构造带的构造格局构造变形样式总体上以向南逆冲的褶席和逆冲叠瓦扇为特征,以由东北往西南构造变形样式依次出现复式背形堆垛和叠瓦扇组合,到侏罗山式语皱构造带的变化其中用皱样式依次出现大型紧闭平卧卜倒转招皱、中尺度尖棱招皱和具圆筒状转折端的开阔格皱,而断层变化则由断层面产状几乎水平的多重复杂冲精席系到缓倾的顶板冲断层-底板冲断层组合样式到叠瓦扇冲断层．大地构造相分析表明研究区构造变形强度自北东向南西呈递减趋势,与库地一麻扎一带的增生楔杂岩可能组成了复杂的增生楔造山作用的增生楔和前陆招皱冲断带的复杂组合因此,研究区的构造格局并非简单的“塔什库尔干一甜水海地体”,而是复杂的增生弧造山带．     

the faulting characteristics of 2008 wenchuan ms8.0 earthquake and its relation with strong ground motion

The 2008 M_S8 Wenchuan earthquake occurs on a high angle listric thrust fault. It is the first time that the near and far field strong ground motion was observed for such special type thrust earthquake. This paper jointly interprets the distribution of peak acceleration of ground motion data with seismogenic structure and slip propagating process to investigate how high angle listric thrust fault controls the pattern of strong ground motion. We found that the distribution of peak acceleration of strong ground motion during the Wenchuan earthquake has four distinctive features: 1)The peak acceleration of ground motion inside the Longmenshan fault zone is large, that is, nearly twice as strong as that outside the fault zone; 2)This earthquake produces significant vertical ground motion, prevailing against horizontal components in the near field; 3)The far field records show that the peak acceleration is generally higher and attenuates slower versus station-fault distance in the hanging wall. It is doubtful that the attenuation of horizontal components also has the hanging wall effect since no evidence yet proving that the unexpected high value at long distance need be omitted; 4)As to the attenuation in directions parallel to the source fault(Yingxiu-Beichuan Fault), the far field records also exhibit azimuthal heterogeneity that the peak acceleration of horizontal components decreases slower in the north-northeastern direction in which the co-seismic slip propagates than that in the backward way. However, the attenuation of vertical component displays very weak heterogeneity of this kind. Synthetically considered with shallow dislocation, high dip angle, and prevailing vertical deformation during co-seismic process of the Wenchuan earthquake, our near and far field ground motion records reflect the truth that the magnitude of ground motion is principally determined by slip type of earthquake and actual distance between the slipping source patches and stations. As a further interpretation, the uniqueness of high angle listric thrust results in that the ground motion effects of the Wenchuan earthquake are similar to that due to a common thrust earthquake in some components while differ in the others.     

斜俯冲板块边界变形分配的力学分析

本文研究斜俯冲板块边界由俯冲的逆掩断层和平行于火山弧的走滑断层所切割出的岩石层窄条的宏观力学状态．分析计算表明，当断层强度小而构造压应力大时，海沟处板块俯冲形成的逆掩断层地震滑动方向与斜收敛板块运动方向将不一致，震源机制反映的主压应力和断层错动方向变得近乎垂直于海沟；板块间的强耦合和大陆板块边缘软弱带的存在为窄条滑动提供了有利条件．给定俯冲倾角和震源机制滑动角的观测值，可以计算逆掩和走滑断层上的有效摩擦系数．计算结果表明，两种断层上的有效摩擦系数一般都不大于０．２，反映或存在着高孔隙水压，或存在断层泥之类弱物质，或两者兼而有之．     

斜俯冲板块边界变形分配的力学分析

本文研究斜俯冲板块边界由俯冲的逆掩断层和平行于火山弧的走滑断层所切割出的岩石层窄条的宏观力学状态．分析计算表明,当断层强度小而构造压应力大时,海沟处板块俯冲形成的逆掩断层地震滑动方向与斜收敛板块运动方向将不一致,震源机制反映的主压应力和断层错动方向变得近乎垂直于海沟;板块间的强耦合和大陆板块边缘软弱带的存在为窄条滑动提供了有利条件．给定俯冲倾角和震源机制滑动角的观测值,可以计算逆掩和走滑断层上的有效摩擦系数．计算结果表明,两种断层上的有效摩擦系数一般都不大于０．２,反映或存在着高孔隙水压,或存在断层泥之类弱物质,或两者兼而有之．