Bends and Ends of Surface Ruptures by-猫眼课题宝

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2017

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G. Biasi;S. Wesnousky

为了改进在已绘制的活动断层上估计未来地震破裂可能长度的经验基础，我们测量了67个历史断层的地图尺度复杂性，包括断层弯曲、不连续破裂、重叠和断层间破裂，并分析了与地震危险性分析相关的统计关系。我们观察到，走滑断层表面破裂末端的弯曲角度系统地大于内部弯曲（IBs），而相应的内部和末端种群在倾滑事件中是相似的。走滑破裂通过弯道的概率随弯道角的增大而系统减小，大致为PR 3:1 ~ 0:0 . 83 × a， PR为通过比，a为弯道角，取值范围为5°~ 30°。回归表明走滑破裂通过25°弯曲传播的可能性约为50%。断裂传播的最大IB角，以及断裂末端断层段的净方向差异，可以用局部恒定的区域应力方向下断层走向变化引起的摩擦阻力变化来解释。断层破裂的平均曲率是用断裂中弯曲的绝对值之和除以断裂长度来定义的。走滑破裂的中位曲率为0:5°=km， 95%曲率为1:5°=km；倾滑破裂对应值分别为1:6°=km和5:6°=km。我们发现，大多数断层到断层的破裂连接跳转到类似机制的断层，如走滑到走滑。在总共42次走滑破裂中，只有2次向构造反转，并持续了相当长的距离。本文的研究结果为断层破裂动力学研究提供了经验数据，并改进了用于地震危险性评估的断裂长度估计。电子补充：破裂图注释显示解释线性段和测量弯曲在破裂。概率地震危险性分析（PSHA）直接依赖于对活动断层上地震的频率和大小的估计。在PSHA中，通常的做法是根据断层的几何形状和长度，在绘制的活动断层上开发可能发生的潜在破裂的集合。在危险分析中越来越多地考虑到滑跃跨越不连续的破裂，这些不连续是沿着绘制的断层迹出现的步骤。因此，PSHA要求对未来通过这些不连续面破裂的可能性分配概率。这激发了人们对获得进一步观察和开发方法来定义和改进这些概率的兴趣。以往的研究侧重于断层道中的不连续面，这些不连续面在地图视图中表现为步骤（例如，Wesnousky, 1988, 2006; Lettis et al., 2002； Biasi和Wesnousky, 2016；下文为BW16）；在这里，我们从过去地震破裂处的弯曲处得到了类似的经验观察。利用经验观察表明，断层轨迹中的弯曲在断层破裂中起作用并不是新的。例如，King和Nabelek（1985）认为断层轨迹中的弯曲可能与地震破裂的开始或结束有关。Fliss等人（2005）和Bhat等人（2007）使用物理模型探索了在断层轨迹中导致分支的弯曲可能反映破裂方向性的可能性。安藤等人（2009）研究了加利福尼亚的圣安德烈亚斯断层系统，观察到从断层轨迹弯曲的裂缝角度最常见的方向约为17°。由此，他们认为断层尖端应力可能在裂缝形成中起作用。克林格（2010）考虑了两者的混合*也是在内华达州里诺的内华达大学地震实验室，MS-174，内华达州里诺89557。2543美国地震学会通报，2017年12月，第107卷，第6期，pp. 2543 - 2560, doi: 10.1785/0120160292

To improve the empirical basis for estimating the likely length of future earthquake ruptures on mapped active faults, we measure map-scale complexities including fault bends, discontinuous rupture, overlaps, and fault-to-fault rupture from 67 historical ruptures and analyze the measurements for statistical relationships relevant to seismic hazard analysis. We observe that angles of bends at the ends of surface ruptures on strike-slip faults are systematically larger than interior bends (IBs), whereas corresponding interior and ending populations are similar for dip-slip events. The probability of a strike-slip rupture passing a bend decreases systematically with increasing bend angle roughly as PR 3:1 − 0:083 × A, in which PR is the passing ratio and A is the bend angle, with values ranging between 5° and 30°. The regression shows the likelihood of a strike-slip rupture propagating through a bend of 25° is about 50%. The maximum IB angles through which ruptures propagate, and the net orientation differences of fault segments at the end of ruptures, may be explained to first order by changes in frictional resistance due to changes in fault strike in a locally constant orientation of regional stress. The average curvature of a fault rupture is defined by dividing the sum of absolute values of bends in the rupture by rupture length. Median and 95% curvatures of strike-slip ruptures are 0:5°=km and 1:5°=km, respectively; corresponding values for dip-slip ruptures are 1:6°=km and 5:6°=km, respectively. We find that most fault-to-fault rupture connections jump to a fault of like mechanism, such as strike slip to strike slip. Only two strike-slip ruptures out of a total of 42 jump to reverse structures and continue for a significant distance. Results here provide empirical data to support study of the dynamics of fault rupture and to improve rupture-length estimates for use in seismic hazard assessment. Electronic Supplement: Rupture maps annotated to show interpreted linear segments and measurements of bends in the ruptures. Introduction Probabilistic seismic hazard analysis (PSHA) depends directly on estimates of the rate and size of earthquakes on active faults. It is common practice in PSHA to develop an ensemble of potential ruptures that may occur on mapped active faults, based on fault geometry and length. Ruptures where slip jumps across discontinuities that appear as steps along a mapped fault trace are increasingly being considered in hazard analysis. PSHA thus requires assigning probabilities to the likelihood of a future rupture passing through these discontinuities. This motivates interest in acquiring further observations and developing methods to define and refine these probabilities. Previous studies focused on discontinuities in fault trace that appear as steps in a map view (e.g. Wesnousky, 1988, 2006; Lettis et al., 2002; Biasi and Wesnousky, 2016; hereafter, BW16); we here develop similar empirical observations from bends in past earthquake ruptures. The use of empirical observations to suggest that bends in fault trace play a role in fault rupture is not new. For example, King and Nabelek (1985) suggested bends in a fault trace might correlate with the initiation or termination of earthquake ruptures. Fliss et al. (2005) and Bhat et al. (2007) used physical models to explore the possibility that bends leading to branches in a fault trace might reflect rupture directivity. Ando et al. (2009) examined the San Andreas fault system of California to observe that the angles of splays bending from a fault trace are most commonly oriented about 17°. From this, they suggest that fault-tip stress may play a role in splay formation. Klinger (2010) considered a mix of both *Also at University of Nevada, Reno, Nevada Seismological Laboratory, MS-174, Reno, Nevada 89557. 2543 Bulletin of the Seismological Society of America, Vol. 107, No. 6, pp. 2543–2560, December 2017, doi: 10.1785/0120160292

Bends and Ends of Surface Ruptures by

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