Elastic Contrast, Rupture Directivity, and Damage Asymmetry in an Anisotropic Bimaterial Strike‐Slip Fault at Middle Crustal Depths
Elastic Contrast, Rupture Directivity, and Damage Asymmetry in an Anisotropic Bimaterial Strike‐Slip Fault at Middle Crustal Depths
复制标题
DOI：
10.1029/2021jb023821
复制
发表时间：
2022-06
期刊：
影响因子：
--
通讯作者：
B. Song;W. Song;S. E. Johnson;C. Gerbi;S. Vel
中科院分区：
文献类型：
--
作者：
B. Song;W. Song;S. E. Johnson;C. Gerbi;S. Vel
具有大累积滑移的成熟断层通常会分离具有不同弹性特性的岩石，并表现出不对称的损伤分布。这种双材料断层的弹性对比可以显着改变地震破裂动力学的各个方面，包括正应力变化、破裂传播方向、地面运动分布和断层损伤的演变。因此，分析双材料断层的弹性对比对于理解地震物理和相关的潜在危险非常重要。各向同性材料之间的弹性对比对破裂动力学的影响已得到相对充分的研究。然而，大多数断层岩石是弹性各向异性的，并且人们对各向异性如何影响破裂动力学知之甚少。我们研究了 Sandhill Corner 剪切带的微观结构，该剪切带将石英长石岩和云母片岩分开，损伤区分别较宽和较窄。该剪切带是诺伦贝加断层系统的一部分，诺伦贝加断层系统是从中地壳深处挖掘出来的古生代、大位移、发震、走滑断层系统。我们计算了具有不同比例的云母颗粒与断层平行排列的每个单元的弹性各向异性岩石的弹性特性和地震波速度。我们的研究结果表明，由于断层法向顺应性，平行于双材料断层（具有断层法向粒子运动）传播的水平极化剪切波是最慢的，因此对于确定影响各向异性介质中破裂动力学的弹性对比度可能很重要。根据各向同性介质中亚剪切破裂传播模型的结果，我们的结果与优先沿片岩滑移方向传播的破裂一致，片岩具有较慢的水平剪切波和较大的断层法向柔量。
Mature faults with large cumulative slip often separate rocks with dissimilar elastic properties and show asymmetric damage distribution. Elastic contrast across such bimaterial faults can significantly modify various aspects of earthquake rupture dynamics, including normal stress variations, rupture propagation direction, distribution of ground motions, and evolution of off‐fault damage. Thus, analyzing elastic contrasts of bimaterial faults is important for understanding earthquake physics and related hazard potential. The effect of elastic contrast between isotropic materials on rupture dynamics is relatively well studied. However, most fault rocks are elastically anisotropic, and little is known about how the anisotropy affects rupture dynamics. We examine microstructures of the Sandhill Corner shear zone, which separates quartzofeldspathic rock and micaceous schist with wider and narrower damage zones, respectively. This shear zone is part of the Norumbega fault system, a Paleozoic, large‐displacement, seismogenic, strike‐slip fault system exhumed from middle crustal depths. We calculate elastic properties and seismic wave speeds of elastically anisotropic rocks from each unit having different proportions of mica grains aligned sub‐parallel to the fault. Our findings show that the horizontally polarized shear wave propagating parallel to the bimaterial fault (with fault‐normal particle motion) is the slowest owing to the fault‐normal compliance and therefore may be important in determining the elastic contrast that affects rupture dynamics in anisotropic media. Following results from subshear rupture propagation models in isotropic media, our results are consistent with ruptures preferentially propagated in the slip direction of the schist, which has the slower horizontal shear wave and larger fault‐normal compliance.