Bimaterial Interfaces, Damage Zones, Earthquake Directivities and Signatures of Rupture Opening in the Parkfield Section of the San Andreas Fault

重点研究了Parkfield地区圣安德烈亚斯断层系统各断层段双材料界面、低速断裂带层、地震破裂方向和开放破裂模式的系统高分辨率成像。总体目标是提供基于大型地震波形和目录数据集的多信号测试，以验证大型走滑断层上的地震具有由断层上的速度对比控制的首选传播方向这一假设，以及此类断层上的地震破裂具有断层断裂的拉伸成分的可能性。双材料界面成像采用沿双材料断层折射的断裂带头波和直接P波。断裂带头波为尖锐双材料界面的存在提供了最具诊断性的信号，也是对其地震特性进行深度成像的最高分辨率工具。断裂带捕获波用于量化不同位置的断裂层相对于主破裂面和其他特征的对称性。破裂方向的分析是将每个事件沿相反走向方向的余震频率叠加起来，观察叠加数据在不同时空尺度上的动态触发模式的对称性。对开口破裂模式的检验利用了最近理论计算提出的几种潜在地震信号。研究还重点澄清了SAFOD站点周围的结构和地震性质，澄清了1966年和2004年M6事件震源之间的主SAF性质，并澄清了主SAF与西南断裂带在深度上的连通性。这些研究可以对地震和断层力学的关键方面产生深远的影响，包括有效本构定律、震源和破裂特性、产生的摩擦热、断层之间的相互作用等。这些分析有助于解决现有的争议。关于地震传播方向的统计偏好的积极结果可能导致对主要断层附近地震震动危险性的精确估计。

The studies focus on systematic high-resolution imaging of bimaterial interfaces, low velocity fault zone layers, rupture directivities of earthquakes, and seismic signatures of opening rupture mode along various fault segments of the San Andreas Fault system in the Parkfield area. The overall goal is to provide multi-signal tests, based on large seismic waveform and catalog data sets, for the hypothesis that earthquakes on large strike-slip faults have preferred propagation directions that are controlled by the velocity contrasts across the faults, and for the possibility that earthquake ruptures on such faults have tensile component of faulting. The imaging of bimaterial interfaces employ fault zone head waves that refract along bimaterial faults and direct P waves. The fault zone head waves provide the most diagnostic signal for the existence of sharp bimaterial interfaces and the highest-resolution tool for imaging their seismic properties at depth. Fault zone trapped waves are used to quantify symmetry properties with respect to the main rupture surface and other characteristics of damaged fault zone layers at different locations. The analysis of rupture directivities involves stacking aftershock rates at the opposite along-strike directions of each event and observing symmetry properties of dynamic triggering patterns of the stacked data at different space-time scales. The examination for opening rupture mode utilizes several potential seismic signals that are suggested by recent theoretical calculations. The studies also focus on clarifying the structural and earthquake properties around the SAFOD site, clarifying properties of the main SAF between the hypocenters of the 1966 and 2004 M6 events, and clarifying the connectivity between the main SAF and the Southwest Fracture zone at depth. The studies can have profound implications for key aspects of earthquake and fault mechanics including effective constitutive laws, earthquake source and rupture properties, generated frictional heat, interaction between faults and more. The analyses can help resolving existing controversies. Positive results on statistical preference of earthquake propagation directions may lead to refined estimates of seismic shaking hazard near major faults.