Collaborative Research: Integrating Observations of Low-Velocity Fault Zones With Models of Spontaneous Dynamic Earthquake Rupture

通过断层陷波和同震形变的大地测量，发现了许多活动断裂周围的低速断裂带，其地震波速度相对于围岩的减速幅度在几十%到几十%之间。这些低地震区与周围的地壳岩石在力学上是不同的，这从它们对附近同震破裂的反应中得到了证明。由于1999年赫克托雷地震附近的同震破裂，1992年兰德斯断裂带的愈合过程的时间反转，以及由于2004年帕克菲尔德6级地震的同震破裂，圣安德烈亚斯断裂带内的地震速度净下降至少2.5%。大地测量也观察到，由于附近的同震破裂，在几个LVFZ上发生了几毫米到几厘米的位移。以往研究中用于检验低位移带观测结果的震源模型要么是位错(运动滑移)模型，要么是点源模型，它们不能准确地描述近源动态应力场和变形场。该项目利用最先进的自发性动态破裂模型来研究低空腹地带对附近同震破裂的响应。自发动态破裂模型建立在物理原理的基础上，如弹性、弹塑性和摩擦定律，并为真实地震的观测提供物理见解。用于模拟几何复杂断层上的自发动态破裂和复杂速度结构中的波传播的有限元程序已在社区范围内的程序验证工作中得到验证，并被用于本研究。最近对有限元的修改，包括增加了对动态破裂的弹塑性脱断层响应，使得PI能够更准确地计算动态应力场，并更好地检查LVFZ对同震破裂的响应。该项目还研究了破裂断层周围的低频带如何影响断层上的自发动态破裂和近场地面运动。将弹塑性断层外响应包含在自发性动态破裂模型中，PI检验了非均匀材料分布、断层外损伤和自发性动态破裂的相互作用以及对近场地面运动的影响。该项目结合了对低空泡沫区的观测和自然动态破裂模型。与运动学或点源模型相比，自发动态破裂模型能够更准确地定量地描述低空夹层带对地震同震破裂的响应，这对于提高我们对断裂带力学和断层系统中应力应变传递的认识具有重要意义。观测到的LVFZ对动态破裂模型中的参数提供了约束，包括LVFZ的速度结构和材料强度(即内聚力和内耗)。这些受观测约束的模型参数可用于支持对其他重要问题的研究，例如预测关键设施场地和地震多发地区的强烈地面运动。这项研究的成果有望促进地震科学中观测与理论的结合，并对地震科学界以观测为导向和以理论为导向的社区产生影响。

Low-velocity fault zones (LVFZs) around many active faults with a reduction in seismic wave velocities of several to several tens percent relative to wall rocks have been detected by fault zone trapped waves and geodetic observations of the coseismic deformation. These LVFZs are mechanically distinct from the ambient crustal rock, as evidenced by their responses to nearby coseismic ruptures. A temporal reversal of the healing process of the 1992 Landers fault zone due to the nearby coseismic rupture of the 1999 Hector Mine earthquake and a net decrease in seismic velocities of at least 2.5% within the San Andreas fault zone due to the coseismic rupture of the 2004 M6 Parkfield earthquake have been documented. Displacements of several millimeters to several centimeters across several LVFZs due to nearby coseismic ruptures have also been observed by geodetic surveys. Earthquake source models used in previous studies to examine these observations of LVFZs are either dislocation (kinematic slip) models or point source models, which cannot accurately characterize the near-source dynamic stress and deformation fields. This project utilizes the most advanced spontaneous dynamic rupture model to investigate responses of LVFZs to nearby coseismic ruptures. Spontaneous dynamic rupture models build upon physical principles such as elasticity, elastoplasticity, and friction laws and provide physical insights to observations from real earthquakes. A finite element method (FEM) code for simulating spontaneous dynamic ruptures along geometrically complex faults and wave propagation in complex velocity structures has been verified in a community-wide code validation effort and is used in this study. Recent modifications to the FEM, including addition of elastoplastic off-fault response to dynamic ruptures, allow the PIs to more accurately calculate dynamic stress field and to better examine responses of LVFZs to coseismic ruptures. This project also investigates how a LVFZ surrounding a rupturing fault affects spontaneous dynamic rupture on the fault and near-field ground motion. With elastoplastic off-fault response included in spontaneous dynamic rupture models, the PIs examine interactions of inhomogeneous material distribution, off-fault damage and spontaneous dynamic rupture and effects on near-field ground motion. This project integrates observations of LVFZs with models of spontaneous dynamic ruptures. Compared with kinematic or point source models, spontaneous dynamic rupture models provide more accurate quantification of how LVFZs respond to earthquake coseismic ruptures, which has significant implications for improving our understanding of fault zone mechanics and stress and strain transfer in fault systems. Observations of LVFZs provide constraints to parameters in dynamic rupture models, including velocity structure of LVFZs and material strength (i.e., cohesion and internal friction). These observation-constrained model parameters can be used to support studies of other significant questions, such as prediction of strong ground motion at critical facility sites and in earthquake-prone areas. Results from this study are expected to promote the integration of observation and theory in earthquake sciences, and to impact both observation-oriented and theory-oriented communities of earthquake sciences.