Collaborative Research: Integrating Observations of Low-Velocity Fault Zones with Models of Spontaneous Dynamic Earthquake Ruptures

低速断层带（LVFZs）周围的许多活动断层的地震波相对于围岩的速度降低了几个到几十个百分点已被检测到的断层带捕获波和大地测量观测的同震变形。这些LVFZ的机械不同，从周围的地壳岩石，证明了他们的反应附近同震破裂。1992年兰德斯断层带愈合过程的时间逆转，由于附近的同震破裂的1999年赫克托矿地震和净减少至少2.5%的地震速度在圣安德烈亚斯断层带内，由于同震破裂的2004年M6帕克菲尔德地震已被记录。大地测量也观测到了由于附近同震破裂而造成的几个LVFZ的几毫米到几厘米的位移。在以往的研究中使用的震源模型来检查这些观测LVFZ的位错（运动学滑动）模型或点源模型，这不能准确地描述近源动态应力和变形场。本计画利用最先进的自发动态破裂模式，探讨低层自由带对邻近同震破裂的反应。自发动态破裂模型建立在弹性、弹塑性和摩擦定律等物理原理基础上，并为真实的地震观测提供物理见解。有限元法（FEM）代码模拟自发动态破裂沿着几何形状复杂的故障和波在复杂的速度结构的传播已在社区范围内的代码验证工作进行了验证，并在这项研究中使用。最近的修改有限元法，包括增加弹塑性断层外动态破裂的响应，允许PI更准确地计算动态应力场，并更好地检查LVFZ同震破裂的响应。本计画亦探讨在破裂断层周围的低层不稳定带如何影响断层上的自发动态破裂及近场地震动。在考虑弹塑性离断层响应的情况下，PI分析了非均匀材料分布、离断层损伤和自发动力破裂的相互作用以及对近场地震动的影响。该项目将LVFZ的观测与自发动态破裂模型相结合。与运动学模型或点源模型相比，自发动态破裂模型能更准确地定量地描述LVFZ对地震同震破裂的响应，这对提高我们对断裂带力学和断裂系统应力应变传递的理解具有重要意义。LVFZ的观测为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，凝聚力和内部摩擦）。这些观测约束的模型参数可以用来支持其他重要问题的研究，如预测强地面运动在关键设施的网站和地震多发地区。本研究的结果将促进地震科学中观测与理论的结合，并对地震科学中的观测导向和理论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.