COLLABORATIVE RESEARCH: Eathquake rupture dynamics on non-planar faults with off-fault damage

地震在破裂前沿附近和几何不规则性附近产生动应力集中，可能会超过周围材料的强度，并在整体中产生非弹性脆性变形。同震离层破坏导致不可逆的变形(塑性)和弹性模数(损伤)的降低，在辐射波场和断裂带结构上具有潜在的不同特征。动态损伤可以放大近断层的运动，并产生双材料界面，从而影响后续的破裂行为。在弹性模数不变的塑性屈服表示断层外耗散的模型中，没有考虑断层外损伤和动态断裂之间的这些反馈机制。此外，天然断层在很大的长度范围内包含几何特征(粗糙度)。断层的多尺度非平面几何结构会增加动态破裂和高频波辐射的复杂性。断层粗糙度还会引起应力集中，这可能会导致动态断层外破坏的产生，并影响整体地震能量平衡。该项目的总体目标是通过理论和计算模拟，定量预测断层外脆性破坏和断层粗糙度(及其反馈)对地震破裂、地震波辐射和断裂带结构短期演化的可观测性质的影响。这项研究直接解决了以下关于地震物理和断层动力学的基本问题：震源周围波速的同震降低的强度和空间范围是什么？断层外的非弹性和断层粗糙度如何影响地震破裂能的表观标度？损伤对最大滑移率和最大地面速度的限制是什么？动态损伤的跨断层不对称性是否会对断裂产生显著的双材料效应？动态脆性损伤能产生更强的波辐射和可观察到的非双耦合地震辐射吗？断层粗糙度如何影响地震破裂的复杂性、损伤的分布和高频辐射的性质？断层的宏观响应与损伤和粗糙度的介观耦合特性有何关系？目前几个小组对地震动力学的研究正在推动前沿，超越弹性介质中平面断层上摩擦滑动的经典模型，以了解更现实和完整的一套因素的作用。涉及的物理过程的多样性和复杂性要求采取一步一步的方法，一次测试几种候选成分。这些研究旨在研究两个基本因素对动态破裂和地震辐射的综合影响：非平面断层几何结构(在很大范围内的断层粗糙度)和断层外材料损伤(弹性模数降低)。其他相关研究目前只考虑了断层几何形状的大尺度、公里特征，并用理想的塑性流变学来表示断层外的非弹性。目前的项目将考虑从米到公里的广泛范围内的断层几何学，并将在断层外屈服过程中包括弹性模数的同震减少。这些结果可以对动态破裂性质的研究、对最大预期地面运动的限制以及对观测地震数据的震源性质的反演产生变革性的影响。该研究可能为约束地震过程和断裂带构造的动态演化提供新的靶向信号。更好地了解动态破裂、高频地震辐射和地面运动的物理极限的性质，将有助于新出现的以物理学为基础的地震工程和减轻地震灾害的方法。在震源过程中纳入材料损伤可对许多基于导出的震源参数(例如，断层滑动和震源机制)的地球物理研究产生重大影响。研究结果将有助于将断层性质和过程从实验室扩展到天然断裂带。虽然我们的重点是地震动力学，但这些研究也可以对固体力学和材料科学界产生影响。

Earthquakes generate dynamic stress concentrations near their rupture front and near geometrical irregularities that can exceed the strength of the surrounding material and produce inelastic brittle deformation in the bulk. Co-seismic off-fault failure induces both irreversible deformation (plasticity) and reduction of elastic moduli (damage) with potentially distinct signatures on the radiated wavefield and fault zone structure. Dynamic damage can amplify near-fault motions and produce bimaterial interfaces that affect the subsequent behavior of rupture. These feedback mechanisms between off-fault damage and dynamic rupture are not accounted for in models that represent off-fault dissipation by plastic yielding with unchanged elastic moduli. Moreover, natural faults contain geometrical features (roughness) over a broad range of length scales. The multiscale non-planar geometry of faults can enhance the complexity of dynamic rupture and high frequency wave radiation. Fault roughness also induces stress concentrations that can contribute to the generation of dynamic off-fault damage and affect the overall earthquake energy balance. The overarching goal of this project is to provide, through theoretical and computational modeling, quantitative predictions of the impact of off-fault brittle damage and fault roughness (and their feedback) on observable properties of earthquake rupture, seismic wave radiation and short-term evolution of fault zone structure. The research addresses directly the following fundamental questions about earthquake physics and fault dynamics: What are the intensity and spatial extent of the coseismic reduction of wave velocities around the earthquake source? How do off-fault inelasticity and fault roughness contribute to the apparent scaling of earthquake fracture energy? What are the limits imposed by damage on maximum slip rate and peak ground velocity? Can the across-fault asymmetry of dynamic damage induce significant bimaterial effects on rupture? Can dynamic brittle damage produce stronger wave radiation and generate observable non-double-couple seismic radiation? How does fault roughness affect the complexity of earthquake rupture, the distribution of damage and the properties of high frequency radiation? How is the macroscopic response of a fault related to the coupled mesoscopic properties of damage and roughness? Current studies of earthquake dynamics by several groups are pushing the frontier beyond the classical model of frictional sliding on a planar fault in elastic media to understand the role of a more realistic and complete set of ingredients. The diversity and complexity of the physical processes involved calls for a step-by-step approach in which a few candidate ingredients are tested at a time. The proposed studies aim to investigate the combined effects on dynamic ruptures and seismic radiation of two fundamental ingredients: non-planar fault geometry (fault roughness on a broad range of scales) and off-fault material damage (reduction of elastic moduli). Other related studies consider at present only large scale, kilometric features of fault geometry, and represent off-fault inelasticity by an ideal plastic rheology. The current project will consider fault geometry over a broad range of scales, from meters to kilometers, and will include co-seismic reduction of elastic moduli in the off-fault yielding process. The results can have transformative impact on studies of properties of dynamic ruptures, limits to the maximum expected ground motion, and inversions of observed seismic data for earthquake source properties. The research may provide new target signals for constraining earthquakes processes and dynamic evolution of fault zone structure. An improved understanding of properties of dynamic ruptures, high frequency seismic radiation and physical limits to ground motion will contribute to emerging physics-based approaches for earthquake engineering and mitigation of seismic hazard. The incorporation of material damage in the source process can have significant impact on many geophysical studies that are based on derived earthquake source parameters (e.g., fault slip and source mechanisms). The results will help to scale fault properties and processes from the laboratory to natural fault zones. Although our focus is on earthquake dynamics, the studies can also have impact on the solid mechanics and material science communities.