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

地震在其破裂前沿和几何不规则附近产生动应力集中，这些应力集中可能超过周围材料的强度，并在整体中产生非弹性脆性变形。同震断层外破坏引起不可逆变形（塑性）和弹性模量降低（损伤），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运动，并产生影响破裂后续行为的双材料界面。在弹性模量不变的非故障塑性屈服耗散模型中，未考虑非故障损伤与动态破裂之间的反馈机制。此外，天然断层在很宽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特征增加了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，从而导致动态断层破坏的产生，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建模，定量预测断层外脆性损伤和断层粗糙度（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特性的影响。该研究直接解决了以下关于地震物理学和断层动力学的基本问题：震源周围波速同震衰减的强度和空间范围是什么？断层外非弹性和断层粗糙度对地震断裂能的明显标度有何影响？损坏对最大滑移率和峰值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破裂产生显著的双材料效应？动态脆性损伤是否能产生更强的波辐射并产生可观测的非双偶地震辐射？断层粗糙度如何影响地震破裂的复杂性、损伤分布和高频辐射的性质？断裂的宏观响应如何与损伤和粗糙度的耦合介观特性相关？目前，一些研究小组对地震动力学的研究正在超越弹性介质中平面断层摩擦滑动的经典模型，以了解更现实和完整的一套成分的作用。所涉及的物理过程的多样性和复杂性要求采取循序渐进的方法，一次测试几种候选成分。本研究旨在探讨两种基本因素对动态破裂和地震辐射的综合影响：非平面断层几何形状（大范围尺度上的断层粗糙度）和断层外物质损伤（弹性模量的减少）。其他相关研究目前只考虑断层几何的大尺度、千米特征，用理想塑性流变学来表示断层外非弹性。目前的项目将考虑从米到公里的广泛尺度的断层几何形状，并将包括断层外屈服过程中弹性模量的同震减少。研究结果可以对动态破裂特性的研究、最大预期地面运动的限制以及对震源特性的观测地震数据的反演产生变革性影响。该研究可为约束地震过程和断裂带结构动力演化提供新的目标信号。对动态破裂特性、高频地震辐射和地面运动物理极限的更好理解将有助于地震工程和减轻地震危害的基于物理的新方法的出现。在震源过程中纳入物质破坏可能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。结果将有助于从实验室到自然断裂带的断层性质和过程的规模。虽然我们的研究重点是地震动力学，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.