Rupture Propagation and Arrest in Geometrically Complex Fault Systems: Branches, Stepovers, and Damaged Border Zones

项目组重点研究地震科学中的一个重大问题，即了解地震滑裂与断裂带几何和结构复杂性的相互作用。这种相互作用包括断层链在弯曲、分支和阶梯处的破坏路径转换，断裂停止，以及断层边缘地带的非弹性变形，这些区域通常是损坏的（高度开裂和/或颗粒化）和流体饱和的。该小组先前的工作，即当前研究的基础，提供了新的理解，即在枝状几何复杂性中如何选择破裂路径，以及断层边缘带的非弹性响应如何影响破裂传播和剪切局部化。目前研究的理论和模型研究的新领域有：(1)了解与地流体的变形和损伤区摩擦弹塑性响应的相互作用如何与破裂传播动力学耦合。这包括解释不同类型断层间材料差异（弹性特性、强度和破坏程度以及断层附近流体渗透率）的影响；这种差异在成熟的、高度滑动的断层中很常见。(2)评估当前对断裂路径如何在分支交叉点选择，以及断裂是否通过或在台阶处停止的理解是否以及在何种程度上受到断裂连接点附近具有弹塑性响应能力的大面积损坏材料的影响。(3)确定先前破裂在断层边界材料中留下的残余应力状态如何影响下一次事件的响应，以及这如何取决于过去和未决事件的破裂方向性；(4)设计程序，严格分析受损/颗粒状断层边界带非弹性响应建模中出现的应变局域化，通过施加局域化限制程序，消除网格依赖性，从而最终发展出一种方法，可以预测局部断层破裂路径通过受损材料的自发发展。理论和建模与现场实例和实验室实验的相关性是该小组工作的一个标志，该方向的新重点如下：(1)采用上文(4)所述的方法来理解一个受损的拉分阶梯，如1992年约翰逊和霍姆斯特德山谷断层之间的兰德斯地震，以及霍姆斯特德山谷和爱默生断层之间的地震，是什么时候被一个完整的破裂所破坏的，同样地，1920年中国海原M8事件也是通过一系列的拉分而破裂的。（II）了解通过增生俯冲带沉积物覆盖的大冲断断裂到展断层上的巨型分支，就像在阿拉斯加、卡斯卡迪亚、南开和苏门答腊岛记录或怀疑的那样，以及何时以及通过什么过程分支到向陆或向海的展断层可能发生，以及这对海啸的产生意味着什么。（三）根据旨在解决相同问题的实验室实验（由其他地方的同事进行）的结果，测试对破裂分支和与受损边界区域相互作用的不断发展的理论理解。了解地震破裂何时以及如何停止，这通常涉及我们所讨论的几何复杂性，是了解地震风险的核心。利用残余断层几何来约束过去事件的指向性和其他特征的新方法也是一个潜在的有价值的成果。

The project group focuses on a major problem in earthquake science, namely, to understand the interaction of seismic slip-rupture with geometrical and structural complexities of fault zones. Such interactions include transitions of the failure path among fault strands at bends, branches and stepovers, rupture arrest, and induced inelastic deformations in fault border zones, which are generally damaged (highly cracked and/or granulated) and fluid-saturated. Prior work of the group, on which the current studies build, provided new understanding of how rupture paths are chosen at branch-like geometric complexities, and of how inelastic response of the fault bordering zone affects rupture propagation and shear localizations. The new areas for theory and modeling in current work are as follows: (1) Understanding how interactions of deformations with ground fluids and frictional elastic-plastic responses in the damage zone couple to the dynamics of rupture propagation. That includes explaining the effects of different types of across-fault material dissimilarity (in elastic properties, strength and extent of damage, and near-fault permeability to fluids); such dissimilarities are common for mature, highly slipped faults. (2) Assessing if, and to what extent, current understanding of how rupture paths are chosen at branch intersections, and of whether rupture passes through or arrests at step-overs, is affected by the presence of extensively damaged material, capable of elastic-plastic response, near such fault junctions. (3) Determining how residual stress states imprinted in fault-border material by the previous rupture affects response in the next event, and how that depends on rupture directivity in the past and pending events; (4) Devising procedures to rigorously analyze strain localizations that arise in modeling inelastic response of damaged/granulated fault border zones, by imposing localization-limiting procedures that eliminate grid dependence, thus ultimately evolving a methodology that can predict spontaneous development of localized fault-rupture paths through damaged material. Correlation of theory and modeling with field examples and lab experiments is a hallmark of the group's work, and new thrusts in that direction are as follows: (I) Adopting methodology like in (4) above to understanding when a damaged pull-apart stepover, like in the 1992 Landers earthquake between the Johnson and Homestead Valley Faults, and between the Homestead Valley and the Emerson Fault, is breeched by a through-going rupture, and similarly for the 1920 M8 Haiyuan, China event, which ruptured through a sequence of pull-aparts. (II) Understanding mega-branches of great thrust ruptures onto splay faults through the sediment cover of accretionary subduction zones, like documented or suspected at Alaska, Cascadia, Nankai and Sumatra, as well as when and by what processes branching onto landward- versus seaward-vergent splays can occur, and what that means for tsunami generation. (III) Testing the evolving theoretical understanding of rupture branching and interactions with damaged border zones against results of lab experiments (conducted by colleagues elsewhere) which are devised to address the same issues. The understanding of when and how earthquake ruptures stop, which often involves geometric complexities of the type we address, is central to understanding seismic risk. New ways of using relic fault geometries to constrain directivity and other features of past events is also a potentially valuable outcome.