Mechanical Erosion of Frictionally Locked Fault Patches Due to Creep: ObservationalEvidence and Modeling

这项研究的目的是增加我们对地震是如何在摩擦闭锁的断层块上孕育地震的理解，这些断层块是由无地震蠕变导致的边界上日益增长的应力集中所加载的。我们将从分析观测到的地震活动模式开始，这些地点可能发生蠕变介导的机械侵蚀：部分蠕动断层上的(一些)微震条纹，以及主要走滑断层的孕震带的底部。条纹是紧密聚集的小地震的近乎水平的条带，首先在加利福尼亚州北部的爬行断层上大量观察到，那个邻居显然是无地震的洞，可能是被摩擦锁定的，或者是无震的蠕动的。我们将分析条纹上的地震活动模式，以寻找可能暴露出邻近锁定斑块的逐渐机械侵蚀的变化。这些变化可能包括加速地震活动，增加瞬间释放率，或增加重复地震的震级或频率。通过将条纹上的地震活动模式与邻近的洞是否存在中等地震(即很可能是锁定的)相关联，锁定的洞可能(在统计上)是可识别的。以前曾用锁定斑块的机械侵蚀来解释基拉韦厄-S东裂谷走滑带上地震活动的加速和最大震级的增加，并可能通过从孕震带下方蠕动而对主要锁定走滑断层的加载起到作用。因此，搜索范围将扩大到加利福尼亚州南部地壳规模的走滑断层底部有希望的地区。这些观察结果将与数值模型进行比较，这些模型旨在增加我们对地震成核的理解，这些模型位于被外部蠕变(速度增强环境)机械侵蚀的卡住(速度减弱)凹凸体的边界上，在具有速率和状态摩擦的断层上。地震如何成核仍然是地震学中一个尚未解决的主要问题。考虑到目前被认为描述断层摩擦的方程中的不确定性，地震成核的数值模型必须不断地与观测对峙。一个标准的概念性模型是，许多地震是由周围断裂区缓慢的、无地震的滑动引起的，这些断裂区逐渐加载被摩擦粘住的断裂区，直到最终发生大地震。例如，一个大型的、锁定的、垂直走滑的断层预计会受到逐渐增加的应力，因为该断层上的无震滑动-S深伸展，由于其较高的温度，直接响应于板块运动而缓慢滑动。简单的力学模型预测，由于在粘滞滑动区和抗震滑动区之间的过渡处应力越来越大，微震活动应该通过随着时间的推移变得更强(例如，速率增加，震级增加)来反映渐进加载，直到大地震释放积累的能量。目标是寻找可能支持这一模型及其预测的观测数据，同时根据当前的摩擦定律与观测数据进行地震周期的数值模拟。已开发的表征地震活动模式的方法揭示了闭锁断层斑块的机械侵蚀，这可能有助于识别那些即将结束地震周期的走滑断层，因此在大地震中更有可能破裂。这一假说最终可能对改进破坏性地震的预报做出有意义的贡献。

The aim of this study is to increase our understanding of how earthquakes nucleate on frictionally locked fault patches that are loaded by the growing stress concentrations at their boundaries due to aseismic creep. We will begin with an analysis of observed seismicity patterns at locations where creep-mediated mechanical erosion is likely to be occurring: on (some) streaks of microearthquakes on partially creeping faults, and at the base of the seismogenic zone of major strike-slip faults. Streaks are near-horizontal ribbons of tightly clustered small earthquakes, first observed in large numbers on northern Californias creeping faults, that neighbor apparently aseismic holes that might be frictionally locked or aseismically creeping. We will analyze the seismicity patterns on streaks to search for changes that might betray the gradual mechanical erosion of neighboring locked patches. Such changes might include accelerating seismicity, increased moment release rates or increases in the magnitudes or frequencies of repeating earthquakes on streaks. By correlating seismicity patterns on the streaks with whether or not neighboring holes have hosted moderate earthquakes (i.e., are probably locked), locked holes might be (statistically) identifiable. Mechanical erosion of locked patches has previously been invoked to explain accelerating seismicity and increases in maximum earthquake magnitude on a strike-slip streak in Kilauea?s East rift, and might also play a role in the loading of major locked strike-slip faults by creep from below the seismogenic zone. The search will therefore be extended to promising regions at the base of crustal-scale strike-slip faults in southern California. These observations will be compared to numerical models designed to increase our understanding of earthquake nucleation on the boundaries of stuck (velocity-weakening) asperities that are being mechanically eroded by external creep (velocity strengthening surroundings), on faults endowed with rate-and-state friction.How earthquakes nucleate remains a major unsolved problem in seismology. Given the uncertainty in the current equations that are presumed to describe friction on faults, it is essential that numerical models of earthquake nucleation be continually confronted by observations. A standard conceptual model is that many earthquakes are caused by slow, aseismic sliding of the surrounding fault area, which progressively loads fault regions that are frictionally stuck until eventually a large earthquake occurs. For example, a large, locked, vertical strike-slip fault is expected to experience progressively increasing stresses because of aseismic sliding on the fault?s deep extension, which because of its higher temperature slides slowly in direct response to plate motion. Simple mechanical models predict that, because of the growing stresses at the transition between the stuck and aseismically sliding regions, micro-seismicity should mirror the progressive loading by becoming stronger over time (e.g., rates increase, magnitudes grow), until a large earthquake releases the built-up energy. The goal is to search for observations that might support this model and its predictions, while at the same time confronting numerical simulations of earthquake cycles based on current laws of friction with the observations. The developed methods to characterize seismicity patterns that reveal such mechanical erosion of locked fault patches might help in identifying those strike-slip faults that are nearing the end of their earthquake cycle and that are therefore more likely to rupture in large earthquakes than others. This hypothesis could eventually contribute meaningfully to improved forecasts of damaging earthquakes.