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

蠕变引起的摩擦锁定断层块的机械侵蚀：观测证据和建模

• 批准号: 1214900
• 负责人: Allan Rubin
• 金额: $ 20.06万
• 依托单位: Princeton University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2012
• 资助国家: 美国
• 起止时间: 2012-10-01 至 2014-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1214900&HistoricalAwards=false
• 关键词: Mechanical; Erosion; Frictionally; Locked; Fault

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.

这项研究的目的是增加我们对地震如何在闭锁的断层块上成核的理解，这些断层块由于无源蠕变而在其边界处受到不断增长的应力集中的加载。我们将开始分析在蠕动介导的机械侵蚀可能发生的位置观测到的地震活动模式：在部分蠕动断层上的（一些）微地震条纹上，以及在主要走滑断层的孕震区底部。条纹是紧密聚集的小地震的近水平带状物，首先在加利福尼亚州北方蠕动断层上大量观察到，这些断层邻近可能是蠕动锁定或抗震蠕动的明显洞穴。我们将分析条纹上的地震活动模式，以寻找可能暴露出相邻锁定斑块逐渐机械侵蚀的变化。这种变化可能包括地震活动加速、力矩释放率增加或条纹上重复地震的震级或频率增加。通过将条纹上的地震活动模式与相邻孔是否发生过中等地震（即，可能是锁定的），锁定的孔可能是（统计学上）可识别的。锁定补丁的机械侵蚀以前被用来解释加速地震活动和增加最大地震震级的走滑条纹在基拉韦厄？的东裂谷，也可能发挥作用，在加载的主要锁定走滑断层从下面的孕震区蠕变。因此，研究将扩展到加州南部地壳规模的走滑断层底部的有希望的地区。这些观测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，这些地震成核发生在被外部蠕变（速度增强环境）机械侵蚀的粘滞（速度减弱）粗糙体的边界上，发生在具有速率和状态摩擦的断层上。由于目前假定用来描述断层摩擦力的方程的不确定性，地震成核的数值模型必须不断地面对观测结果。一个标准的概念模型是，许多地震是由周围断层区域缓慢的、不稳定的滑动引起的，这种滑动逐渐加载被卡住的断层区域，直到最终发生大地震。例如，一个大的，锁定的，垂直的走滑断层预计将经历逐步增加的应力，因为在断层上的滑动？由于温度较高，它直接响应板块运动而缓慢滑动。简单的力学模型预测，由于在卡住和抗震滑动区域之间的过渡处的应力不断增加，微震活动应该随着时间的推移而变得更强，从而反映出渐进的载荷（例如，速率增加，震级增加），直到大地震释放出积累的能量。我们的目标是寻找可能支持这个模型及其预测的观测结果，同时面对基于当前摩擦定律的地震周期数值模拟。开发的方法来描述地震活动模式，揭示这种机械侵蚀锁定故障补丁可能有助于识别那些走滑断层接近其地震周期的结束，因此更有可能在大地震比其他破裂。这一假设最终可能有助于改善破坏性地震的预测。