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.

这项研究的目的是提高我们对地震如何在摩擦锁定断层斑块上成核，这些断层斑块是由于无性蠕变而在其边界上增长的压力浓度所加载的。我们将从对蠕变介导的机械侵蚀的位置上观察到的地震性模式进行分析：在部分蠕变断层上的微夸氏Quake的（某些）条纹上，以及在重大滑移断层的地震生成区的基础上。条纹是几乎紧密聚集的小地震的近端丝带，首先在北加州北部爬行的断层中大量观察到，邻居显然可能在摩擦上锁定或在无线上爬行。我们将分析条纹上的地震性模式，以寻找可能背叛相邻锁定贴片的机械侵蚀的变化。这样的变化可能包括加速地震性，增加的力矩释放速率或在条纹上重复地震的大小或频率增加。通过将条纹上的地震性模式与相邻的孔相关联（即可能是锁定的），锁定的孔可能（统计上）可识别。以前已经调用过锁定斑块的机械侵蚀来解释地震性的加速性，并在基拉韦阿的西方裂痕的走滑条纹上增加了地震幅度的增加，并且也可能在从下方地下抗震区下方通过蠕变加载大的锁定式滑滑断层。因此，搜索将扩展到南加州地壳尺度滑滑断层底部的有前途的地区。这些观察结果将与旨在提高我们对地震成核的理解的数字模型进行比较，这些观察因处于蠕变（速度增强的速度）的尖锐性（速度 - 效率）的垂直度（速度加强周围环境），对质量和状态的缺陷。鉴于当前方程中描述断层摩擦的不确定性，至关重要的是，地震成核的数值模型必须与观测值不断面对。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，无性滑动引起的，该区域逐渐加载了摩擦下的断层区域，直到最终发生大地震。例如，由于在断层的深延伸范围上滑动的大型，锁定的垂直滑移断层有望逐渐增加压力，因为它的温度较高，因此在直接响应板运动时，其温度较高。简单的机械模型预测，由于卡住和亚洲滑动区域之间过渡的应力增长，微观性应该通过随着时间的推移变得更强大（例如，速率增加，大小增长）来反映渐进式载荷，直到大地震发出构建能量。目的是搜索可能支持该模型及其预测的观察结果，同时基于当前的摩擦定律与观测值面对地震周期的数值模拟。开发的表征地震性模式的方法揭示了锁定断层贴片的机械侵蚀，这可能有助于识别那些接近地震循环结束的滑移断层，因此比其他地震中更有可能在大地震中破裂。该假设最终可能有意义地改善了破坏性地震的预测。