An Observational Study of Microearthquakes on a Bimaterial Interface

PI已经积累了大量使用波形互相关的精确地震位置目录。 以前的工作与此目录表明，余震的小地震沿着圣安德烈亚斯断层附近的圣胡安鲍蒂斯塔分布非常不对称。 在主震边缘的1-2个半径范围内，西北方向的余震数量几乎是东南方向的3倍。 相反，在主震之上或之下的余震数量上，或者沿着沿着卡拉韦拉斯断层的余震数量上，没有明显的不对称性，卡拉韦拉斯断层没有明显的跨断层速度对比。 这些观测结果与与圣安德烈亚斯地震速度对比相关的不对称性一致，该地区的地震速度可能达到35%甚至50%。 根据先前的NSF资助，在双材料界面上进行的弹性动力学破裂的数值研究表明，由滑动弱化摩擦引起的破裂会以轻微不对称的裂纹形式增长。 值得注意的是，有一个大的减少，正应力立即后方的破裂前移动（在圣安德烈亚斯的背景下）向东南方向。 当这个破裂前缘被应力障减缓时，张应力继续向前，并且在很宽的参数范围内，张应力携带着一个垂死的滑移脉冲。滑移停止后，破裂的东南边缘远低于破坏阈值，因为携带滑移脉冲的张应力消失了。 与此相反，西北破裂锋逐渐减慢，并停止，因为它会在一个均匀的介质，与破裂前仍然非常接近的故障阈值后，滑动已经停止。 这为余震的不对称性提供了一个有吸引力的解释。目前的项目在两个重要方面扩展了这项工作。 首先，因为数值模型暗示了复合地震中子事件之间的明显不对称性，而且因为在较早的工作中发现了这方面的诱人证据（但只是回顾性地认识到），所以PI正在整个地区系统地寻找这种不对称性。 其次，由于余震的不对称性被认为是断层两侧物质对比如何影响主震动力学的一种表现，因此PI正在寻找微震本身的方向性，因为这也被认为是断层两侧物质对比的副产品。 准静态和弹性动力学模型正在应用于解释这些observation.The主要目标的工作是进一步描述一个相当显着的观察，并把它放在更大的背景下破裂沿着双材料界面。 有理论上的理由假设双材料界面上的破裂导致在特定方向上的优先传播，但是对此的地质证据是混合的。 由于这种方向性对地震危险的影响，人们对这个问题很感兴趣。 虽然这些观测只涉及微震，但它们具有统计意义的优势，并且表征材料对比对微震的影响将有助于我们对这一现象的全面理解。

The PI has amassed a large catalog of precise earthquake locations using waveform cross-correlation. Previous work with this catalog has shown that aftershocks of microearthquakes along the San Andreas fault near San Juan Bautista are distributed very asymmetrically. Within an estimated 1-2 radii of the margin of the mainshock, there are nearly 3 times as many aftershocks to the northwest as to the southeast. In contrast, there is no discernible asymmetry in the number of aftershocks above or below the mainshock, or along the Calaveras fault, which has no significant across-fault velocity contrast. These observations are consistent with the asymmetry being related to the contrast in seismic velocities across the San Andreas, which in this region may reach 35 or even 50%. Numerical studies of elastodynamic ruptures on a bimaterial interface, carried out under a previous NSF grant, show that ruptures governed by slip-weakening friction grow as slightly asymmetric cracks. Significantly, there is a large reduction in normal stress immediately behind the rupture front moving (in the context of the San Andreas) to the southeast. When this rupture front is slowed by a stress barrier the tensile stress continues onward, and for a wide range of parameters carries a dying slip pulse with it. After slip ceases, the southeast edge of the rupture is left far below the failure threshold because the tensile stress that carried the slip pulse there is gone. In contrast, the northwest rupture front slows gradually and stops much as it would in a homogeneous medium, with the rupture front remaining very nearly at the failure threshold after slip has stopped. This provides an appealing explanation for the aftershock asymmetry. The current project extends this work in two important ways. First, because the numerical models are suggestive of a pronounced asymmetry among subevents in compound earthquakes, and because tantalizing evidence for this was uncovered in older work (but recognized only retrospectively), the PI is systematically searching for such asymmetry throughout this region. Second, because the aftershock asymmetry is thought to be a manifestation of how the material contrast across the fault affects the dynamics of the mainshock, the PI is searching for directivity of the microearthquakes themselves, as this has also been proposed to be a byproduct of the material contrast across the fault. Quasi-static and elastodynamic models are being applied to interpret these observations.The main goal of this work is to further characterize a rather remarkable observation, and to place it in the larger context of rupture along a bimaterial interface. There are theoretical reasons to suppose that rupture on a bimaterial interface leads to preferential propagation in a particular direction, but the geologic evidence for this is mixed. Because of the implications of such directivity for seismic hazards, there is considerable interest in this question. Although these observations concern only microearthquakes, they have the advantage of statistical significance, and characterizing the effect of the material contrast on microearthquakes will contribute to our overall understanding of this phenomenon.