An Observational Study of Microearthquakes on a Bimaterial Interface

PI利用波形相互关系收集了大量精确的地震位置目录。先前的工作表明，沿着圣胡安包蒂斯塔附近的圣安德烈亚斯断层，微地震的余震分布非常不对称。在主震边缘估计1-2个半径范围内，西北方向的余震几乎是东南方向的3倍。相比之下，主震上方或下方以及沿卡拉维拉斯断层的余震数量没有明显的不对称性，这就没有明显的断层间速度对比。这些观测结果与这种不对称性是一致的，这种不对称性与整个圣安德烈亚斯断层的地震速度对比有关，在这个地区，地震速度可能达到35%甚至50%。先前在NSF资助下进行的双材料界面弹性动力学破裂的数值研究表明，由滑移减弱摩擦控制的破裂以轻微不对称裂纹的形式发展。值得注意的是，在断裂锋向东南移动（在圣安德烈亚斯的背景下）的后面，正应力大大减少。当这个破裂锋被应力屏障减缓时，拉应力继续向前，并且在大范围的参数中携带一个垂死滑动脉冲。滑动停止后，断裂的东南边缘远低于破坏阈值，因为携带滑动脉冲的拉应力消失了。相反，西北断裂锋逐渐减缓并停止，就像在均匀介质中一样，在滑动停止后，断裂锋仍然非常接近破坏阈值。这为余震的不对称性提供了一个令人信服的解释。当前的项目从两个重要方面扩展了这项工作。首先，由于数值模型表明复合地震的子事件之间存在明显的不对称性，并且由于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），因此PI正在系统地在整个地区寻找这种不对称性。其次，由于余震的不对称性被认为是断层上物质对比如何影响主震动力学的一种表现，因此PI正在寻找微地震本身的指向性，因为这也被认为是断层上物质对比的副产品。准静态和弹性动力学模型被用来解释这些观察结果。这项工作的主要目标是进一步描述一个相当显著的观察结果，并将其置于沿双材料界面破裂的更大背景下。理论上有理由认为，双材料界面上的破裂会导致在特定方向上的优先传播，但地质证据对此并不一致。由于这种指向性对地震危险的影响，人们对这个问题非常感兴趣。虽然这些观测只涉及微震，但它们具有统计显著性的优势，表征材料对比对微震的影响将有助于我们对这一现象的整体理解。

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.