An Observational Study of Microearthquakes on a Bimaterial Interface

双材料界面微地震的观测研究

• 批准号: 0710896
• 负责人: Allan Rubin
• 金额: $ 24.8万
• 依托单位: Princeton University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2007
• 资助国家: 美国
• 起止时间: 2007-07-01 至 2012-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0710896&HistoricalAwards=false
• 关键词: Observational; Study; Microearthquakes; Bimaterial; Interface

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.

使用波形互相关，PI已积累了大型地震位置的大量目录。 该目录的先前工作表明，沿圣胡安包蒂斯塔（San Juan Bautista）沿圣安德烈亚斯（San Andreas）断层沿线的微观震动非常不对称。 在估计的主震边缘的1-2半径内，西北部的余震几乎是东南部的3倍。 相比之下，在主震动或下方或沿calaveras断层的余震中没有明显的不对称性，该余震没有显着的跨率速度对比。 这些观察结果与与圣安德烈亚斯的地震速度的对比有关的不对称性是一致的，圣安德烈亚斯的地震速度可能达到35％甚至50％。 在以前的NSF赠款下进行的双层界面上弹性动力学破裂的数值研究表明，由于摩擦摩擦的裂纹略有不对称的裂纹而受到的破裂。 值得注意的是，在破裂前部移动（在圣安德烈亚斯的背景下）向东南移动后，正常应力却大大减轻。 当这种破裂的前部因应力屏障而减慢时，拉伸应力会继续向前，并且对于广泛的参数会带有垂死的滑移脉冲。 滑动停止后，破裂的东南边缘远低于故障阈值，因为带有滑动脉冲的拉伸应力消失了。 相比之下，西北破裂的前部逐渐减慢并停止，就像在均匀的介质中一样，破裂的前部几乎保持在滑动停止后的故障阈值处。 这为余震不对称提供了一个吸引人的解释。当前项目以两种重要方式扩展了这项工作。 首先，由于数值模型暗示了复合地震中亚事件之间的明显不对称性，并且因为在较旧的工作中发现了诱人的证据（但仅回顾性地认识到），因此PI正在系统地寻找整个该地区的不对称性。 其次，由于余震不对称被认为是对故障的材料对比度如何影响主震动的一种表现，因此PI正在寻找微进行微夸克本身的方向性，因为这也被认为是跨断层材料对比的副产品。 准静态和弹性动力学模型正在应用于解释这些观察结果。这项工作的主要目标是进一步表征相当出色的观察结果，并将其置于沿双层界面的更大的破裂环境中。 理论上的理由假设双层界面上的破裂会导致在特定方向上的优先传播，但是混合了地质证据。 由于这种方向性对地震危害的影响，因此对这个问题引起了很大的兴趣。 尽管这些观察结果仅涉及微夸克，但它们具有统计意义的优势，并且表征材料对比度对微量的影响将有助于我们对这一现象的整体理解。