Earthquake travel time tomography of the southern Santa Cruz Mountains: Control of fault rupture by lithological heterogeneity of the San Andreas Fault Zone
Earthquake travel time tomography of the southern Santa Cruz Mountains: Control of fault rupture by lithological heterogeneity of the San Andreas Fault Zone
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圣克鲁斯山脉南部的地震走时层析成像：圣安德烈亚斯断层带岩性非均质性对断层破裂的控制
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发表时间：
1993
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通讯作者：
T. McEvilly
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作者：
W. Foxall;A. Michelini;T. McEvilly
1989年洛马普列塔地震发生在沿着圣安德烈斯断层带延伸的南部圣克鲁斯山脉，最后一次失败的大地震是在1906年。1989年破裂的东南端标志着从稳定的过渡，在圣安德烈亚斯断层的中央蠕动部分不稳定的故障锁定1906年部分的滑动。我们调查这种转变和破裂的特点，1989年地震使用三维P波速度模型的南部圣克鲁斯山脉部分的断裂带。速度模型是通过对173次地震的5422个P波到达时间的联合渐进反演确定的，这些地震包括1989年地震的余震和美国地质调查局中心加州网络记录的主震“背景”事件。这个速度模型被用来重新定位424个背景地震，洛马普列塔主震和292个余震。利用加州海岸山脉岩石类型的实验室数据和可用的地震折射剖面，根据岩性对速度模型进行了校准，结果表明，该模型与地表地质情况吻合良好。该模型的图像大异常高速体中地壳深度内的破裂带的1989年地震，现有的证据表明，可能有辉长岩或其他镁铁质成分。从速度模型解释的地震活动性和表面蠕动的岩性特征的关系的基础上，我们提出了一个模型，在该模型中，高速体是主要负责从稳定过渡到不稳定的断层滑动在Pajaro缺口。圣安德烈亚斯断层的活动面贯穿整个身体。断层系统试图通过将滑动转移到次级断层来绕过这一障碍，次级断层包括沿高速岩体和方济各围岩之间的有利接触面沿着传播的展布断层。然而，稳定滑动的几乎停止导致应力集中在体内，并且体内的高强度，不稳定接触从1989年地震中失败的障碍物演变为粗糙体。该模型预测的1989年破裂的一般特征与地震计算的几个破裂历史一致。该模型意味着，正如其他工作者所提出的，洛马普列塔地震并不涉及1906年滑动的重复，这对断层的圣克鲁斯山脉段的地震复发估计有重要影响。
The 1989 Loma Prieta earthquake occurred along the stretch of the San Andreas fault zone within the southern Santa Cruz Mountains that last failed as a major earthquake in 1906. The southeastern end of the 1989 rupture marks the transition from stable, aseismic slip on the central creeping section of the San Andreas fault to unstable failure on the locked 1906 segment. We investigate this transition and the rupture characteristics of the 1989 earthquake using a three-dimensional P wave velocity model of the southern Santa Cruz Mountains section of the fault zone. The velocity model was determined by joint progressive inversion of 5422 P wave arrival times from 173 earthquakes, which included aftershocks of the 1989 earthquake and premainshock “background” events, recorded by the U.S. Geological Survey central California network. This velocity model was used to relocate 424 background earthquakes, the Loma Prieta mainshock, and 292 aftershocks. The velocity model is calibrated in terms of lithology using laboratory data for California Coast Ranges rock types and available seismic refraction profiles and is shown to be in good agreement with the surface geology. The model images a large anomalous high-velocity body at midcrustal depths within the rupture zone of the 1989 earthquake that the available evidence suggests might have gabbroic or other mafic composition. On the basis of the relationship of the lithological features interpreted from the velocity model to the seismicity and surface creep we propose a model in which the high-velocity body is primarily responsible for the transition from stable to unstable fault slip at Pajaro Gap. The active plane of the San Andreas fault cuts throughout the body. The fault system attempts to circumvent this barrier by transferring slip to secondary faults, including splay faults that have propagated along the frictionally favorable contact between the high-velocity rock mass and Franciscan country rocks. However, the near arrest of stable sliding causes stress to concentrate within the body, and the high-strength, unstable contact within it evolves from a barrier to the asperity that failed in the 1989 earthquake. The general features of the 1989 rupture predicted by this asperity model agree with several rupture histories computed for the earthquake. The model implies that as proposed by other workers, the Loma Prieta earthquake did not involve a repeat of the 1906 slip, which has an important bearing on earthquake recurrence estimates for the Santa Cruz Mountains segment of the fault.