Asperities, Barriers, Fault Geometries and Their Relationship to Earthquake Precursors and Rupture Initiation

地震破裂起始的两个方面将被研究：1）断层面上的应力分布与前兆之间的关系; 2）预先存在的断层几何形状与破裂起始之间的关系。与大地震有关的断层面上的凹凸不平和应力分布可能与这类地震的短期前兆和可预测性有关。如果一个粗糙体受到足够的压力产生扩张，然后破裂，短期的前兆可能会被观察到。如果破裂起源于断层的低应力（“弱”）区域，并传播和破坏粗糙体，则可能没有与应力相关的前兆。对几次大地震的初步调查支持了上述观点。有人认为，预先存在的断层几何形状和物理障碍物的分布可能会强烈影响地震破裂的起始和总范围。特别是，King和Nabelek（1985）指出了断层弯曲对破裂的开始和终止的强烈影响，并给出了几个例子。如果地震发生在弯道（或其他障碍物）附近，并且这种行为足够普遍，则可以用于确定需要加强监测的区域。同样，如果断裂倾向于局限于弯曲之间的区域，断层带的几何形状可以用于估计地震危险性。我们建议对上述假设进行检验。我们将通过地震波形的反演来确定大地震断层面上的微凸体的位置。然后将高应力粗糙体的位置与观察到的前体的位置（如果观察到任何前体）进行比较。通过反演断层机制和应力释放的细节，我们将确定断裂在哪里开始以及地震期间断层的哪些部分移动。将与地质观测进行详细的比较。我们将分析全球范围内的大型事件，这些事件在良好的地质和/或地质数据可用的监测区域。首先，我们将集中在中国的九个大事件，我们已经收集了地质，地震和地震数据。我们还将分析最近（1986年）在加州和阿拉斯加发生的事件。

Two aspects of the rupture initiation of earthquakes will be investigated: 1) the relation between the stress distribution on the fault surface and precursors and 2) the relationship between pre- existing fault geometry and the initiation of ruptures. Asperities and stress distribution on the fault surface associated with large earthquakes may be related to short-term precursors and predictability of such earthquakes. If an asperity gets stressed enough to produce dilantancy and then ruptures, short-term precursors are likely to be observed. If rupture originates in a low-stress ("weak") region of the fault and propagates and breaks asperities, there may be no stress-related precursors. Preliminary investigations of several large earthquakes support the above concept. It has been suggested that pre-existing fault geometry and the distribution of physical barriers may strongly influence the initiation and the total extent of earthquake ruptures. In particular, King and Nabelek (1985) pointed out the strong influence fault bends can have on the initiation and termination of rupture and showed several examples. If earthquakes initiate near bends (or other barriers) and if such behavior is sufficiently common, it can be used to identify areas for intensified monitoring. Similarly, if ruptures tend to be confined to regions between bends, the geometry of the fault-zone can be used in estimating earthquake hazard. We propose to test the above hypotheses. We shall determine the locations of asperities on the fault surfaces of large earthquakes by inversion of teleseismic waveforms. The locations of high-stress asperities will then be compared to the locations of observed precursors if any were observed. By inverting for the details of the faulting mechanisms and stress release, we will determine where the ruptures initiated and what segments of faults moved during the earthquakes. Detailed comparisons with geological observations will be made. We will analyze large events world-wide in well monitored areas for which good geological and/or precursory data are available. Initially, we will concentrate on nine large events in China for which we have already collected geological, seismological and precursory data. We will also analyze the most recent (1986) events in California and Alaska.