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

将从两个方面研究地震的破裂起因：1)断层面上的应力分布与前兆的关系；2)既有断层几何形状与破裂起因的关系。与大地震有关的断裂面上的凹凸体和应力分布可能与这类地震的短期前兆和可预测性有关。如果一个粗糙的物体受到足够大的压力而产生膨胀，然后破裂，那么很可能会观察到短期的前兆。如果破裂起源于断层的低应力(“弱”)区域，并传播并破坏凹凸体，则可能不存在与应力相关的前兆。对几次大地震的初步调查支持上述观点。有人认为，预先存在的断层几何形状和物理屏障的分布可能强烈地影响地震破裂的起始和总程度。特别是，King和Nabelek(1985)指出了断层弯曲对破裂的开始和终止可能产生的强烈影响，并列举了几个例子。如果地震发生在弯道(或其他障碍物)附近，如果这种行为足够普遍，就可以用来识别需要加强监测的地区。同样，如果破裂倾向于局限于弯曲之间的区域，则断裂带的几何形状可用于估计地震风险。我们建议对上述假设进行检验。我们将通过远震波形的反演来确定大地震断裂面上的凹凸处的位置。然后，将高应力粗糙的位置与观测到的前兆的位置进行比较，如果观察到任何前兆的话。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，以及地震期间哪些断层段移动。将与地质观测进行详细的比较。我们将在监测良好、有良好地质和/或前兆数据的地区分析世界范围内的大型事件。首先，我们将集中关注中国发生的九个大事件，我们已经收集了这些事件的地质、地震和前兆数据。我们还将分析加州和阿拉斯加最近(1986)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.