中文摘要：

为了理解地震的发生和地壳结构不均匀性的关系，利用南加州地震台网的固定台和临时台所记录的2863个兰德斯余震和区域地震，共计107401个P波和19624个S波高质量的到时数据，采用地震层析成像方法得到了兰德斯地震区P波和S波的精细的三维速度结构和泊松比分布.结果显示，地震的发生和分布与地壳结构的横向不均匀性有密切的关系.总体上看，兰德斯地震区余震成丛分布，并被低速块体截断，其中4级以上地震大多分布于P波高、低速异常过渡区域或偏向高速块体一侧，这可能是因为高速区多属地壳脆性介质，易于造成应力集中，导致地震；反之，低速度区则可能代表破碎程度较高、富含流体或温度较高区域，因而更倾向于产生无震变形.基于兰德斯地震区强震震源位置、地震区P波、S波速度异常与泊松比分布推断，兰德斯地震区可能有流体存在.地壳流体易使地壳岩石弱化，从而引发大地震.

英文摘要：

Abstract In order to understand the relationship between earthquake occurrence and crustal heterogeneities, we used 107401 P- and 19624 S-wave high-quality arrival times from the 1992 Landers aftershocks and other local earthquakes which were recorded by both permanent and temporary stations in Southern California to determine detailed three-dimensional P- and S-wave velocity and Poisson＇ s ratio structures beneath the Landers earthquake area. Our results show a close correlation between the seismic rupture zones and crustal heterogeneities. The distribution of the Landers aftershocks is cluster-like and separated by or terminated in the area where lowvelocity anomalies exist. Most of the large earthquakes with magnitudes bigger than 4 occur in or around areas with high P-wave velocity .The possibility is that high-velocity areas are probably brittle and strong parts which can sustain seismogenic stress, and so can generate earthquakes. In contrast, low-velocity areas may have either higher degree of fracture, high fluid content, or higher temperatures where deformation is more likely to be aseismic. In addition, we infer that fluids exist in the Landers earthquake area from the distribution of P- and S- wave velocity and Poisson＇ s ratio. The existence of fluids may weaken the surrounding crustal rocks, then trigger strong earthquakes.