The 1992 Landers earthquake: Effect of crustal heterogeneity on earthquake generation

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 low-velocity 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 L-inders 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.