Consistency of seismogenic stress field of preshocks to the tectonic stress field before eight earthquakes (M_W≥6.0) in southern California of United States from 1981 to 2011

This paper defines an index called “concentration ratio of P axis distribution” used for analyzing the variation of focal mechanisms of small earthquakes before moderate-strong earthquakes. And then based on the focal mechanism solutions of earthquakes in southern California in the period of 1981—2011, this paper investigates the seismogenic stress field of small events concentrating on the local tectonic stress field before eight main shocks (M_W≥6.0) in the southern California. The results show a phenomenon that the “concentration ratio of P axis distribution” has a minimum was observed before seven earthquakes, that is, the seismogenic stress field of small events concentrates on the local tectonic stress field. It provides a method for the risk prediction of moderate-strong earthquakes.