Seismological evidence for the convergence of crustal stress orientation before large earthquakes

Earthquakes are caused by the failure of faults, driven by tectonic stress build-up in the Earth's crust. To study the earthquake preparation process and assess regional earthquake potentials, it is vitally important to understand the crustal stress evolution process and identify its change in pattern associated with the seismogenic process. In this study we investigate the focal mechanism orientations of earthquakes in southern California from 1982 to 1999, basing on a focal mechanism catalog from Hauksson. We find that for the two large earthquakes occurred in southern California, the 1992 M_W7.3 Landers and the 1999 M_W7.1 Hector Mine, the orientations of focal mechanisms near the coming earthquake tend to converge to the stress direction promoting the rupture of the coming earthquake and align with its focal mechanism about half-year before its occurrence, suggesting that the tectonic stress field gets more organized in favor of the rupture of the event pre-seismically. The degree of stress convergence is measured by the orientation angle RMS (root mean square) between the preshocks' focal mechanisms and the focal mechanism of the large event studied, and its time series recorded the stress convergence process. The degree of anomalies, measured by the F-tests, indicates that the convergence of stress orientations become significant at 90% confidence about half-year prior to both the Landers and Hector Mine quakes, and it becomes even more prominent at 99% confidence right before the occurrences. Our study may be of significance for assessment of regional seismic potentials.