Lawrence Berkeley National Laboratory

Two recent strike-slip earthquakes on the San Andreas Fault (SAF) in California, the M6.0 2004 Parkfield and M7.0 1989 Loma Prieta events, revealed peaks in the acoustic emission (AE) activity in the surrounding crust several months prior to the main events. Earthquakes directly within the SAF zone were intentionally excluded from the analysis. The observed increase in AE is assumed to be a signature of the increasing stress level in the surrounding crust, while the peak and subsequent decrease in AE starting several months prior to the main events is attributed to damage-induced softening processes as discussed herein. Further, distinctive zones of low seismic activity surrounding the epicentral regions in the pre-event time period are present for the two studied events. Both AE increases in the crust surrounding a potential future event and the development of a low-seismicity epicentral zone can be regarded as promising precursory information that could help signal the arrival of large earthquakes.