Seismicity Precursors of the M6.0 2004 Parkfield and M7.0 1989 Loma Prieta Earthquakes

The M6.0 2004 Parkfield and M7.0 1989 Loma Prieta strike-slip earthquakes on the San Andreas Fault (SAF) were preceded by seismicity peaks occurring several months prior to the main events. Earthquakes directly within the SAF zone were intentionally excluded from the analysis because they manifest stress-release processes rather than stress accumulation. The observed increase in seismicity is interpreted as a signature of the increasing stress level in the surrounding crust, whereas the peaks and the subsequent decrease in seismicity are attributed to damage-induced softening processes. Furthermore, in both cases there is a distinctive zone of low seismic activity that surrounds the epicentral region in the pre-event period. The increase of seismicity 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. The Gutenberg-Richter relationship (GRR) should allow extrapolation of seismicity changes down to seismic noise level magnitudes. This hypothesis is verified by comparison of seismic noise at 80 Hz with the Parkfield M4 1993-1994 series, where noise peaks 5 months before the series to about twice the background level