Analysis of foreshock sequences in California - implications of foreshock triggering process

Abstract:

We examine precursory occurrence patterns for M ≥ 5 earthquakes in southern California from 1981 to 2011 and in northern California from 1984 to 2009. 53% of the 64 mainshocks are preceded by at least one foreshock within 2 days and 5 km. Foreshock occurrence appears correlated with mainshock faulting type and depth. Foreshock area is correlated with the magnitude of the largest foreshock but only weakly correlated with mainshock magnitude. We also examine small seismicity clusters that have at least 10 events within 2 days and 5 km, resembling swarm-like foreshock sequences. Only a small fraction of the small clusters lead to a larger cluster. About 70% of the large clusters don't start with their largest event, and the spatial distribution pattern is similar to M ≥ 5 mainshocks, with lower occurrence rates in the Transverse Range and central California and higher occurrence rates in the eastern California Shear Zone and the Hayward fault zone. These results suggest that foreshock occurrence is largely controlled by the regional tectonic stress field and fault zone properties. Foreshocks properties do not seem useful in predicting the magnitude of the eventual "mainshock".