Occurrence patterns of foreshocks to large earthquakes in the western United States

OBSERVATIONS of foreshocks preceding large earthquakes provide one of the few well documented cases of premonitory events that are clearly related to a subsequent earthquake. Unfortunately, the apparent randomness of foreshock occurrence-they precede some events and not others-has severely hampered their use in reliable earthquake prediction. Understanding the factors that control foreshock occurrence is critical for determining how large earthquakes initiate and whether reliable short-term prediction will ever be possible¹. Here we report the results of a comprehensive study of the occurrence patterns of foreshocks to large earthquakes in the western United States. The incidence of foreshocks decreases with increasing depth of the mainshock, and also depends on the mainshock slip orientation. This pattern of occurrence may be explained by a decrease in small-scale crustal heterogeneity with increasing depth, and suggests that increasing normal stress (both regional tectonic stress and lithostatic load) inhibits the occurrence of foreshocks. No relationship is observed between any aspect of foreshock occurrence and the magnitude of the subsequent mainshock, suggesting that the eventual size of the mainshock may be independent of the earthquake nucleation process, or that foreshocks are not part of this process.