Creep events preceding small to moderate earthquakes on the San Andreas fault

Abstract

THE San Andreas fault in central California is unusual in that much of the fault displacement occurs as aseismic slip, also known as creep. Changes in creep have been suggested as a possible earthquake precursor^1–6, but clear associations between creep changes and subsequent earthquakes are uncommon. Here I report a test of the hypothesis that episodes of rapid creep ('creep events'^1–3) precede small to moderate earthquakes on the creeping portion of the San Andreas. The test consists of a comparison of archival creep data to the earthquake catalogue for this area, and an earthquake prediction experiment. Between 1970 and 1994, creep events occurred in the five days preceding half or more of the earthquakes of magnitude > 3.3 in three 1-yr periods of above-average seismic activity. Of five predictions based on the occurrence of creep events between October 1995 and mid-January 1996, four were fulfilled; there were also two 'misses' (earthquakes not preceded by a creep event). If the relationship reported here holds for the entire creeping segment of the San Andreas, and applies to larger earthquakes, these results suggest that a prediction based on precursory creep may be possible at Parkfield, California.