CREEP EVENTS PRECEDING SMALL TO MODERATE EARTHQUAKES ON THE SAN-ANDREAS FAULT

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 asa possible earthquake precursor (1-6), but clear associations between creep changes and subsequent ear thquakes are uncommon. Here I report a test of the hypothesis that epi sodes of rapid creep ('creep events'(1-3)) precede small to moderate e arthquakes on the creeping portion of the San Andreas. The test consis ts of a comparison of archival creep data to the earthquake catalogue for this area, and an earthquake prediction experiment. Between 1970 a nd 1994, creep events occurred in the five days preceding half or more of the earthquakes of magnitude greater than or equal to 3.3 in three 1-yr periods of above-average seismic activity, Of five predictions b ased on the occurrence of creep events between October 1995 and mid-Ja nuary 1996, four were fulfilled; there were also two 'misses' (earthqu akes 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.