A SLOW EARTHQUAKE SEQUENCE ON THE SAN-ANDREAS FAULT

EARTHQUAKES typically release stored strain energy on timescales of th e order of seconds, limited by the velocity of sound in rock Over the past 20 years, observations(1-13) and laboratory experiments(14) have indicated that rupture can also occur more slowly, with durations up t o hours. Such events may be important in earthquake nucleation(15) and in accounting for the excess of plate convergence over seismic slip i n subduction zones. The detection of events with larger timescales req uires near-field deformation measurements. In December 1992, two boreh ole strainmeters close to the San Andreas fault in California recorded a slow strain event of about a week in duration, and rye show here th at the strain changes were produced by a slow earthquake sequence (equ ivalent magnitude 4.8) with complexity similar to that of regular eart hquakes. The largest earthquakes associated with these slow events wer e small (local magnitude 3.7) and contributed negligible strain releas e. The importance of slow earthquakes in the seismogenic process remai ns an open question, but these observations extend the observed timesc ale for slow events by two orders of magnitude.