CLUSTERING AND PERIODIC RECURRENCE OF MICROEARTHQUAKES ON THE SAN-ANDREAS FAULT AT PARKFIELD, CALIFORNIA

The San Andreas fault at Parkfield, California, apparently late in an interval between repeating magnitude 6 earthquakes, is yielding to tec tonic loading partly by seismic slip concentrated in a relatively spar se distribution of small clusters (<20-meter radius) bf microearthquak es. Within these clusters, which account for 63% of the earthquakes in a 1987-92 study interval, virtually identical small earthquakes occur red with a regularity that can be described by the statistical model u sed previously in forecasting large characteristic earthquakes. Sympat hetic occurrence of microearthquakes in nearby clusters was observed w ithin a range of about 200 meters at communication speeds of 10 to 100 centimeters per second. The rate of earthquake occurrence, particular ly at depth, increased significantly during the study period, but the fraction of earthquakes that were duster members decreased.