Abstract
The San Andreas fault at Parkfield, California, apparently late in
an interval between repeating magnitude 6 earthquakes, is yielding
to tectonic loading partly by seismic slip concentrated in a relatively
sparse distribution of small clusters (<20-meter radius) of microearthquakes.
Within these clusters, which account for 63\% of the earthquakes
in a 1987-92 study interval, virtually identical small earthquakes
occurred with a regularity that can be described by the statistical
model used previously in forecasting large characteristic earthquakes.
Sympathetic occurrence of microearthquakes in nearby clusters was
observed within a range of about 200 meters at communication speeds
of 10 to 100 centimeters per second. The rate of earthquake occurrence,
particularly at depth, increased significantly during the study period,
but the fraction of earthquakes that were cluster members decreased.
10.1126/science.267.5197.503
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