More than half of the microearthquakes that occur near Parkfield, California, when located with high-resolution methods, are seen to define some 80 small clusters of 2 to 12 similar events. Each cluster occupies a patch typically 100 to 200 m in length within the fault zone. Cluster members have nearly identical waveforms (correlation coefficient of 0.9 or greater) to frequencies of 50 to 100 Hz, as recorded by the borehole-installed seismographic network. The clusters are distributed throughout the fault zone around the presumed nucleation region, in the locked section to the SE and in the creeping part of the NW of the previous M 6 hypocenter. They are also found in the Salinian block several kilometers SW of the fault zone. The total area occupied by all of the clusters constitutes only a small fraction (1 to 2%) of the fault zone approaching failure at Parkfield. Such clusters provide insight into the dynamics of the failure process through their spatial-temporal characteristics and their mechanisms. They also serve as highly repetitive sources distributed throughout the fault zone suitable for monitoring the nucleation zone for possible precursory changes in physical properties that affect wave propagation. In this article we demonstrate these applications with a detailed analysis of selected clusters.

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