SEISMOLOGICAL STUDIES AT PARKFIELD .3. MICROEARTHQUAKE CLUSTERS IN THE STUDY OF FAULT-ZONE DYNAMICS

More than half of the microearthquakes that occur near Parkfield, Cali fornia, when located with high-resolution methods, are seen to define some 80 small clusters of 2 to 12 similar events. Each cluster occupie s a patch typically 100 to 200 m in length within the fault zone. Clus ter members have nearly identical waveforms (correlation coefficient o f 0.9 or greater) to frequencies of 50 to 100 Hz, as recorded by the b orehole-installed seismographic network. The clusters are distributed throughout the fault zone around the presumed nucleation region, in th e locked section to the SE and in the creeping part of the NW of the p revious M 6 hypocenter. They are also found in the Salinian block seve ral 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-tempora l characteristics and their mechanisms. They also serve as highly repe titive sources distributed throughout the fault zone suitable for moni toring the nucleation zone for possible precursory changes in physical properties that affect wave propagation. In this article we demonstra te these applications with a detailed analysis of selected clusters.