Studies at very high resolution of microearthquakes at Parkfield, California, since 1987 reveal a systematic organization in space and time, dominated by clustering of nearly identical, regularly occurring microearthquakes (characteristic events) on 10 to 20-m-wide patches within the fault zone. More than half of the 4000+ events in our 1987 to 1996 catalog exhibit this trait. In general, recurrence intervals (0.5 to 2 yr) scale with the magnitude of the repeating events for the on-scale range (Mw 0.2 to 1.3) in this study. The similar waveforms, superimposed locations, quasi-periodic recurrence, and uniform size of these characteristic events permit relative hypocenter location accuracy of meters and predictable occurrence times within windows of a few months. Clustered characteristic events occur at depths as shallow as about 3 km, and these are feasible targets for deep scientific drilling and observation at the focus of a subsequent small earthquake within an active plate-boundary fault zone. At Parkfield, the achievable location accuracy to which a hypocenter can be specified as well as the predictability of its occurrence time appear to be uniquely favorable for in situ fault-zone measurements.

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