SEISMOLOGICAL STUDIES AT PARKFIELD V - CHARACTERISTIC MICROEARTHQUAKESEQUENCES AS FAULT-ZONE DRILLING TARGETS

Studies at very high resolution of microearthquakes at Parkfield, Cali fornia, since 1987 reveal a systematic organization in space and time, dominated by clustering of nearly identical, regularly occurring micr oearthquakes (characteristic events) on 10 to 20-m-wide patches within the fault zone. More than half of the 4000 + events in our 1987 to 19 96 catalog exhibit this trait. In general, recurrence intervals (0.5 t o 2 yr) scale with the magnitude of the repeating events for the on-sc ale range (Mw 0.2 to 1.3) in this study. The similar waveforms, superi mposed 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 month s. 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 a ctive plate-boundary fault zone. At Parkfield, the achievable location accuracy to which a hypocenter can be specified as well as the predic tability of its occurrence time appear to be uniquely favorable for in situ fault-zone measurements.