Abstract

Microearthquake activity during the 4-year interval 1968 to 1971 in a region of 25 km radius, centered on the San Andreas fault near Hollister, has been monitored by means of high-gain high-frequency seismographs which allow detection of events as small as magnitude zero within the region. The distribution of the 4764 microearthquakes shows a high degree of clustering. The series of time intervals between consecutive shocks can be described approximately by a gamma distribution whose shape parameters for different samples change from 0.3 to 0.8. The exponential probability plots of the {Δti} series for 265 larger regional earthquakes over 10 years result in a constant value of the hazard function except for small values of time where the hazard is higher. The variance-time and intensity functions show a varying degree of interdependence between events for each year and a long-term dependence up to 80 days for the period 1968-1971. The time scale of clustering has been estimated. Inferences based on short-term microearthquake studies may be grossly misleading.

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