Seismicity preceding moderate earthquakes in California

Regional seismicity data have been examined to determine the nature of seismicity in the years preceding these moderate earthquakes in California: 1952 Kern County (M = 7.7), 1963 Watsonville (M = 5.4), 1964 and 1967 Corralitos (M = 5.0, 5.3), 1966 Parkfield-Cholame (M = 5.1, 5.5), 1968 Borrego Mountain (M = 6.4), 1969 Santa Rosa (M = 5.6, 5.7), and 1971 San Fernando (M = 6.4). In each instance the moderate earthquakes occurred in areas characterized by a relatively high level of small earthquake activity. In most cases the preceding activity in the area immediately surrounding the epicenter was high relative to other segments of the same fault zone or other nearby fault zones that, on the basis of geologic criteria, would be considered equally likely locations for moderate earthquakes. Detailed observations for a moderate earthquake that occurred in 1972 near Bear Valley (M = 5.0), where seismograph station coverage is adequate to obtain reliable focal depths for small earthquakes in, dicate that concentrations of small earthquakes were present in the immediate hypocentral region in the months before the earthquake. The regional observations have potential applications for the analysis of seismic risk in that fault zones or segments of fault zones characterized by a relatively large number of small earthquakes seem more likely to sustain moderate earthquakes than do adjacent fault zones or fault segments with equivalent geologic evidence for recency of movement. The detailed observations provide a potential tool for mapping stress concentrations or areas of anomalously low strength along the parts of fault zones capable of seismic slip.