Hypocenters and magnitudes of more than 600 aftershocks of the 1966 Parkfield-Cholame earthquake were determined from recordings of a dense network of portable seismograph stations operated in the epicentral region from 3 to 82 days after the main shock. Hypocenters were virtually confined to a nearly vertical zone extending downward from the zone of visible ground fracturing at the Earth's surface to a depth of 12 to 14 km. Aftershocks were concentrated in patches on the slip surface, with large numbers of events in depth ranges of 2 to 4 km and 8 to 10 km and relatively few at depths of 5 to 7 km. First-motion patterns suggest that simple, nearly horizontal right-lateral strike-slip displacement was the source for an overwhelming majority of the aftershocks. The coefficient “b” in the frequency versus magnitude equation appears to be depth-dependent: it is about − 0.6 for events between 8- and 10-km depth but averages about −0.95 for events at other depths. Individual stations with consistently late P-wave arrivals were also found to record abnormally large amplitudes, with both anomalies increasing, apparently, with increasing thickness of sediments beneath the site.

First Page Preview

First page PDF preview
You do not currently have access to this article.