abstract

The three largest earthquakes to occur within 50 km of San Francisco during 1970 were centered near the community of Danville, California, and spanned a time interval of 50 sec starting on June 12, at 0330 hr GMT. These earthquakes were greater than magnitude 4.0 and were part of a shallow, tightly-clustered earthquake swarm that persisted for approximately 2 months starting in mid-May 1970. During the same period of time, continuous tilt at tidal sensitivity (10−8 radian) was being recorded in two vaults at distances of 25 and 45 km west of the epicentral area. Two types of anomalous tilting preceded the principal events: (1) a long-term (1-month) constant tilt rate of 8 × 10−10 radian/hr that was recorded by both tilt stations and (2) a short-term (1-day) accelerated micro-tilt change which occurred just prior to the events and was most pronounced on the station closest to the epicentral area. Neither type of anomaly was recognized in the post-Danville record. The lack of a strong correlation with meteorological phenomena to account for the long-term coherency between two tilt stations of 25-km separation and the apparent azimuthal correlation of the Berkeley residual tilt vector with the earthquake epicenters suggests that the anomalous tilt preceding the Danville events were tectonic.

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