An example of how first-motion polarities can precisely fix lateral velocity variations along active faults is given by an earthquake (ML = 4.4 and Mo = 3.4 × 1021 dyne-cm) that occurred on 28 April 1979, at 0044 UTC along the San Andreas fault zone in the vicinity of Pacifica (37°38.7′N, 122°28.0′W) near the western coastline of San Francisco Peninsula. This moderate earthquake is of particular interest in that it is the largest and best-recorded earthquake that has occurred in the region in the past 20 yr. The well-constrained fault plane solution indicates that there is approximately a 20 per cent velocity contrast across the fault zone with the velocity increasing toward the southwest and that the earthquake had a right-lateral strike-slip focal mechanism orientated parallel to the strike of the San Andreas fault. The distribution of the duration of the first half-cycle of the P wave with azimuth indicated that the rupture proceeded unilaterally northwestward along the San Andreas fault at a velocity of (1.6 ± 0.3) km/sec. In contrast, the fault plane solution for the ML 5.3 earthquake of 22 March 1957, centered about 4 km northwest of the 28 April 1979 earthquake, had a reverse faulting mechanism with the continental (northeast) side rising with respect to the oceanic side.

Unlike other moderate earthquakes which have occurred in the region in the past 22 yr, this earthquake had no detectable foreshocks or aftershocks. The depth of focus is estimated to be (12.9 ± 1.0) km.

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