At 19h00m09.46s UTC, on 24 January 1980, a strong earthquake (ML = 5.5) that caused a surprising amount of damage occurred north of Livermore Valley about 12 km to the southeast of Mt. Diablo, and was associated with surface rupture along the Greenville Fault. There was a foreshock (ML = 2.7) a minute and a half earlier and a sequence of 59 events (ML ≧ 2.5) in the ensuing 6 days. On 27 January at 02h33m35.96s, a larger magnitude earthquake occurred in the sequence (ML = 5.6). This second principal shock was located 14 km to the south of the first principal earthquake toward the southern end of the Greenville Fault. Preliminary estimates of the seismic moments of the two principal shocks are 5.3 × 1024 and 1.3 × 1024 dyne-cm, respectively. In addition to the lower seismic moment, the ML = 5.6 shock on 27 January exhibits a clearly focused radiation pattern, with large amplitudes toward the northeast.

Field investigations after the first principal shock indicated surfaced rupture along the Greenville fault zone for at least 6 km, with both right-lateral strike-slip and some dip-slip motion with the northeast side up. Variable offsets on surface cracks suggested displacements of a few centimeters (with evidence of increases in some places after the second 27 January earthquake). There were eight earthquakes with ML ≧ 4.0 in the sequence up to 5 February 1980. No foreshocks near the Greenville Fault (ML ≧ 1.5) were observed by the University of California Seismographic Stations in the prior 3 months.

Rapid deployment of field seismographs by a number of seismological organizations permitted precise locations and fault-plane solutions. Some results on seismicity are as follows.

  1. The rupture propagated over 15 km to the southeast along the Marsh Creek-Greenville faults on 24 January and stopped in the vicinity of Highway 580. This southern progression may have had some causal connection with the relatively high intensities reported near the southwest end of the Greenville Fault.

  2. The two principal shocks of the sequence have slight but significant differences in the fault-plane solutions; both are predominantly right-lateral strike-slip, but the strike of the northern one is N13°W, whereas the strike of the southern one is N39°W. This change in strike is not evident in the mapped strikes of the Marsh Creek and the Greenville faults.

  3. In contrast to the second principal earthquake, the first principal shock was followed by two others (ML > 4.0) in rapid succession, one 53 sec and the other 97 sec after. This repetition gave a relatively long duration to the shaking on 24 January, and was commented on in felt reports. It may explain the greater intensity reported in many localities on 24 January compared to 27 January.

  4. The b value (0.64 ± 0.13) for the sequence is somewhat lower than the b = 0.70 ± 0.17 for the recent Coyote Lake earthquake sequence on the Calaveras Fault on 5 August 1979. There are fewer earthquakes than normal in the range 3.0 < ML < 4.0 in the Greenville sequence.

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