The Morgan Hill Earthquake of April 24, 1984—Seismological Aspects

Abstract

On April 24, 1984, at 21:15 UTC, a moderate earthquake occurred on the Calaveras fault in the vicinity of Halls Valley, California (epicenter 37° 19.2'N, 121° 41.9'W). The earthquake had a mean local magnitude of 6.2 ± 0.20 and a maximum Modified Mercalli Intensity of VIII. It is probably a repeat of the 1911 Coyote earthquake. The rate of seismicity from historical catalogs, compiled at the U. C. Seismographic Stations yields an average recurrence time of 160±60 years for an ML≥6.2 event occurring in a 30 km segment of the Calaveras fault. Other evidence from the repeat times of earthquakes suggests a recurrence period of 75 years for a total magnitude of 6.0 or larger.. These estimates are in reasonable agreement. A seismic moment of 1.1×1025 dyne-cm for the mainshock was estimated from broadband displacement seismograms recorded at Berkeley (Δ ≈ 79 km). The moment and the rupture area inferred from the aftershocks suggest a mean fault displacement of 30 cm. The aftershock sequence through December 31, 1984, contained four earthquakes of magnitude 4.0 or greater and the sequence b-value was 0.81 ± 0.03.