An earthquake swarm designated S20020514.1 occurred 5 km southwest of Gilroy in Santa Clara County, California. The sequence began at 12:03 on 13 May 2002 and concluded at 03:59 on 18 May 2002, spanning 111 hours and 55 minutes. During this period, 54 earthquakes were recorded, providing a clear example of swarm behavior in the region.
The events clustered tightly in both space and time, with the majority occurring on 14 May. Magnitudes ranged from 0.8 to a peak of 4.9, while focal depths remained shallow, predominantly between 3 km and 7 km. The largest event, magnitude 4.9 at 05:00 on 14 May, was followed by numerous smaller aftershocks that declined in frequency over subsequent days. Notable additional shocks included magnitudes of 3.2 and 2.8, both at depths near 6 km. Most activity concentrated between 4 km and 7 km depth, consistent with shallow crustal faulting.
This swarm reflects typical behavior along the Calaveras Fault zone, a major strand of the San Andreas Fault system. The Gilroy area lies within the broader transform boundary between the Pacific and North American plates, where right-lateral strike-slip motion produces frequent seismic sequences. Historical records document repeated moderate earthquakes in the vicinity, including the 1984 Morgan Hill event and earlier activity tied to the same fault system. The shallow depths observed align with the known seismogenic zone in this segment of the fault, where brittle failure occurs in the upper crust.
Geological mapping confirms the presence of Quaternary sediments overlying Franciscan Complex bedrock, which influences local ground motion amplification during seismic events. Ongoing GPS and seismic monitoring by regional networks continues to track strain accumulation along these faults, underscoring the persistent seismic hazard in the southern San Francisco Bay Area.
The 2002 swarm did not produce significant surface rupture or damage, yet it illustrates the value of dense seismic catalogs for understanding fault interaction and precursory patterns. Such sequences often occur without a single dominant mainshock, distinguishing them from typical aftershock sequences.
References
USGS Earthquake Catalog (historical data for Santa Clara County)
California Geological Survey Fault Activity Map
USGS Professional Paper on the San Andreas Fault System