M 4.9 - 5 km SSW of Gilroy, California

Tectonic Summary

The May 13, 2002 Gilroy earthquake occurred near the south end of the Sargent fault and was followed by several small aftershocks located nearby. This moment magnitude M 4.9 earthquake was not unusual, but it ended a post-1989 Loma Prieta earthquake period of relative seismic quiescence in the south San Francisco Bay region. The energy released in an M4.9 event is about 1/1000 that of the M6.9 Loma Prieta event. The M 5.3 Pacheco Pass earthquake near San Juan Bautista in 1998 was the only nearby M> 5.0 shock since 1990. The Gilroy earthquake occurred at normal mid-crustal depth and had a typical strike-slip faulting mechanism. The region where the earthquake occurred has been seismically active at the magnitude 5 level at least since detailed recording began about 50 years ago.

The epicenters of the earthquake and its aftershocks are located near the Castro fault, a minor mapped fault which parallels the Sargent fault. The mapped length of the Castro fault is about 15 km and the maximum magnitude earthquake on a fault of that length is about 6.2. The largest known event that might have occurred on the Castro fault is the magnitude 5.3 earthquake that occurred on March 2, 1959.

The Gilroy earthquake was reported as felt from near San Miguel on the south to Santa Rosa on the north but did little damage. Magnitude 5 earthquakes in the San Francisco Bay region are typically felt throughout the region by people in tall buildings and at sites susceptible to ground amplification. In comparison, ground motion recordings in the epicentral region by newly-installed instruments in the ANSS network are about 20% as strong as those recorded in the epicentral region of the 2000 magnitude 5.2 Yountville earthquake.

The strong-motion coverage of the Gilroy earthquake was doubled by the records from ANSS instruments installed during the past three years. This coverage was especially improved in the Silicon Valley area. ANSS funding has been limited to date; implementation of full ANSS funding will provide more than a five-fold increase in the number of new instruments that would have recorded this earthquake. Fulfilling the promise of ANSS is a high priority for the USGS.

Dense urban recording of earthquakes provides a detailed description of shaking severity, enabling rapid identification of locales of potential damage to structures toward which emergency response efforts can be directed to saving lives. After well-recorded earthquakes, analysis of the dense data sets leads to more accurate inputs to seismic design codes and practices.