Magnitude 4.9 near Gilroy, CA
Monday, May 13, 2002 at 10:00:29 PM (PDT)
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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.
Preliminary Earthquake Report
USGS, Menlo Park, CA
U.S. Geological Survey, Menlo Park, California
U.C. Berkeley Seismological Laboratory,