Central California was shaken by a M6.5 earthquake on December 22, 2003.
This event was located 6 miles NE of San Simeon, CA, and has a
This event is being followed by an active aftershock sequence. As of March 18, 2003
there have been 28 events over magnitude 4, and 230 events of magnitude 3.0 and
higher as determined by the automatic systems. We have put together a
figure illustrating the distribution of aftershocks with time,
comparing the decay to the 1983 Coalinga and 1989 Loma Prieta earthquakes.
The San Simeon sequence appears to be behaving more like Coalinga than
Loma Prieta, as the Coalinga sequence also had a vigorous aftershock
sequence with a number of larger events.
Several people in the Cambria area have reported "shaking" or "rumbling" around 10:40 AM on Dec 22,
prior to the M6.5. Analysis of the seismic data indicates that this is most
likely a sonic boom, and not an earthquake.
This figure shows a portion
of an "electronic helicorder" for the period 10AM-11AM 12/22 PST from station PCB (near Cambria)
which shows a small signal at 2003/12/22 10:38 PST. The impulse-response signal at 18:46 is a
daily calibration signal. A blow up of the trace at that
time shows the detail of the signal.
The M6.5 earthquake is the largest earthquake in California since the
M7.1 1999 Hector Mine earthquake.
In 1952, an M6 earthquake occurred in this general area.
UC Berkeley Seismological Lab, USGS/Menlo Park, LLNL, SCEC, State of CA
The CISN Northern California Management Center produces
for events of M3.5 and higher. ShakeMaps are based on the observed ground motions from
seismic instruments combined with predicted motions in areas without sensors. In this sequence
ShakeMaps have been produced for several events.
March 24, 2004
The CISN has updated the ShakeMap for the M6.5 San Simeon earthquake. The updated map
reflects several changes. First, the map was updated using the new version of the
ShakeMap software - version 3.0. This new version introduces some minor
changes in appearence of the map as well as a number of "under the hood"
changes that improve the speed and distribution of the maps. Second,
we corrected a problem in the code used to compute distances to a
rupture segment. As a result, previous maps underestimated ground
motions near the fault trace. The new map shows the corrected estimates
of ground motion.
The CISN Engineering Data Center
produces reports showing details of
ground motions for events of interest.
The M6.5 earthquake was widely felt, from San Francisco and Santa Rosa to Los Angeles
and Oceanside. Many of the aftershocks have been widely felt as well. People with Internet
access can report their observations through the Did You Feel It?
Web site. In the link below, you can see the map illustrating the reported
intensities for the M6.5 event. Unlike the ShakeMaps - which
are based on data from seismic instruments - these maps are based on human