February 2, 14:00 PST: Happy Groundhog Day!
The residents of the Dublin-San Ramon area awakened to a small sequence
of earthquakes this morning. This earthquake sequence is located within the
Calavears fault zone and immediately to the south of the
November 2002 swarm.
reveal a trend parallel to the Calaveras fault.
In contrast, the November sequence showed a trend that was oriented
perpendicular to the Calaveras and northeast of the fault. The current
sequence locates less than a mile to the east of the Calaveras fault.
Given the uncertainty in hypocenter locations, it suggests that the
quakes are occurring on the Calaveras. The various
which reveal the
and sense of motion on the fault are also consistent with this
interpretation. However, the computed fault planes dip slightly to the
southwest, rather than the northeast as would be expected for locations
that are northeast of the fault. At present, we believe the discrepancy
can be accounted for by computed uncertainties in earthquake locations
As of this time, the largest event in this sequence is an M4.2 which
ocurred at 10:22 local time. Like the events in November, this sequence
shows the characteristics of a swarm - it includes an M3.6 at 8:22, M3.0
at 08:45, M4.2 at 10:22, M4.0 at 10:47, and M3.5 at 11:02. Past swarms of
earthquakes in this vicinity have persisted for weeks. During the last
30 years the maximum magnitudes of other swarms in the region have not
exceed M5. However, since these quakes appear to be on the Calaveras
fault, we remind the public that the Calaveras fault has the capability
to rupture in a M7. The probability
that the northern Calaveras will rupture in an earthquake of magnitude
greater or equal to M6.7 is 18% over the next 30 years. Historically,
the northern section of the Calaveras fault produced a ~M6 earthquake in 1861.
David Oppenheimer, Lind Gee, and Bill Ellsworth
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 swarm,
ShakeMaps have been produced for several events.
The CISN Engineering Data Center
produces reports showing details of
ground motions for events of interest. They have issued an
Quick Report (IQR) for the M4.2 showing records from the Dublin Fire
Station (3.5 km away) as well as other sites of interest:
Many earthquakes in this swarm have been widely felt in the San Francisco
Bay Area. People with Internet access can report their observations
of shaking and damage through the
Did You Feel It?
Web site. In the links below, you can see the maps illustrating the reported
intensities for several events in this sequence. Unlike the ShakeMaps - which
are based on data from seismic instruments - these maps are based on human
observations. You can contribute to these data by filling out the form for each event.