Special Reports
Regional Location:

Preliminary Magnitude:

Greenwich Mean Date:
Greenwich Mean Time:
Focal Depth:


M 4.2 (NCSN)

16.7 km (10.4 miles)


M 4.0 (NCSN)

16.5 km (10.3 miles)

The San Ramon Valley earthquake returns, in a big way!

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Foreshocks and aftershocks: the sequence of events
Small bursts and occasional quakes have occurred in the San Ramon Valley since the November swarm that generally ended on December 2. See list of M 1.0s from January 2002 to February 3, 2003. One such series occurred on January 5-8 and consisted of 11 events, including one as large as M 2.7 that was locally felt. Following that series, activity declined significantly, producing only two minor quakes till the end of the month.

On February 1st, two minor quakes measuring M 1.9 and M 1.5 occurred in San Ramon Valley. Although both were small, there was a distinct difference between the two: the M 1.5 occurred at 15 km deep while the other occurred 6 km shallower, at 9 km deep.

Like an out of tune car trying to start, an other M 1.9 sputter occurred at 2:47 a.m. on February 2. It too occurred at about 15 km deep. Then at 19 minutes past 8 a.m. the car, er... earthquake swarm started with a M 2.9 jolt that was followed by seven more events by the end of the hour, including a M 3.6 at 8:22 a.m. and a M 3.0 at 8:45 a.m. All of the action occurred at depths greater than 12 km.

The 9 o'clock hour saw a slight reduction in activity, producing only five quakes in the M 1.0 range. But that was short lived because at 10:22 a.m., a M 4.2 earthquake rocked the region. It did not spur on any aftershocks or kick-start the swarm to any significance, as only a few M 1.0s occurred between it and the M 4.0 jolt at 10:47 a.m. - some 25 minutes later.

The swarm sputtered four quakes during the 11 o'clock hour, including one as large as M 3.5, but declined rapidly after that, producing 0-2 events per hour till about 4 p.m. The last event in the swarm occurred at 12:53 a.m. on February 3 and measured M 1.2.

The brief 19-hour swarm consisted of 31 events, including a pair of M 4.0s, three M 3.0s and seven M 2.0s. Most residents in San Ramon Valley felt anything above M 2.0 and a few exclaimed they felt the smaller ones as well.

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Report update: 7:53:53 AM, Tuesday, February 4, 2003
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