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!

Seismo-Watch More Special Earthquake Reports

Sunday, February 2, 2003

Two moderate earthquakes registering M 4.2 and M 4.0 occurred this morning east of San Francisco in San Ramon Valley, shaking a wide area but not causing any damage or injuries. The activity was centered close to a swarm that occurred in November of last year.

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According to the Northern California Seismic Network (NCSN) the first earthquake measured M 4.2 occurred at 10:22:58 a.m. PST (18:22:58 GMT), February 2, and the second measured M 4.0 and occurred at 10:47:39 a.m. PST (18:47:39 GMT). They were centered about 26 miles east of San Francisco, 3 miles northwest of Dunlin, and 3 miles southeast of San Ramon along the eastern side of San Ramon Valley, in eastern San Ramon Village. See a Topozone topographic map M 4.2 or Topozone topographic map M 4.0 a MapQuest street map. They occurred in essentially the same location.

The earthquakes occurred slightly to the south of an earthquake swarm in late November that produced at least 122 quakes, including seven M 3.0s and one as large as M 3.9 (see epicentral plot). See the Seismo-Watch Central California earthquake reports for November 21 - 27, 2002 and November 27 - December 4, 2002.

The focal depths were rather deep for events of this region, both M 4.0s triggering at about 16 km. Seismo-Watch has prepared two plots of the focal points of earthquakes from January 2002 to February 2003, one along a north-south line and the other along an east-west line. Data shows that the most of the February 2003 sequence has occurred between 12-17 km whereas the series from November 2002 occurred at depths of mostly between 7-9 km.

Note the tight cluster formed by the November 2002 swarm along the north-south projection, suggesting the orientation of the slip plane. Conversely, the February 2003 sequence is scattered in both north-south and east-west projections.

Although tremors from 10-17 km have occurred here from time to time in the past, most of the tectonic action has triggered between 4-9 km deep. Action at >17 km deep is rare and has only occurred 13 times since 1970, once in 1970, another in 1990, and 11 times since 1997.

A preliminary fault plane solution by UC Berkeley for the first event indicated strike-slip motion along an either a northwest or northeast trending plane, with a slight oblique-slip component. The November 2002 swarm showed a simmilar solution and after plotting up the sequence, it was found that the trend of events implicated a northeast-trending fault. This orientation is perpendicular to the Calaveras Fault, which is located on the west side of the San Ramon Valley and about 1.25 miles from this current sequence.

Continued on Page 2.

Report update: 3:12:59 PM, Monday, February 3, 2003
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