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
Back to Page 1
Back to Page 2

Continued from Page 2

Foreshocks and aftershocks (Cont.)

Earthquake swarms or bursts of seismicity are not uncommon for the San Ramon Valley. The most noteworthy swarm occurred in June-July of 1970 and consisted of 159 events, including 10 M 3.0s and three M 4.0s, the largest of which registered M 4.3. See list of M +3.0s from 1970 to February 3, 2003

A little further north and near Danville seems to be another location of earthquake swarms, with a prominent sequence in 1976 producing nine quakes registering M 3.0 or stronger, including two as large as M 4.1. Both areas are close to the Calaveras Fault and are probably within its loosely defined "fault zone", but have occurred on smaller splays or adjacent structures.

The main trace of the Calaveras Fault has not ruptured (produced a large, ground surface-displacing earthquake) in historic times. Studies show that the Calaveras Fault has the capability of producing a M 7.0+ earthquake and the probability that the northern Calaveras Fault will produce an earthquake of magnitude greater or equal to M 6.7 in next 30 years is 18%. Although the data is slim, the northern section of the Calaveras Fault probably produced a ~M 6.0 earthquake in 1861, but no surface displacements have ever been found.

Felt Reports
Both earthquakes were widely felt in the Bay Area, but there have been no reports of damage or injuries at this time. Felt reports came in from as far away as Guerneville to the north, Sacramento-Stockon-Manteca to the east, and Santa Cruz to the south. See USGS Community Intensity Report M 4.2 event. Many aftershocks were also felt but were felt more locally.

In describing the largest events, most people in the surrounding areas felt a sharp jot, followed by brief, vigorous ground shudders. Some reported low rumble sounds while others said the quakes sounded like a loud explosions that also shook the ground. Many said they heard creaking sounds from their house.

Did you feel this quake? Where were you and what were you doing? Send us a note.

There have been no reports of items toppling from tables or shelves, although people exclaimed that pictures were knocked askew, hanging plants swung and dish and glassware tinkled.

Some people noticed their pets being a little skittish.

One person said that during the largest event, the M 4.2 at 10:22 a.m., the shaking became strong enough to think about doing something, but just as he rose to his feet, it stopped.

There were no reports of car alarms going off.

Additional seismic information links:
Outside links will open a new browser window.

Mainstream media news reports:
Outside news stories will open a new browser window.
Report update: 7:57:44 AM, Tuesday, February 4, 2003

About Us || Terms of Use || Contact Us || Jobs || Advertise
©2003 Seismo-Watch, Inc. All rights reserved.