Special Reports
Regional Location:

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Greenwich Mean Date:
Greenwich Mean Time:
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M 4.0 (NCSN)

1.1 km (0.7 miles)

M 4.2 quake jolts The Geysers

Seismo-Watch More Special Earthquake Reports

Tuesday, May 20, 2003

An earthquake registering M 4.0 occurred this morning in the Northern California Coast Range at The Geysers. Although the quake was felt over a wide area, there have been no reports of damage or injuries at this time.

Seismo-Watch EQ Flash! Alert Bulletins notified subscribers of this earthquake within minutes after it happened. You should subscribe to the service too! It's FREE and Fast! NOW! Wireless EQ Flash! via pager, cell phone or PDA's! More info.

According to the Northern California Seismic Network (NCSN) the M 4.0 earthquake struck at 9:50:42 a.m. PDT (16:50:42 GMT), May 20, about 25 miles north of Santa Rosa, 6 miles northwest of Anderson Springs, and 5 miles west of Cobb Mountainin the Big Sulphur Creek drainage, near the defunct Old Geysers Resort. See a Topozone topographic map or a MapQuest street map.

The earthquake occurred in the heart of The Geysers geothermal steam production field.

The magnitude 4.0 is a moment magnitude calulation. The local surface wave or Richter magnitude was only 3.7. Sometimes variations between the moment and surface wave magnitudes can be explained as "slow" earthquakes, that is, they proceed at a slower rupture rate, releasing more energy but not creating as large seismic waves. In this case, the lower magnitude reading better reflects what the local people "felt" on the surface. See felt reports.

The focal depths were shallow and calculated to be only 1.1 km (0.7 mi.) beneath the base elevation of The Geysers sub-seismic network, which is at about the 2700' elevation. This places the hypocenter of the earthquake at about 1000 feet below mean sea level.

A preliminary fault plane solution by UC Berkeley indicated a complex dip-slip motion (both up-down and side-ways) along a north-northwest trending fault plane. This generally agrees with other fault motions recorded for this area.

Continued on Page 2.

Report update: 9:37:02 AM, Sunday, May 25, 2003
If you knew where a M 7.0+ earthquake was going to strike and you knew you could be safe, would you go there to feel it?
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