Seismo-Watch Significant Earthquake Report - No. 01-040
Regional Location:

Preliminary Magnitude:

Greenwich Mean Date:
Greenwich Mean Time:
Focal Depth:


ML 4.5 (NCSN), Mw 5.1 (UCB), Mw 5.3 (HRV)

08:02:23 (01:02 a.m. local time)
8 km

NORTHERN CALIFORNIA, SEPTEMBER 20, 2001 (Seismo-Watch) -- A moderate earthquake measuring ML 4.5 (NCSN), Mw 5.1 (UCB), Mw 5.3 (HRV) occurred in the wee hours of the morning off the Humboldt County coast of Northern California. There have been no felt reports at this time

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The quake occurred at two minutes past 1 a.m., Thursday morning, September 20, about 63 miles west-southwest of Eureka and 52 miles west of Petrolia in the Pacific Ocean.

The preliminary location suggests it occurred along the Mendocino Fault. The Mendocino Fault is the tectonic boundary between the Gorda and Pacific plates.

Analysis from the University of California, Berkeley Seismological Laboratory (UCB) indicated the type of motion was strike-slip along an east-west trending plane which correlates with the orientation and typ of motion expected for the Mendocino Fault (See the UCB solution). Harvard Geophysical Laboratory (HRV) obtained a similar result but with a slight oblique component See the HRV solution.

The large magnitude difference between the Local (ML or Richter) magnitude and the Moment (Mw) magnitude is common for earthquakes along the Mendocino Fault and is probably caused by a"slow" rupture process.

Seismo-Watch perfers the U. C. Berkeley (UCB) Moment tensor solution and Moment magnitude analysis for this event because the quake was centered too far from shore for a good Local magnitude analysis and Harvard's

Foreshocks and Aftershocks
There were no foreshocks and no aftershocks have been recorded at this time.

Felt Reports
There have been no felt earthquake reports at this time probably because it was centered too far from shore. Did you feel this quake?

Brief Background
This was the first
M4+ temblor off the Cape Mendocino coast since May 2, 2001 - more than four months ago. Gaps in M4+ activity of this length are unusual for this area with the last happening in 1997 (see list of events since 1990).

Since 1990, at least 96 tremors have measured M4.0 or stronger along the Mendocino Fault or within the southern Gorda tectonic plate, including eight measuring M5.0 or stronger. Interestingly, only 12 have occurred in the last four years.

Additional Seismic information links:

Seismograms of the earthquake series from nearby USGS stations:
Mainstream media news reports:
  • None at this time
Report update: 12:19:00 PM, Thursday, September 20, 2001
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