Seismo-Watch Significant Earthquake Report - No. 01-028
Regional Location:

Preliminary Magnitude:

Greenwich Mean Date:
Greenwich Mean Time:
Focal Depth:


ML 5.5 (NCSN), ML 5.5 (UNRSL), Mw 5.2 UCBSL

20:19:26 (1:19 p.m. local time)
14.5 km

PORTOLA, CALIFORNIA AUGUST 10, 2001 (Seismo-Watch) -- An earthquake measuring Ml 5.5 (NCSN), Ml 5.5 (UNRSL) occurred at 20:19:26 UTC (1:19 p.m. local time), August 10 about 50 miles northwest of Reno, 11 miles northwest of Portola in a remote area of northeastern California between Quincy and Portola. Although earthquakes of this size are capable of significant damage, there have been no reports of damage or injuries at this time.

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A preliminary location indicates the quake was centered in the along the northern ridge of the Middle Fork Feather River canyon about 5 miles north of Graeagle, in the Bonta Ridge area. See a Topo Zone location map. The focal point was centered 14.5 km (9 miles) beneath the surface.

The U.C. Berkeley fault plane solution indicated the quake was caused by strike-slip motion along a northwest trending plane. (See UCBSL Moment Tensor Solution) Harvard Geophysical Laboratory also determined strike-slip motion for a fault plane solution and confirmed a Moment magnitude analysis of Mw 5.5 (See HRV Solution here), but Berkeley's preliminary Moment magnitude analysis was slightly less strong and registered Mw 5.2.

Foreshocks and aftershocks
There were no foreshocks and a few small aftershocks have been recorded at the time of this report, the largest of which measured
M3.9 and occurred at 5:01 a.m. the morning after the main jolt. It was locally felt. Below is a list of the earthquakes in Greenwich Mean Time (subtract 7 hours for local time):

2001/08/10 20:19:26.970 39.828 N 120.632 W 14.5 5.5 <<main shock
2001/08/10 20:29:23.939 39.801 N 120.636 W 15.0 1.4
2001/08/10 20:36:34.361 39.802 N 120.577 W 5.0
2001/08/10 20:52:18.195 39.848 N 120.578 W 5.0
2001/08/11 03:05:44.683 39.801 N 120.636 W 15.0
2001/08/11 03:52:04.650 39.847 N 120.637 W 15.0
2001/08/11 05:43:42.270 39.709 N 120.694 W 15.0
2001/08/11 12:01:30.904 39.893 N 120.579 W 5.0
3.9 <<Felt
Source: UNRSL

Official Aftershock Forecast
The US Geological Survey has issued an Aftershock Forecast for the region. It indicates the statistical chance of moderate aftershocks, aftershocks measuring larger than the main shock, and the probility of weak aftershocks. The information is based upon similar magnitude earthquakes happening elsewhere in California.

Continued on Page 2

Report update: 11:50 p.m., August 11, 2001
Wednesday, August 22, 2001
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