Seismo-Watch Significant Earthquake Report - No. 01-029
Regional Location:

Preliminary Magnitude:

Greenwich Mean Date:
Greenwich Mean Time:
Focal Depth:


Ml 4.4 (SCSN)

18:04:32 (11:04 a.m. local time)
0.0 km (near surface)

SAN CLEMENTE ISLAND, CALIFORNIA, AUGUST 16, 2001 (Seismo-Watch) -- An earthquake measuring Ml 4.4 (SCSN) occurred this morning at 18:04:16 UTC (11:04 p.m. local time), about 63 miles west of San Diego and just south of San Clemente Island, in the Pacific Ocean. It was followed about four hours later by another quake registering M4.2 (SCSN). Although both were widely felt onshore, there were no reports of damage.

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Preliminary data indicates the quake was centered about 7 miles southeast of Pyramid Point, the southern tip of San Clemente Island at the base of the San Clemente Escarpment, in about 700 fathoms of water. It probably occurred along the San Clemente fault zone and although we do not have a fault plane solution as of yet, the San Clemente fault zone typically shows strike-slip movement with a slight oblique (dip-slip) component.

We do not have a Moment magnitude analysis for this event as of yet.

Since this is a remote location for seismic monitoring, the focal depth may be a little suspect as this early stage of data analysis. This is also apparent from the focal depths of some of the aftershocks which showed depths of 31 and 25 km and are not real and will be modified after further analysis.

Foreshocks and aftershocks
The main jolt was preceded a week earlier by a
M3.0 foreshock on August 9 UTC (August 8 PDT). There were no other foreshocks.

At least three aftershocks have been recorded thus far, two in the M3 range and a M4.2 at 3:06 p.m. - a little more than four hours after the main shock.

Below is a list of the foreshocks and aftershocks in Greenwich Mean Time (subtract 7 hours for local time). (Note the unusul depths for the M3.1 and M4.2 aftershocks. These will undoubtably change after further data review.):

2001 08 09 01 13 06.65 32 46.07-118 12.33 7.00 3.0 << foreshock
2001 08 16 18 04 33.77 32 45.57-118 17.29 6.94
4.4 << main shock
2001 08 16 20 04 18.75 32 45.61-118 19.26 7.00
2001 08 16 20 36 59.64 32 43.65-118 23.58 31.39
2001 08 16 22 06 28.05 32 44.01-118 20.05 25.22
4.2 << Felt
Source: Trinet

Continued on Page 2

Report update: 9:20 p.m., August 16, 2001
Friday, August 24, 2001
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