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M5.1 (SCSN), Mw 4.7 (SCSN)

15.2 km (9.4 miles)

Moderate M5.1 earthquake strikes Southern California

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October 30, 2001

ANZA, SOUTHERN CALIFORNIA - A moderate earthquake measuring ML 5.1 (SCSN) Mw 4.7 (SCSN) was recorded just before midnight, shaking a wide area of Southern California but not causing any damage or injures. A robust aftershock sequence has consisted of nearly a 100 events, but none have registered in the M3 range.

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According to the Southern California Seismic Network (SCSN), a cooperative between the United States Geological Survey (USGS - Pasadena) and Caltech, the earthquake occurred at 11:56 p.m., PST, October 30 (07:56 UTC, October 31), about

  • 24 miles south of Palm Springs
  • 19 miles northwest of Borrego Springs,
  • 9 miles southeast of Anza at the base of Santa Rosa Mountain.

    (See a Topozone topo map, see a MapQuest road map).

Preliminary data places the epicenter about 2.5 miles southwest of Santa Rosa Mountain on the ridge between Horse and Coyote Creeks and just inside the Anza Borrego Desert State Park.

The focal depth was placed at 15.2 km (9.4 miles) beneath the surface, rather deep for quakes in this area, but probably the reason why it was so widely felt.

A preliminary fault plane solution by TriNet indicated predominately strike-slip (sideways) motion with a slight oblique component. The preliminary TriNet moment tensor solution was Mw 4.7, considerably less than the local surface wave (Richter) magnitude of ML 5.1. Further analysis may refine these early determinations.

The quake probably occurred along the Clark Fault, the major component to the Anza segment of the San Jacinto fault zone. The quake was too weak and too deep to cause surface fault ruptures.

Continued on Page 2...

Report update: 10:07:22 PM, Wednesday, October 31, 2001

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