Seismo-Watch

More Special Earthquake Reports

December 8, 2001

An earthquake registering a magnitude Ml 5.2 (SCSN) occurred south of Yuma, Arizona, in the Colorado Delta region of Baja California, Mexico. A preliminary analysis by the National Earthquake Information Center (NEIC) and Harvard Geophysical Laboratory (HRV) determined a Moment magnitude Mw 5.7 and M5.8, respectively.

The quake was centered about 54 miles south of Yuma, Arizona, and 60 miles south-southeast of Mexicali and close to where the Colorado River enters the Gulf of California, some 15 miles northwest of Montague Island. See location maps.

Preliminary data indicates the quake was centered at less the a kilometer below the surface. There is a slight chance that surface fault ruptures have occurred.

Harvard's preliminary fault plane solution suggests a complex oblique-slip (strike-slip with a vertical component) motion along a northwest trending plane. The NEIC solution suggests an even stronger oblique component, making the slip movement predominantly normal (down and away) motion.

The earthquake was centered about 8 miles west of the main trace of the Cerro Prieto Fault and probably the strong oblique-slip component suggest the quake occurred along a feature not related to the Cerro Prieto fault zone. This prominent fault zone ruptured violently in 1934 with a M7.1 earthquake that caused stunning fault ruptures.

This earthquake was sent out on the Global M5+ Seismo-Watch EQ Flash! Earthquake Alert Bulletin list within minutes of the quake being reported. It was not located in California or the USA, so subscribers to those lists did not receive bulletins. Seismo-Watch will change the parameters for the California list to include Baja California in the future. Sorry for the inconvenience.

Felt Reports
Although there have been no reports from the epicentral area, felt reports have been received from as far away as Oceanside and Palm Springs. It is likely that items were tossed from tables and shelves, pictures were knocked from walls, and some windows may have broken. This is a relatively remote area with relatively small settlements. Did you feel the quake?

People in San Diego said they felt a rolling motion that caused pots to clang, dishware to rattle and walls to vibrate. One early report from Imperial Valley said there was a sharp jolt followed by about 10 seconds of shaking that was strong enough to cause people to rise and get ready to move to safety. But then the shaking stopped.

With access to the USGS because of a court dispute involving the Department of Interior, the USGS have set up an alternative path to their Southern California "Community Intensity Maps".

Aftershocks
Three aftershocks have been recorded thus far, the largest of which measured M4.5 at 5:42 p.m. The other two aftershocks were in the M2 range.

Background
Although more than 140 earthquakes have registered M4.0 or stronger in the greater Mexicali Valley-Colorado Delta region since 1983, this is the largest quake since a M5.3 temblor struck on August 31, 1988. See list of M4.0+ quakes.

The Mexicali Valley-Colorado Delta region was quite active from 1932-1970, producing 31 quakes measuring M5.5 or stronger, including eight quakes in the M6 range and one as large as M7.1. However since then, only two M5.5+ plus quakes have been recorded. (See Big Quake List)

Additional Links

Seismo-Watch Quick Links in Southern California

Southern California Seismic Network

Wirenews Reports

Moderate Quake Rattles US, Mexico (AP) Dec. 8, 2001

5.2 earthquake jolts the Valley (Imperial Valley Press) Dec. x, 2001