Southern California Seismic Network/TriNet: cooperative project of Caltech and USGS

Lucy Jones and Doug Given USGS, Pasadena, CA 91106 jones@usgs.gov

Egill Hauksson and Kate Hutton, Caltech, Pasadena, CA91125 hauksson@gps.caltech.edu

A moderate earthquake of M5.7 occurred near the southern end of the Laguna Salada fault at 11:32 a.m. (local time) on the morning of Friday, February 22, 2002 (see Figure ). The earthquake was felt over much of the U.S. border region from the Imperial Valley to San Diego and as far north as Orange County. It was followed by a very robust aftershock sequence including 13 events of magnitude 3 or greater as of 7:00 p.m. The largest aftershock so far was a M3.9 event at 12:33 p.m. This type of robust aftershock sequence is typical of the area and more M~3 and even M~4 events should be expected in the next few days.

The Laguna Salada fault was the probable source of a M~7 earthquake 110 years ago on February 23, 1890. The last significant (M5) earthquake in this region was a M5.4 earthquake on February 7, 1987. This fault is an extension to the south of the Elsinore fault of California and has a slip rate on the order of 4 mm/yr. This earthquake has a focal mechanism suggest strike-slip motion on a fault with an orientation similar to the Laguna Salada.