Preliminary Report 3 Sept. 2002

California Integrated Seismic Network (CISN): Cooperative Project of Caltech, USGS, CGS, & UCB

Southern California Seismic Network/TriNet

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

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

A ML4.6 mainshock occurred at 00:08 am on 3 September northeast of Yorba Linda in Orange County at a depth of 10 km. It was preceded by two foreshocks at 09:50pm (ML2.6) and 10:23pm (ML1.5) on Sept 2nd. It was also followed by 23 aftershocks during the next 9 hours, with the two largest aftershock of ML2.8 at 00:15am and 04:28am.

We use seismic records to determine the orientation of the fault on which an earthquake occurs. The mainshock exhibited strike-slip faulting (horizontal movement) on a vertical plane striking N30°W. This mechanism is consistent with the mainshock being near the Whittier fault, one of the fastest moving faults (~2 to 3 mm/yr) in the Los Angeles basin. However, preliminary locations of the aftershocks appear to form a northeast trend thus suggesting that this sequence is occurring on a small conjugate fault, adjacent to the Whittier fault. Alternatively, this sequence is occurring near a jog in the Whittier fault itself.

This sequence is located along the eastern part of the Los Angeles basin where the Whittier fault and the buried thrust faults to the west from a complex zone of deformation. The 1987 ML5.9 Whitter Narrows earthquake that showed vertical movement on a west striking thrust fault, occurred near the north end of this zone. The last previous earthquake of magnitude greater than 4, to occur in the greater Los Angeles area, was located near Compton ML4.2 on 28 October 2001.

This is the largest earthquake to occur in the greater Los Angeles region since the ML5.1 and ML4.9 Northridge aftershocks on 26 and 27 April 1977.