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.<\/p>\n
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 ML<\/sub>5.9 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 ML<\/sub>4.2 on 28 October 2001.<\/p>\n This is the largest earthquake to occur in the greater Los Angeles region since the ML<\/sub>5.1 and ML<\/sub>4.9 Northridge aftershocks on 26 and 27 April 1977.<\/p>\n 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…<\/span><\/p>\n![](\"\/wp-content\/uploads\/sprep_images\/2002yorbalinda.jpg\")
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