Magnitude 4.8 near Yorba Linda, CA

Preliminary Earthquake Report

A M4.8 (updated from M4.6) 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 aftershocks 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 Whittier Narrows earthquake occurred near the north end of this zone. The last previous M4 earthquake to occur in the greater Los Angeles area was located near Compton on 28 October 2001. Today’s M4.6 event is the largest in the Los Angeles metropolitan area since a M5.1 Northridge aftershock in April 1997.

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