Egill Hauksson, Caltech, Pasadena, CA91125 Lucy Jones, USGS, Pasadena, CA 91106 10/28/2001

The M3.7 Compton earthquake in the Los Angeles basin occurred Sunday morning, October 28, at 8:27 a.m. The mainshock was followed by more than 40 aftershocks during the first 8 hours, the largest of which was M3.0 at 8:29 a.m. The aftershocks define a steep southwest dipping plane in the depth range of 17 to 19 km. near the Newport-Inglew ood fault. This is a particularly robust aftershock sequence for the Los Angeles basin. In addition, preliminary locations suggest that several microearthquakes (M<2.0) occurred in the following few hours at several other sites around the Los Angeles basin.

The mechanism for the Compton earthquake showed thrust faulting on a northwest striking plane, dipping steeply to the southwest. Thus, although this sequence is occurring close to a major strike-slip fault, the Newport-Inglewood fault, it is occurring on the deep thrust fault beneath the sediments of the Los Angeles basin. This thrust fault could be a restraining bend in the Newport-Inglewood fault. It could also be a part of a deeper system of thrust faults that operated independently of the shallower strike-slip system. The last thrust event to occur in this area near Compton had a magnitude of M3.3 and occurred in 1982.

The 28 October Compton earthquake sequence differs from the recent M4.2 Beverly Hills sequence (Sept 9th) and Silver Lake sequence (Oct. 7th) both in terms of depth and style of faulting. Both of those sequences were shallower than 8 km depth and involved mostly shallower strike-slip faults, in some cases containing a small thrust component.

The earthquake activity of the last two months is high but not unprecedented in the seismic history of the Los Angeles basin. Since 1940, the Los Angeles basin area (latitude from 33.5° to 34.15° and longitude from 118° to 118.5°) has averaged about 3 earthquakes of magnitude 3.0 or larger each year. Four such earthquakes have occurred in 2001, all in the last two months. One possible interpretation of this earthquake activity is the emergence of southern California from the stress release shadow of the 1992 Mw7.3 Landers earthquake and Mw6.7 Northridge earthquakes. The Landers earthquake released stress across the whole region, including the Los Angels basin. This year, the tectonic stress in the region may have increased to levels as demonstrated by the onset of normal level of background seismicity.