Escape tectonics in the Los Angeles metropolitan region and implications for seismic risk

Abstract

Recent damaging earthquakes in California, including the 1971 San Fernando1, 1983 Coalinga2, 1987 Whittier Narrows3 and 1994 Northridge4 events, have drawn attention to thrust faults as both potentially hazardous seismic sources and as a mechanism for accommodating shortening in many regions of southern California. Consequently, many geological studies5,6 have concluded that thrust faults in Southern California pose the greatest seismic hazard, and also account for most of the estimated 5–7 mm yr−1 of contraction across the greater Los Angeles metropolitan area7,8 indicated by Global Positioning System geodetic measurements9. Our study demonstrates, however, that less than 50% of the geodetically observed contraction is accommodated on the principal thrust systems across the Los Angeles region. We integrate the most recent geological, geodetic and seismological data to assess the spatial distribution of strain across the Los Angeles metropolitan region. We then demonstrate that a significant component of seismic moment release and shortening in this region is accommodated by east–west crustal escape ‘extrusion’ along known strike-slip and oblique-slip faults.