Late quaternary activity and seismic potential of the Santa Monica fault system, Los Angeles, California

Abstract

The Santa Monica fault is a 40-km-long, oblique left-lateral reverse fault that extends through the densely urbanized northwestern Los Angeles, California, area and offshore parallel to the Malibu coast. The fault exhibits near-surface strain partitioning in paleoseismologic trenches, and has undergone at least six surface ruptures during the past ∼50 k.y. Although events may be missing from the older part of the record, and the ages of older events are based on poorly constrained soil age estimates, at least two and probably three events occurred after the well-dated burial of a prominent paleosol ca. 16–17 ka. The resulting ∼7–8 k.y. latest Pleistocene–Holocene average recurrence interval for events large enough to cause surface rupture is much longer than the ≤1.9–3.3 k.y. maximum interval we calculate for hypothetical Mw 6.9–7.0 earthquakes generated by rupture of the entire Santa Monica fault. The pronounced disparity between the measured and calculated average recurrence intervals suggests that the Santa Monica fault undergoes infrequent, and therefore probably very large, earthquakes (Mw ≥ 7.0). If such large earthquakes have occurred, we speculate that they may have been generated by simultaneous rupture of the Santa Monica fault with other faults in the Transverse Ranges Southern Boundary fault system, such as the Hollywood or Anacapa-Dume faults. The most recent definite Santa Monica fault surface rupture occurred between ca. 10 and 17 ka, although a more recent event probably occurred between ca. 1 and 3 ka.