The 1930 Santa Monica and the 1979 Malibu, California, earthquakes

Abstract

The Malibu earthquake (M_L = 5.0) that occurred 1 January 1979 in Santa Monica Bay was located at 33°56.9′N and 118°41.3′W and at a depth of 10 km. The focal mechanism of the main shock as determined from first motions recorded by short-period, high-gain seismic stations in southern California shows reverse faulting with one plane dipping 55° to 60° to the north striking 275° to 290°, and the second plane dipping 30° to 38° to the south and striking 100° to 120°. More than 400 aftershocks were recorded during 1979. Focal mechanisms for 20 aftershocks of M ≧ 3.0 show similar reverse mechanisms as the main shock. The main shock and aftershocks (M ≧ 2.5) were relocated using a local velocity model and a set of corresponding station delays. The aftershocks define a volumetric hypocentral zone, 4 km × 6 km, west-trending with depths ranging from 6 to 11 km. This 1979 Malibu main shock-aftershock sequence probably occurred on the eastern end of the Anacapa-Dume fault. The Santa Monica earthquake (M_L = 5.2) that occurred 31 August 1930 was relocated using station delays calculated from the 1979 date set. The relocated 1930 epicenter is located near the western end of the Santa Monica fault. The 1979 and 1930 main shocks can be interpreted to define a 8- to 10-km north-south sidestep between the Anacapa-Dume and Santa Monica faults. Possible segmentation of these two fault systems suggests that they are unlikely to rupture simultaneously in one large earthquake.