Seismicity and Active Compressional Tectonics in Santa Monica Bay, Southern California

Abstract

During 1973–1986 the seismicity in Santa Monica Bay consisted of an average of four events per year of M_L ≥ 2.5, in addition to the 1979 Malibu (M_L = 5.0) mainshock-aftershock sequence. The 1979 sequence ruptured a north dipping thrust fault at 10–15 km depth beneath the bay. A diffuse distribution of seismicity extends across the bay from the terminus of the Palos Verdes fault to Point Dume. Two thirds of the single-event focal mechanisms of 36 events that are located within the bay show mostly thrust or reverse faulting on west to west-northwest striking planes. A third of the focal mechanisms exhibit strike-slip faulting on north to north-northwest striking planes. A stress inversion of the focal mechanism data shows that the maximum principal stress is oriented N13°E in a horizontal plane and the minimum principal stress is oriented N102°E and plunging 47°. Hence the tectonics in Santa Monica Bay are dominated by compression, with thrust, reverse, and strike-slip faulting. The coexistence of thrust and strike-slip faults suggests that the bay is not a part of the Peninsular Ranges terrane but forms a transition zone that accommodates the change from strike-slip in the Peninsular Ranges to the south to reverse faulting in the Transverse Ranges to the north. Mapped anticlines and the previously unrecognized thrust faults beneath the bay form two fold and thrust belts that present additional seismic hazards to Los Angeles and the southern California coastal zone through seismic shaking and possible generation of small tsunamis.