Analysis of the microseismicity of the Santa Monica Mountains and associated Malibu Coast, Santa Monica-Dume, and Santa Monica Bay faults

The Santa Monica Mountains, in the western Transverse Ranges, are separated from Los Angeles and offshore Santa Monica sedimentary basins by the E-W, now predominantly left-lateral Raymond-Hollywood-Santa Monica-Dume fault system (Dolan et al, 2000). The western ~80 km-long stretch of this fault system has been investigated by Sorlien et al (2006) using seismic reflection and earthquake data. The fault system is primarily left-lateral with a strain accumulation rate of2.4 +/- 1.1 mm/yr, based on GPS data (Meade and Hager, 2005). Thrust slip on a low-angle blind fault beneath the Santa Monica-Dume fault is proposed to account for the Santa Monica anticlinorium (Dolan et al, 1995). The onshore Malibu Coast fault (MCF) and the onshore Santa Monica fault are probably oblique left-reverse faults (Dolan et al, 2000). The Malibu Coast fault shows evidence of reverse-oblique slip with a left-lateral strike-slip component along north-dipping strands ranging from 30-70 degrees (Cronin and Sverdrup, 1998). The convergence rate across the MCF is estimated to be about 18 mm/yr (Huftile and Yeats, 1995), and the slip rate is estimated to be between 0.04 to 1.5 mm/yr (Dolan et al, 1995). Convergence is evident in focal mechanisms showing mostly reverse faulting. Though Holocene surface displacements have been officially recognized across only two strands of the MCF zone to date, the MCF is still considered active and capable of producing a magnitude 6.5 to 7.0 earthquake (Cronin and Sverdrup, 1998). The microseismicity (1996-present; M=l-3, 307 events) for the region has been relocated using HYPOINVERSE 2000 and the SCEC/LARSEII crustal velocity structure. The results show seismicity (map view and cross-sections) associated with the Malibu Coast, Santa Monica-Dume, and Santa Monica Bay faults, as well as scattered events in the eastern region of the Santa Monica Mountains. These focal mechanisms show primarily reverse, and some left-lateral slip.