RECENT DEFORMATION ALONG THE OFFSHORE MALIBU COAST, DUME AND RELATED FAULTS WEST OF POINT DUME, SOUTHERN CALIFORNIA

Offshore faults west of Point Dume, southern California, are part of an important regional fault system that extends for about 200 km, from near the city of Los Angeles westward along the south flank of the Santa Monica Mountains and through the northern Channel Islands. This boundary fault system separates the Western Transverse Ranges, on the north, from the California Continental Borderland, on the south. Previous research showed that the fault system includes many active fault strands; consequently the entire system is considered a serious potential earthquake hazard to nearby Los Angeles. We present an integrated analysis of multichannel seismic- and high-resolution seismic-reflection data and multibeam bathymetric, gravity and magnetic information to focus on the central part of the fault system that lies west of Point Dume. We show that some main offshore faults within the boundary fault system have displacements of 3-5 km, and many faults are currently active because they deform the seafloor or very shallow sediment layers. The main offshore fault is the Dume fault, a large reverse or thrust fault that dips north. In the eastern part of the study area, this fault offsets the seafloor. Onshore, the Malibu Coast fault dips steeply north, is active, and shows left-oblique slip. The probable offshore extension of this fault is a large fault that dips steeply in its upper part; its dip flattens at depth. High resolution seismic data show that this fault deforms shallow sediment making up the Hueneme fan, so this fault probably has been active during the Holocene. We propose that at the west end of the Santa Monica Mountains, where the Oxnard plain begins, a north-south alignment of geologic structures, topographic features and a gravity anomaly indicates a structural zone that strikes north, transversely to the mountains. This structure could have important consequences for the analysis of the regional earthquake hazard because the zone might form a boundary between earthquake-rupture segments.