Tectonic geomorphology, deformation history, and slip-rate estimate along the Palos Verdes Fault, offshore Southern California

Abstract:

The Palos Verdes Fault (PVF) is one of few active faults in Southern California that crosses the shoreline and can be studied using both terrestrial and subaqueous methodologies. Despite its proximity to metropolitan Los Angeles, the recent activity and earthquake hazards associated with the PVF are poorly constrained. To characterize the near seafloor fault morphology, Late Pleistocene–Holocene slip-rate and tectonic influences on slope sedimentary processes, a grid of high-resolution multibeam bathymetry and chirp sub-bottom profiles were acquired with the Monterey Bay Aquarium Research Institute’s (MBARI) Autonomous Underwater Vehicle (AUV). The AUV surveys were focused along the continental slope ~25 km south of Long Beach in water depths between 250 and 600 m, where the PVF crosses the slope. AUV multibeam bathymetry data gridded at 2-m resolution and chirp profiles are merged with 25-m resolution ship-based multibeam bathymetry and seismic-reflection profiles. Vibracores collected with the MBARI Remotely Operated Vehicle (ROV) and ship-based USGS gravity cores provided radiocarbon dates for stratigraphic horizons offset by the PVF. Recent deformation is expressed as a well-defined seafloor lineation and offset Late Pleistocene–Holocene sub-bottom reflections. Curvilinear scarps associated with an upper submarine landslide (~450 m water depth), a buried slump block, and a lower submarine landslide (~525 m water depth) have been right-laterally offset by 55±3, 55±5, and 40±5 meters, respectively. The age of the upper scarp is bracketed between 23–31 kyr BP, which yields an average slip rate across the PVF of 1.6–2.4 mm/yr. However, our best estimate for the age of the upper landslide is ~ 31 kyr BP, which yields a right-lateral slip-rate of 1.8 mm/yr. Vertical growth faulting observed along a subtle transtensional fault-bend suggests that at least two surface ruptures occurred during the Holocene. In summary, these results indicate that the offshore extension of the PVF remains active, may carry as much as 50% of the total slip-budget assigned to the Inner California Borderland, and is critical to understanding the earthquake hazards in Southern California.