Determination of slip rates for offshore fault zones: An example from the Palos Verdes fault zone, southern California

Numerous active and potentially hazardous faults are offshore southern California but not included in probabilistic seismic hazard assessments because no slip rates are available. Where offshore faults cross the coastline, such as the Newport-Inglewood and Palos Verdes (PV) faults, slip rates can be determined using traditional paleoseismological techniques on land, and thus these faults can be included in hazard assessments. An important focus of ongoing research on offshore faults, therefore, is to develop methods to determine slip rates for faults that lie entirely offshore. Both very high-resolution bathymetric and sub-bottom data are needed to be able to resolve and date features offset by offshore faults. Bathymetric surveys using autonomous underwater vehicles (AUV) are now capable of imaging seafloor features on the order of tens of centimeters. The AUV also gather high-resolution seismic reflection data with similar sub-bottom resolution. Key features can then be targeted for sampling and dating studies that can provide information on slip rates. Together, these data can be used to produce an "acoustic trenching" of the submarine fault. The PV fault is an ideal fault with which to evaluate the "acoustic trenching" method, since slip rates determined offshore can be compared with values established from studies of onshore portions of the fault. Portions of the PV fault south of San Pedro Shelf in water depths of 350-500 m were surveyed in 2007-2008 by AUV to obtain high-resolution bathymetric and seismic reflection data. These high-resolution surveys along the fault near the San Gabriel River submarine channel reveal a submarine slide scarp that is offset 30-40 m by the PV fault. Radiocarbon dating of a sediment core taken from the floor of the slide indicates that the age of the slide is about 6000 ka. This suggests a slip rate of about 4-7 mm/yr for the PV fault, in general agreement with current estimates of 2-4 mm/yr based on onshore studies.