Recent studies of the Palos Verdes fault in the Los Angeles Outer Harbor characterize the fault's Holocene activity. The studies included areal and local geophysical surveys, as well as, drilling, sampling, and dating of the subsurface materials. The studies identify the fault's location in the shallow subsurface, document evidence of Holocene strike-slip, quantify dip-slip versus sediment age, and quantify the net slip where an early Holocene paleochannel crosses the fault. The slip rate for the Palos Verdes fault is constrained to between 2.7 and 3 mm/yr for the past 7.8 to 8 ka. Because the new evidence shows that the Palos Verdes fault has one of the highest slip rates in the Los Angeles basin, the studies provide significant new insight into the regional tectonics and the seismic risk of the Los Angeles region. Using this rate of slip and segmentation models for the fault, the coastal Los Angeles region can expect a Mw 7 to 7.2 event about every 400 to 900 years from this potential seismic source. fered an alternat• suggestion that the fault is a backthrust above a blind thrust fault with essentially no strike slip [Davis et al., 1989; J. Shaw and J. Suppe, Earthquake hazards of active blindthrust faults under the central Los Angeles basin, Califomia, submitted to Journal of Geophysical Research, 1995]. Those models, however, predict higher rates of uplift than are indicated by the terrace chronology. Most previous assessments of the fault's slip rate typically assumed that the vertical to horizontal components of slip were approximately equal [Darrow and Fischer, 1983; Freeman et al., 1987]. Models of the terrace deformation, however, led Ward and Valensise [1994] to conclude that the fault slips obliquely at about 3 mm/yr in the peninsular area with a dominant strike-slip component. In addition, recent onshore seismic reflection data and geomorphic features were interpreted by Stephenson et al. [1995] as evidence of a strike-slip rate of 2.5 to 3.8 mm/yr. Their rate assumes that a 300-m deflection in the ancestral channel to the Los Angeles River, which is incised into a 80 to 120 ka marine terrace sequence, is due entirely to slip on the fault. Ward and Valensise [1994] and Stephenson et al. [1995] both resolved largely lateral slip rates that contradict the conclusions of previous studies, including those that incorporate blind thrust models. In this paper, we present new data [Fugro West, Inc., 1994] on the style and rate of Holocene slip of the Palos Verdes fault. The data were collected offshore, within the Outer Harbor and adjacent areas of the Port of ,Los Angeles (Figure 2), where the harbor bottom slopes to the south-southeast from elevations of about -6 to -14 m. We acquired and analyzed nearly 250 km of new seismic reflection data and drilled 124 borings to collect stratigraphic information and datable material (e.g., shell, wood, and charcoal). The integration of the geophysical records and 8317 8318 MCNEILAN ET AL.' STYLE AND RATE OF SLIP, PALOS VERDES FAULT Santa Monica t-tF • Mountains • ß m...
