SEISMIC-REFLECTION AND GEOMORPHIC CHARACTERIZATION OF THE ONSHORE PALOS-VERDES FAULT ZONE, LOS-ANGELES, CALIFORNIA

Two high-resolution (25- to 110-Hz bandwidth) seismic-reflection profi les acquired along roads crossing the Pales Verdes fault zone, Los Ang eles basin, California, strongly suggest that some geomorphic lineamen ts seen on aerial photographs coincide with steeply dipping faults. At roughly the midpoint of the Pales Verdes peninsula, the Pales Verdes fault zone consists of at least three major strands. One of the northe rn strands, which has the most apparent vertical displacement, is inte rpreted to dip 55 degrees SW down to 200 m, steepening at greater dept h. Other strands appear to dip from 50 degrees to 90 degrees. Just 3 k m to the southeast, however, the fault zone is composed of at least fi ve strands and is roughly 1.5-km wide. Apparently, younger strands are located northeast of older, less active strands across the fault zone . Recent tight-lateral offset on the northeastern fault strands is inf erred from a deflection of an ancestral Los Angeles River channel, app roximately 1.8 km west of the Los Angeles Harbor. The channel incises deposits, dated to be as young as 120 to 80 ka, and is deflected appro ximately 300 m. This deflection, if attributed solely to slip on the P ales Verdes fault zone, suggests an average horizontal slip rate betwe en 2.5 and 3.8 mm/yr. Because very little vertical displacement is det ectable in the seismic data acquired near the channel offset that also shows little vertical displacement, we infer that recent onshore move ment in the Pales Verdes fault zone on the eastern Pales Verdes penins ula has been predominantly strike slip.