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Seismic reflection and geomorphic characterization of the onshore Palos Verdes fault zone, Los Angeles, California

Abstract

Two high-resolution (25- to 110-Hz bandwidth) seismic-reflection profiles acquired along roads crossing the Palos Verdes fault zone, Los Angeles basin, California, strongly suggest that some geomorphic lineaments seen on aerial photographs coincide with steeply dipping faults. At roughly the midpoint of the Palos Verdes peninsula, the Palos Verdes fault zone consists of at least three major strands. One of the northern strands, which has the most apparent vertical displacement, is interpreted to dip 55° SW down to 200 m, steepening at greater depth. Other strands appear to dip from 50° to 90°. Just 3 km to the southeast, however, the fault zone is composed of at least five strands and is roughly 1.5-km wide. Apparently, younger strands are located northeast of older, less active strands across the fault zone. Recent right-lateral offset on the northeastern fault strands is inferred from a deflection of an ancestral Los Angeles River channel, approximately 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 approximately 300 m. This deflection, if attributed solely to slip on the Palos Verdes fault zone, suggests an average horizontal slip rate between 2.5 and 3.8 mm/yr. Because very little vertical displacement is detectable in the seismic data acquired near the channel offset that also shows little vertical displacement, we infer that recent onshore movement in the Palos Verdes fault zone on the eastern Palos Verdes peninsula has been predominantly strike slip.

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