Paleoseismology and Geomorphology of the Raymond Fault, Los Angeles County, California

by Kristin D. Weaver** and James F. Dolan

Abstract

The Raymond fault extends for 20 km in a broad convex-to-the-south arc across the densely populated San Gabriel Valley northeast of Los Angeles. Although south-facing scarps characterize much of the fault, geomorphic and seismological evidence indicate a predominance of left-lateral motion. An apparent 3.4-km left-lateral offset of a crystalline basement ridge at the east end of the fault may approximate total slip on the fault. Data from one of our paleoseismologic trenches, in combination with published data, indicate that the most recent Raymond fault surface rupture occurred ~1-2 ka. Estimates of fault-plane area suggest that rupture of the entire Raymond fault could generate an earthquake of M_w ~ 6.7. The similar kinematics of the Raymond fault and the Hollywood fault, along strike to the west, together with a pronounced fault-parallel gravity lineament and sparse geomorphic evidence, suggests that there may be a through-going mechanical connection between the two faults, and that they could conceivably rupture together in large earthquakes. Published data, however, indicate that the Hollywood fault ruptured to the surface most recently ~6-9 ka, indicating that the most recent Raymond fault surface rupture apparently did not extend westward onto the Hollywood fault. Data from another of our paleoseismologic trenches yielded evidence for at least five latest Pleistocene earthquakes, including at least four surface ruptures that occurred during a brief, ≤10,000-year-long period between ~31.5 and 41.5 ka. The ≤3300 year-long, average recurrence interval for these events is much shorter than the interval suggested for the past ~40,000 years by the frequency of paleoearthquakes recognized in previous trenches. Thus, either the 31.5 ka to 41.5 ka events represent a temporal cluster, and the recurrence interval for the fault is highly irregular, or at least half of all Raymond fault earthquakes that have occurred since ~31.5 ka have not yet been recognized.

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