Paleoseismology and geomorphology of the Raymond fault, Los Angeles County, California

The Raymond fault extends for 20 km in a broad convex-to-the-south are acro ss the densely populated San Gabriel Valley northeast of Los Angeles. Altho ugh south-facing scarps characterize much of the fault, geomorphic and seis mological evidence indicate a predominance of left-lateral motion. An appar ent 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 similar to1-2 ka. Estimates of fault-plane area suggest that rupture of the entire Raymon d fault could generate an earthquake of M-w similar to 6.7. The similar kin ematics of the Raymond fault and the Hollywood fault, along strike to the w est, 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 similar to6-9 ka, ind icating that the most recent Raymond fault surface rupture apparently did n ot extend westward onto the Hollywood fault. Data from another of our paleo seismologic trenches yielded evidence for at least five latest Pleistocene earthquakes, including at least four surface ruptures that occurred during a brief, less than or equal to 10,000-year-long period between similar to 3 1.5 and 41.5 ka. The less than or equal to 3300 year-long, average recurren ce interval for these events is much shorter than the interval suggested fo r the past less than or equal to 40,000 years by the frequency of paleoeart hquakes recognized in previous trenches. Thus, either the 31.5 ka to 31.5 k a events represent a temporal cluster, and the recurrence interval for the fault is highly irregular, or at least half of all Raymond fault earthquake s that have occurred since similar to 31.5 ka have not yet been recognized.