EVIDENCE OF A LARGE EARTHQUAKE AND COASTAL UPLIFT OF THE SAN JOAQUIN HILLS, SOUTHERN LOS ANGELES BASIN, CALIFORNIA, SINCE A.D. 1635

The historic record of earthquakes in California begins in 1769 with an account by Spanish explorers of a strong temblor in the southern Los Angeles basin. Since then, the largest known earthquakes in the Los Angeles (LA) basin were the destructive but moderate-size Long Beach (M 6.3), San Fernando (M 6.7) and Northridge (M 6.7) earthquakes in 1933, 1971 and 1994. A significantly larger earthquake may have occurred in the San Joaquin hills, southern LA basin, just prior to or during the early historic period. We have mapped, measured and dated late Holocene shorelines and marsh deposits that are above the active shoreline or marsh at 20 sites around the coastal margin of the San Joaquin Hills and upper Newport Bay. The paleo-shorelines are 1.6 m (average of 60 measurements) above the active shoreline. Several measurements exceed 3 m along the rocky coastline between Laguna Beach and Dana Point, with a maximum of 3.6 m. The average elevation is greater along the open coast between Corona Del Mar and Dana Point (1.8 m from 39 measurements at 13 sites) than in Newport Bay. The elevated shorelines and emergent marsh deposits are best explained by tectonic uplift of the San Joaquin Hills. Radiocarbon dating of plant material and a shell from an elevated "marsh bench" in Newport Bay indicate that emergence occurred sometime after 1635 A.D. Assuming the uplift occurred in a single earthquake, the magnitude can be estimated from empirical relationships of Wells and Coppersmith (1994) with correction factors for application to paleoseismic data (Hemphill-Haley and Weldon, 1999). The estimated magnitude (M) range is 7.0-7.3 from average (1.6 m) and maximum (3.6 m) uplift. Slip on the fault would be greater than uplift at the surface, so the magnitude may be underestimated.