Grant, L.B. & Ballenger, L.J.
Paleoseismic Evidence of a Historic Coastal Earthquake and Uplift of the San Joaquin Hills, Southern California
Poster presented at Southern California Earthquake Center Annual Conference 1999 in Palm Springs, California.
The San Joaquin Hills at the southern margin of the Los Angeles Basin have been rising at a rate of 0.21-0.27 m/ka during the last 122 ka. Grant and others (in press) proposed that the most recent uplift occurred during the Holocene due to seismogenic movement of an underlying active blind thrust fault. Their inference was based primarily on data from a 1954 study of salt marsh in upper New port Bay, a late Pleistocene erosional gap in the northern San Joaquin Hills. We mapped, measured, and sampled remnants of emergent marsh deposits in upper Newport Bay and wave-cut platforms and shorelines along the rocky open coast of the San Joaquin Hills. Paleo-shorelines and marine abrasion platforms are elevated up to 3.6 m above the active shoreline. The elevated deposits and platforms are best explained by co-seismic uplift due to a M 6.7 or large earthquake beneathe the San Joaquin Hills. Radiocarbon dating of marsh deposits demonstrates that the most recent uplift and emergence occurred sometime after deposition of plant material and shells circa 1635 A.D. Historical records suggest that the uplift may have been caused by the first reported earthquake in California in 1769 A.D., or possibly by coastal temblors in 1800 or 1855. Unusual sea waves were reported immediately following the 1855 earthquake at Pt. San Juan along the southern coast of the San Joaquin Hills.