Abstract: Neotectonics, Holocene Uplift and Earthquake Potential of the San Joaquin Hills, Southern Los Angeles Basin, California

GRANT, LISA, University of California, Irvine, Irvine, CA

The San Joaquin Hills in coastal Orange County, California, are the topographic expression of an anticlinal fold parallel to the seismically active Newport-Inglewood fault zone (NIFZ) at the southern margin of the Los Angeles basin. Mapping of emergent Late Quaternary marine terraces in the San Joaquin Hills and Th-230 dating of solitary corals from the lowest terraces reveal that the San Joaquin Hills have risen at an average rate of 0.21 - 0.27 m/k.y. during the past 122, 000 yrs due to partitioned strike-slip and compressive shortening across the southern NIFZ (Grant et al., 1999). The San Joaquin Hills anticline has significantly greater topographic expression than other anticlines within the Newport-Inglewood structural zone and appears to be underlain by a blind thrust fault. Analysis of Quaternary terraces suggests that the underlying fault dips to the southwest and is capable of generating a M 6.8 - 7.3 earthquake (Grant et al., 1999). Analysis of Late Holocene shorelines on the San Joaquin Hills coast and marsh deposits in Newport Bay (Grant and Ballenger, in progress) reveals 1.0 - 3.6 m of recent tectonic uplift. Radiocarbon dating, historic seismicity and analysis of shorelines suggest that the San Joaquin Hills were uplifted by a M 6.7 - 7.4 earthquake between 1635 A.D. and 1855 A.D. Tectonic uplift of the San Joaquin Hills may present a tsunami hazard as well as an earthquake hazard for southern California coastal communities.

AAPG Search and Discovery Article #90911©2000 AAPG Pacific Section and Western Region Society of Petroleum Engineers, Long Beach, California