ABRUPT SUBSIDENCE IN THE SEAL BEACH WETLANDS, SOUTHERN CALIFORNIA

The Seal Beach Wetlands (SBW) are located within the perimeter of the U.S. Naval Weapons Station Seal Beach, in southern California. As part of a study of the late Holocene sedimentary deposits in the SBW, we collected a set of 3 to 5-m long sediment cores and identified multiple buried organic-rich layers. The buried organic-rich layers suggest that the surface of the marsh episodically and rapidly subsided below sea level. Because the SBW straddle the Newport-Inglewood fault zone (NIFZ), we hypothesize that the surface of the wetlands abruptly subsides during large magnitude earthquakes. Primary analysis of three vibra-cores and one piston core reveals abrupt changes in % total organic matter, algal (diatom) remains, and grain size indicating that at least 3 times in approximately the last 2100 years the ground surface subsided abruptly between 40 cm and 100 cm. The location of the subsidence suggests it is a result of coseismic subsidence along the NIFZ. Such vertical movements of the wetlands during future earthquakes along the NIFZ may present serious hazards to the U.S. Naval Weapons Station Seal Beach and the surrounding communities. Results of this study provide insight into the evolution of the SBW during the late Holocene, earthquake recurrence, coseismic deformation, and the seismic hazard associated with the NIFZ in southern California.