Meeting Date: Monday, June 5th, 2017
at the DoubleTree Club Hotel, Santa Ana, CA
University of California, Riverside
Newly identified seismic hazards along the Newport-Inglewood fault zone: Evidence for repeated late Holocene coseismic subsidence events in the Seal Beach saltmarsh
Paleoenvironmental records from the Seal Beach saltmarsh reveal evidence for repeated late Holocene coseismic subsidence events along the Newport-Inglewood fault zone. Field analysis of sediment gouge cores established discrete lithostratigraphic units extend across the saltmarsh. Detailed sediment analyses of vibracores reveal abrupt changes in lithology, percent total organic matter, grain size, and magnetic susceptibility. Diatom and foraminiferal analyses indicate that predominantly freshwater deposits bury relic intertidal deposits at three distinct depths. Radiocarbon dating indicates that the three burial events occurred in the last 2000 calendar years and that two of the three events are contemporaneous with large-magnitude paleoearthquakes along the Newport-Inglewood/Rose Canyon fault system. From these data, and extensive oil and water well data from across the Sunset Gap, it is inferred that during large magnitude earthquakes a step-over along the fault zone results in the vertical displacement of a roughly 5-km2 area that is consistent with the footprint of an estuary identified in pre-development maps. These findings provide new insight on southern California seismic hazards, including coseismic deformation and earthquake recurrence along the Newport-Inglewood/Rose Canyon fault system.