Study reveals wetlands are susceptible to rapid lowering in elevation during large earthquakes

Fullerton faculty-student at California State University found the evidence of sudden sinking of the wetlands at the National Wildlife Refuge Seal Beach, California. It is caused by ancient earthquakes that shook the area at least three times in the past 2,000 years. Researchers believes that it could happen again.

The 500- acre wetlands located within the Naval Weapons Station Seal Beach are susceptible to rapid lowering in elevation during large 7.0 magnitude-earthquakes.

Talking about the new study CSUF professor of geological sciences, Matthew E. Kirby said

“Imagine a large earthquake—and it can happen again—causing the Seal Beach wetlands to sink abruptly by up to three feet. This would be significant, especially since the area already is at sea level,”

The study “Evidence for Coseismic Subsidence Events in a Southern California Coastal Saltmarsh,” is published in Scientific Reports, an open-access, peer-reviewed Nature research journal.

Located off Pacific Coast Highway between Belmont Shores and Sunset Beach, the Seal Beach wetlands likely formed due to complex, lateral movement of the Newport-Inglewood fault, said Leeper,a doctoral student in the earth sciences program at University of California, Riverside, is the lead author of the paper. The wetlands straddle a segment of the fault system, which extends from Beverly Hills in the north to the San Diego region in the south.

“These research findings have important implications in terms of seismic hazard and risk assessment in coastal Southern California and are relevant to municipal, industrial and military infrastructure in the region,” added Leeper, a former USGS geologist whose work focused on natural hazards. He recently left the scientific agency to concentrate on his doctoral studies.

This new study stems from National Science Foundation-funded research on past occurrences of tsunamis along Southern California’s coastal wetlands that Kirby and Rhodes began in 2012. As an undergraduate, Leeper joined their study, which turned up no evidence of previous tsunamis in Orange County or the region.

Soil samples analyzed in Kirby’s lab from mud cores collected from the Seal Beach wetlands, combined with the study of microscopic fossils to identify the past environment, pointed the researchers in a new direction. The analyses revealed buried wetland surface layers, signaling evidence of sinking in the area from past massive earthquakes.

“Since that epiphany in 2013, our research evolved and has involved many other collaborators, each providing a skill or expertise that helped to develop our conclusions,” Kirby said.

CSUF’s Carlin, his students and Leeper are continuing to study the Seal Beach wetlands to further investigate potential seismic hazards, as well as the poorly understood Newport-Inglewood fault system.

“We’re looking to identify other past earthquake events in the sediment record from other cores from the wetlands,” Carlin said. “The goal is to get a better understanding of how often earthquakes may have occurred in the past, the hazards associated with this fault—and the probability of the next earthquake.”

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