SCEC Award Number 14041 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Seal Beach Wetlands: A Potential Paleoseismic and/or Tsunami Record for Southern California
Investigator(s)
Name Organization
Brady Rhodes California State University, Fullerton Matthew Kirby California State University, Fullerton
Other Participants Project will include between 2-4 UG and Graduate students.
SCEC Priorities 1a, 4a SCEC Groups Geodesy, EFP, Geology
Report Due Date 03/15/2015 Date Report Submitted 06/25/2017
Project Abstract
 The Seal Beach wetlands straddle the Newport-Inglewood fault zone within the perimeter of the U.S. Naval Weapons Station Seal Beach. A new paleoenvironmental study on 55 sediment cores collected throughout the wetlands reveals evidence for late Holocene coseismic subsidence events, representing previously unrecog- nized ecological and socioeconomical hazards along the southern California coastline. We present evidence for the coseismic subsidence events based on detailed sedimentological analyses in two vibracores with well- constrained radiocarbon chronostratigraphy. The cores reveal distinct and consistent changes in stratigraphy, percent total organic matter, grain size, magnetic susceptibility, and diatom and foraminiferal assemblages indicating repeated, rapid subsidence and filling of a small basin. Together, the core data indicate that three subsidence events occurred in the wetlands during the past 2000 calendar years. From these data we suggest a releasing step-over along the Newport-Inglewood fault zone results in the vertical displacement of an approx- imately 5-km2 area, consistent with the footprint of an estuary identified in pre-development maps. This study identifies a new seismic hazard in coastal southern California and provides insight to coseismic deformation and earthquake controls on the evolution of the wetlands during the Holocene.
Intellectual Merit This project is one of the first in southern California to evaluate the potential of coast wetland to record an earth- quake chronology. We have found evidence that the Seal Beach Wetlands area was a marine embayment ~ 4,000 cal yrs BP. After formation of intertidal marsh, the wetlands abruptly subsided 3 times in the last 2000 years. After each subsidence event rapid infilling occurred and intertidal marsh formed. We have proposed that the wetlands subsided during large magnitude earthquakes on the NIFZ. This is the first concrete geologic record of earthquake activity on the Newport Inglewood Fault Zone, and provides additional constraints on the seismic hazards of south- ern California.
Broader Impacts 1. Our data suggest that a significant hazard of sudden subsidence may exist for the Seal Beach Wetlands, and the Seal Beach Naval Weapons Station, which stores large numbers of naval weapons including nuclear warheads. Our data suggest that planning for sudden subsidence may be indicated for this installation.
2. Out project has a large educational component. SCEC helped fund 2 completed MS theses and 4 undergraduate theses. Additionally, over 10 additional undergraduate and graduate students assisted with field work and laboratory analyses. Most of these student were woman and/or minorities. Many are currently pursuing advanced degrees.
Exemplary Figure Figure 3. Schematic diagram showing the chronological cycle in the Seal Beach wetlands of marine embayment followed by intertidal saltmarsh accretion, earthquake occurrence (coseismic subsidence) and basin formation, in- filling with allochthonous sediment, and intertidal saltmarsh accretion.