Sedimentary response of a structural estuary to Holocene coseismic subsidence

Laura C. Reynolds, Alexander R. Simms, Thomas K. Rockwell, Yusuke Yokoyama, Yosuke Miyairi, & Alexandra Hangsterfer

Published December 23, 2021, SCEC Contribution #7942

Coseismic subsidence is a recurring hazard along coastlines associated with subduction zones. However, few studies have unequivocally demonstrated coseismic subsidence along other types of active margins. Here we show lithological, geochemical, and biological evidence for an abrupt subsidence event in an estuary along the highly-populated southern California coastline at 1.0 ± 0.1 ka. At an elevation of -2 to -4 m throughout the estuary, an organic-rich mud containing marsh invertebrates and geochemical signatures indicative of terrestrial influence is sharply overlain by a blue-gray sand containing intertidal and subtidal invertebrates and geochemical signatures of marine influence. Using relative sea-level indicators, we interpret this contact to represent up to 3.47 ± 0.5 m of coseismic subsidence, sediment compaction, and subsequent erosion. This event suggests that coseismic subsidence is not confined to coastlines near subduction zones but is also possible along other types of active margins including the densely-populated California coast.

Key Words
Coseismic subsidence, Estuary, Earthquake, Sea Level, Rincon Creek Fault, Ventura Avenue Anticline, Pitas Point Thrust

Citation
Reynolds, L. C., Simms, A. R., Rockwell, T. K., Yokoyama, Y., Miyairi, Y., & Hangsterfer, A. (2021). Sedimentary response of a structural estuary to Holocene coseismic subsidence. GSA Bulletin, 134(7-8), 2037-2050. doi: 10.1130/B35827.1.

Related Projects & Working Groups
Ventura Special Study Area