Evidence for Holocene coseismic subsidence during a non- plate boundary earthquake

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

Published August 11, 2017, SCEC Contribution #7490, 2017 SCEC Annual Meeting Poster #126

Coseismic subsidence is a coastal hazard commonly associated with subduction zones. Although discrete coseismic events have also been proposed for tectonic basins and synclines along non-plate boundary active margins, confidently differentiating the signal of true coseismic subsidence from nonseismic processes such as floods, eustatic sea level or sediment flux-driven transgressions, local morphological changes such as barrier breaching, and anthropogenic-induced subsidence is difficult. Here we show lithological, geochemical, and fossil evidence for an abrupt subsidence event in Carpinteria Marsh, Carpinteria California at 1.0 ± 0.1 ka. At an elevation of -2 to -4 m throughout the estuary, an organic-rich mud containing marsh invertebrates is sharply overlain by a blue-gray sand containing articulated, intertidal to subtidal shells. We interpret this contact to represent up to 3.47 ± 0.5 m of coseismic compaction and subsidence due to movement along the Rincon Creek Fault; an amount similar in magnitude to that observed along subduction zones. The age of the subsidence event at 1.0 ka in Carpinteria overlaps in time with the age of a large uplift event documented at Pitas Point, Ventura. Correlation between the two events may support the potential for large scale ruptures within the Ventura/Santa Barbara regions.

Key Words
subsidence, estuary, Ventura

Citation
Reynolds, L. C., Simms, A. R., Rockwell, T. K., Yokoyama, Y., Miyairi, Y., & Hangsterfer, A. (2017, 08). Evidence for Holocene coseismic subsidence during a non- plate boundary earthquake. Poster Presentation at 2017 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology