Skip to main content

Research Repository

Advanced Search

Timing and amount of southern Cascadia earthquake subsidence over the past 1700 years at northern Humboldt Bay, California, USA

Padgett, JS; Engelhart, SE; Kelsey, HM; Witter, RC; Cahill, N; Hemphill-Haley, E

Timing and amount of southern Cascadia earthquake subsidence over the past 1700 years at northern Humboldt Bay, California, USA Thumbnail


Authors

JS Padgett

HM Kelsey

RC Witter

N Cahill

E Hemphill-Haley



Abstract

Stratigraphic, lithologic, foraminiferal, and radiocarbon analyses indicate that at least four abrupt mud-over-peat contacts are recorded across three sites (Jacoby Creek, McDaniel Creek, and Mad River Slough) in northern Humboldt Bay, California, USA (∼44.8°N, −124.2°W). The stratigraphy records subsidence during past megathrust earthquakes at the southern Cascadia subduction zone ∼40 km north of the Mendocino Triple Junction. Maximum and minimum radiocarbon ages on plant macrofossils from above and below laterally extensive (>6 km) contacts suggest regional synchroneity of subsidence. The shallowest contact has radiocarbon ages that are consistent with the most recent great earthquake at Cascadia, which occurred at 250 cal yr B.P. (1700 CE). Using Bchron and OxCal software, we model ages for the three older contacts of ca. 875 cal yr B.P., ca. 1120 cal yr B.P., and ca. 1620 cal yr B.P. For each of the four earthquakes, we analyze foraminifera across representative mud-over-peat contacts selected from McDaniel Creek. Changes in fossil foraminiferal assemblages across all four contacts reveal sudden relative sea-level (RSL) rise (land subsidence) with submergence lasting from decades to centuries. To estimate subsidence during each earthquake, we reconstructed RSL rise across the contacts using the fossil foraminiferal assemblages in a Bayesian transfer function. The coseismic subsidence estimates are 0.85 ± 0.46 m for the 1700 CE earthquake, 0.42 ± 0.37 m for the ca. 875 cal yr B.P. earthquake, 0.79 ± 0.47 m for the ca. 1120 cal yr B.P. earthquake, and ≥0.93 m for the ca. 1620 cal yr B.P. earthquake. The subsidence estimate for the ca. 1620 cal yr B.P. earthquake is a minimum because the pre-subsidence paleoenvironment likely was above the upper limit of foraminiferal habitation. The subsidence estimate for the ca. 875 cal yr B.P. earthquake is less than (<50%) the subsidence estimates for other contacts and suggests that subsidence magnitude varied over the past four earthquake cycles in southern Cascadia.

Citation

Padgett, J., Engelhart, S., Kelsey, H., Witter, R., Cahill, N., & Hemphill-Haley, E. (2021). Timing and amount of southern Cascadia earthquake subsidence over the past 1700 years at northern Humboldt Bay, California, USA. GSA Bulletin, 133(9-10), 2137-2156. https://doi.org/10.1130/b35701.1

Journal Article Type Article
Acceptance Date Nov 19, 2020
Online Publication Date Feb 12, 2021
Publication Date Sep 1, 2021
Deposit Date Dec 14, 2020
Publicly Available Date Mar 28, 2024
Journal Geological Society of America Bulletin
Print ISSN 0016-7606
Electronic ISSN 1943-2674
Publisher Geological Society of America
Peer Reviewed Peer Reviewed
Volume 133
Issue 9-10
Pages 2137-2156
DOI https://doi.org/10.1130/b35701.1

Files





You might also like



Downloadable Citations