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Diatom-Based Coseismic Subsidence Estimates Spanning a ~4,500 Year History of Cascadia Subduction Zone Ruptures Along the Southcentral Coast of Oregon

Description: 

Estimating coseismic subsidence produced by past Cascadia Subduction Zone (CSZ) megathrust earthquakes is crucial for understanding rupture dynamics, particularly magnitudes of paleoseismic slip. Past CSZ earthquakes are expressed in tidal wetland stratigraphy as sharp contacts between saltmarsh peat and overlying intertidal mud, indicating rapid coseismic subsidence. Recent statistical methods (transfer functions) use microfossils (e.g., foraminifera, diatoms) to quantify coseismic subsidence across these abrupt peat-mud contacts. Transfer functions estimate the amount of coseismic subsidence by analyzing land/sea-level relationships before and after earthquakes. While foraminifera-based estimates reveal variability in subsidence along-strike during the 1700 CE CSZ rupture, equivalent foraminifera-based estimates for pre-1700 CE earthquakes are scarce. Tidal wetlands along the Coquille River estuary in southcentral Oregon preserve evidence of 12 CSZ ruptures over the last ~7,000-years, but preliminary work indicates low numbers of calcareous foraminifera in deposits older than the 1700 CE rupture, complicating reconstructions of subsidence. In this study, we use diatoms, a microscopic algae with resilient biosiliceous hard parts, as a complementary alternative for subsidence reconstructions due to their preservation potential in low pH environments. Using a diatom-based transfer function, we analyze seven CSZ earthquake contacts (1700 CE to ~4,500 yr BP) from Fahys Creek, a Coquille River tributary. Additionally, we present new (between 3-6) radiocarbon age dates for CSZ earthquake contacts, to aid in earthquake correlation. Our diatom analysis reveals variability in subsidence estimates over multiple earthquake cycles, suggesting variable rupture patterns for successive CSZ ruptures. The new subsidence estimates deepen our understanding of the heterogeneous nature of CSZ ruptures through time. Future work will entail combining estimates from Fahys Creek with estimates for pre-1700 CE CSZ earthquakes at other sites to build a comprehensive examination of along-strike rupture characteristics of the CSZ.

Session: From Faults to Fjords: Earthquake Evidence in Terrestrial and Subaqueous Environments - IV

Type: Oral

Date: 5/1/2024

Presentation Time: 04:30 PM (local time)

Presenting Author: David

Student Presenter: Yes

Invited Presentation: 

Authors