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Compilation and Assessment of Data Quality for Onshore and Offshore Paleoseismic Proxies of Great Cascadia Megathrust Rupture

Description: 

The quality of paleoseismic data for megathrust rupture along the Cascadia subduction zon0 (CSZ)e collected over the past three decades varies because analytical capabilities and data collection methodologies have evolved. Since 2019, the USGS Powell Center Cascadia earthquake hazards working group compiled published onshore and offshore paleoseismic data along the 1100-km long CSZ. Here we present our progress, spanning sites from Vancouver Island to the Mendocino triple junction, including a systematic reanalysis of coastal radiocarbon chronology for inferred megathrust ruptures. Evidence for megathrust rupture includes coastal land-level change, tsunami inundation, onshore shaking proxies such as landslides or liquefaction, and offshore shaking proxies such as marine turbidites. As part of the compilation, we also present a ranking scheme to assess the quality of age estimates and evidence for great megathrust rupture. With the age ranking scheme, we ask: "How well is a proposed paleoseismic event dated?" based on the materials and methods used. With the evidence ranking scheme, we ask: "How confident are we that a proposed event is, in fact, the result of a Cascadia megathrust rupture?" based on the sedimentological characteristics, correlation, and mapping. The evidence ranking scheme also helps to evaluate possible alternative mechanisms for creating paleoseismic evidence such as crustal fault, intraslab, or distant tsunamigenic earthquake. We envision the compilation as a “one-stop-shop” for paleoseismic data in Cascadia that was compiled and evaluated by a diverse group of researchers and designed with sufficient detail to be useful for a wide audience. For example, explicit values of coastal deformation with uncertainty, tsunami inundation, and ground motion constraints may be used by modeling colleagues to assess variability in past rupture characteristics. The compilation also identifies data gaps and future research opportunities.

Session: Constraining Seismic Hazard in the Cascadia Subduction Zone

Type: Oral

Date: 4/20/2023

Presentation Time: 03:00 PM (local time)

Presenting Author: Lydia M. Staisch

Student Presenter: No

Invited Presentation: 

Authors