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Precisely Dating Seismically Triggered Debris Avalanches in the Northern California Coast Range

Description: 

Determining the timing and cause for prehistoric hillslope failures has proven difficult in the western US, yet critical as it ties directly into ground motion estimates for future hazardous events. This knowledge gap is important to confront, however, as these avalanche failures are candidates to have been triggered by earthquakes along active plate boundaries, where sources include crustal faults, plate boundaries, and intraslab events. Here, we identify two exceptionally large prehistoric debris avalanche failures in the Coast Ranges of northern California that are well suited for studying the timing (to the exact year) and cause (what triggered the failure) as the densely forested landscape enables the effective use of dendrochronology and high-resolution radiocarbon. This high precision geochronology allows us to identify time of failure, and the legacy of landslide studies in the region provides context for evaluating a climatic versus a seismic trigger as the most likely failure mechanism. We find that the two debris avalanche sites are physiographically suited to accommodate topographic amplification of seismic shaking, and through a suite of multiproxy evidence, we establish that likely time of failure of the two debris avalanches to be 1906 CE and 933 CE, respectively. In the first instance, the year 1906 CE is the year of the San Francisco earthquake on the Pacific/North American plate boundary, and in the second instance, the year of failure, 933 CE, falls within the broad age range (850-966 CE) that probably includes the year of the antepenultimate earthquake on the Cascadia subduction zone as recorded in coastal marshes in Humboldt Bay, California. The precise age on the 933 CE debris avalanche could trim down the uncertainty on a Cascadia subduction zone event from 116 years to one year. Utilizing debris avalanche records from sites suitable to record seismic shaking improves understanding of plate-boundary earthquake timing and extent of shaking.

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

Type: Oral

Date: 5/1/2024

Presentation Time: 03:00 PM (local time)

Presenting Author: Harvey

Student Presenter: No

Invited Presentation: 

Authors