Debris Avalanches in the Northern California Coast Range Triggered by Plate-Boundary

Determining the timing and cause for ancient hillslope failures is challenging in the western United States, yet it is critical because it can inform ground-motion estimates for large-togreat earthquakes on plate-boundary faults. We identify two preinstumental debris avalanches (3 x 106 and 4:28 x 106 m3) in the Coast Ranges of northern California. These debris avalanches are well suited for studying the timing (to the exact year) and trigger, as the densely forested landscape enables effective use of dendrochronology and high-resolution radiocarbon to pinpoint the time of failure, and the legacy of landslide studies in the region provides context for evaluating a seismic trigger as the most likely triggering mechanism. Neither of the debris avalanches was triggered by the largest precipitation event of the instrumental record in 1964 C.E., and the two debris avalanche sites are physiographically suited to experience topographic amplification of seismic shaking. Through a suite of geologic and dendrochronologic evidence, we establish the likely time of failure of the two debris avalanches as 1906 C.E. and 933 C.E. The failure of the younger landslide coincides with the 1906 C.E. San Francisco earthquake on the Pacific/North American plate boundary, whereas the older landslide dates to 933 C.E., within the broad age range (850-966 C.E.) of a likely megathrust earthquake on the Cascadia subduction zone as recorded in coastal marshes in Humboldt Bay, California. If the older landslide was triggered by a subduction earthquake, then the precise age of the 933 C.E. debris avalanche would trim the uncertainty on a Cascadia subduction zone event from 116 yr to 1 yr. Utilizing debris avalanche records from sites suitable to record seismic shaking improves understanding of plate-boundary earthquake timing and extent of shaking.