Assessing local impacts of the A.D. 1700 Cascadia earthquake and tsunami using tree ring growth histories: A case study in South Beach, Oregon, U.S.A.

Abstract

Abstract. We present a spatially focused investigation of the disturbance history of an old-growth Douglass fir stand in South Beach, Oregon for possible growth effects due to tsunami inundation caused by the A.D. 1700 Cascadia subduction zone earthquake. A high-resolution model of the 1700 tsunami run-up heights at South Beach, assuming an L sized earthquake, is also presented to better estimate the inundation levels several kilometers inland at the old-growth site. This tsunami model indicates the South Beach fir stand would have been subjected to local inundation depths from 0–10 m. Growth chronologies collected from the fir stand shows several trees experienced significant growth reductions before, during and several years after 1700, consistent with the tsunami inundation estimates. The +/-1–3 year timing of the South Beach disturbances are also consistent with disturbances previously observed at a Washington state coastal forest ~220 km to the north. Additional comparison of the South Beach chronologies with regional chronologies across Oregon indicates the South Beach stand growth was significantly and unusually lower in 1700. Moreover, the 1700 South Beach growth reductions were not the largest over the 110-year tree chronology at this location. with other disturbances likely caused by other climate drivers (e.g. drought or windstorms). Our study represents a first step in using tree growth history to ground-truth tsunami inundation models by providing site specific physical evidence.