Expanding the Cascadia 1700 CE Paleogeodetic Database With Subsidence Estimates From Northern California and Washington

Date: 4/24/2019

Time: 04:30 PM

Room: Vashon

Quantitative relative sea-level reconstructions derived from foraminifera-based Bayesian transfer function analysis yield precise estimates of coseismic vertical deformation from the 1700 CE Cascadia Subduction Zone (CSZ) earthquake. These estimates inform hypothetical rupture scenarios used in seismic and tsunami hazard models. However, the current CSZ 1700 CE paleogeodetic database primarily consists of estimates from Oregon and comprises conspicuous spatial gaps in northern California and Washington. Therefore, strategically placed transfer function investigations within these geospatial gaps afford the opportunity to progress our understanding of Cascadia rupture and inform hazard characterization.

We examine stratigraphic sequences of CSZ 1700 CE earthquake subsidence (abrupt mud-over-peat contacts), from both northern California and Washington. We quantitatively reconstruct relative sea-level rise across stratigraphic contacts by applying a foraminiferal-based Bayesian transfer function to fossil foraminiferal assemblages. At northern Humboldt Bay, California, coseismic subsidence averaged across nine stratigraphic contacts of the 1700 CE earthquake is 0.58 m (±0.46 m). In southwestern Washington we analyzed 1700 CE earthquake contacts at seven sites; Copalis River, Ocean Shores, Chehalis River, Johns River, Smith Creek, Bone River, and Naselle River. At these sites, coseismic subsidence estimates ranged from 0.39 m (±0.37 m) at Johns River to 1.52 m (±0.51 m) at Smith Creek. Within error, subsidence estimates from northern California and Washington generally agree with a recent geophysical rupture model of the CSZ 1700 CE earthquake, which features heterogeneous fault slip distribution along strike. Our new subsidence data broadens the areal extent of the 1700 CE paleogeodetic database and increases the number of measurements by over a third, which will help improve our understanding of the seismogenic behavior of Cascadia megathrust.

Presenting Author: Jason S. Padgett

Authors