Quantifying coseismic land-level change along the Cascadia subduction zone in central Oregon

Understanding the behavior of past Cascadia Subduction Zone (CSZ) megathrust earthquakes is crucial for assessing rupture dynamics and predicting future seismic hazards. Estimates of the magnitude of coseismic subsidence produced by these great (>M 8.5) earthquakes provide critical constraints for rupture and hazard models. Coastal subsidence from these earthquakes is preserved in tidal wetland stratigraphy, where sharp contacts between peat and intertidal mud indicate rapid, earthquake-induced shifts in land elevation. Transfer functions (TFs) using microfossil assemblages (e.g., diatoms, foraminifera) from these layers allow for precise reconstructions of coseismic subsidence. The two studies in this dissertation employ a new diatom-based transfer function for quantifying coseismic subsidence, advancing our understanding of CSZ earthquake characteristics over both temporal and spatial scales. In Chapter 1, we analyze subsidence over multiple earthquake cycles at a single location in south-central Oregon, seeking to determine if earthquake-induced subsidence varies over time. In Chapter 2, we quantify subsidence along the CSZ margin across earthquake contacts correlated to a single rupture, examining the variability of subsidence along the margin. By capturing this variability, this research improves our understanding of past rupture patterns, which in turn enhances seismic hazard assessments for the Cascadia margin.