Reaches dominated by debris flow scour and incision tend to greatly influence landscape form in steepland basins. Debris flow networks, despite their ubiquity, have not been exploited to develop erosion rate proxies. To bridge this gap, I applied a proposed empirical function that describes the variation of valley slope with drainage area in fluvial and debris flow reaches of steepland channel networks in the Oregon Coast Range. I calibrated a relationship between profile concavity and erosion rate to map spatial patterns of long-term uplift rates assuming steady state. I also estimated the magnitude and inland extent of coseismic subsidence in my study area. My estimates agree with field measurements in the same area along the Cascadia margin, indicating that debris flow valley profiles can be used to make interpretations from spatial patterns of rock uplift that may better constrain physical models of crustal deformation. This thesis includes unpublished co-authored material.

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. / Doctor of Philosophy / Studying past megathrust earthquakes along the Cascadia Subduction Zone (CSZ) is essential for understanding how these massive events occur and what they mean for future seismic hazards. When these great (>M 8.5) earthquakes happen, they cause rapid shifts in coastal land levels, which can be seen in tidal wetlands as sharp layers of peat and mud. By studying the microfossil found in these layers, we can accurately measure the amount of land subsidence caused by the earthquakes. Diatoms are a microscopic algae that lives in all coastal environments, and different groups of diatoms can inform us of past environments, and environmental changes. This research focuses on a new method using diatom microfossils to measure past earthquake-induced land subsidence. The goal is to improve our understanding of how CSZ earthquakes behave over time and across different areas. In Chapter 1, we look at how land subsidence has varied over multiple earthquake cycles at a single location in southern Oregon, aiming to determine whether subsidence changes with each earthquake. In Chapter 2, we measure land subsidence along the entire Cascadia margin during a single earthquake rupture and examine how it varies across the region. By studying these changes, we can better predict future earthquake risks and improve our understanding of the CSZ's long-term behavior.