Most Recent Rupture on the Boulder Creek Fault Triggered Bedrock Landsliding in the Nooksack Watershed, Whatcom County, Washington
Portland State University. Department of Geology
Term of Graduation
Date of Publication
Master of Science (M.S.) in Geology
1 online resource (vi, 59 pages)
Catastrophic bedrock landslides with volumes ranging from 106 to 108 m3 contribute to rapid landscape evolution, often resulting in erosion rates that exceed the long-term average rates caused by tectonic uplift. Understanding the spatiotemporal trends of large bedrock landslides helps us understand previous drivers of landscape evolution as well as predict how the landscape will respond in the future. The Nooksack watershed, Whatcom County, Washington, is particularly susceptible to large slope failures because of its high relief, seismic activity, local geology, and relatively abundant precipitation. Specifically, folded and faulted bedrock structures conducive to landsliding in addition to recent surface rupturing earthquakes on the Boulder Creek fault are both probable mechanisms for widespread landsliding. To determine the relative importance of these driving mechanisms, we investigate spatiotemporal trends of 447 landslides in the Nooksack Watershed using a calibrated relationship between lidar based surface roughness and age. We compare the temporal patterns in the overall landslide chronology to simulated landslide frequency histories with and without incorporated coseismic landslide pulses at the times of the two most recent earthquakes on the Boulder Creek fault. We assess spatial patterns by conducting a regional kinematic analysis to define areas susceptible to planar sliding and toppling failures. We find surface roughness values of bedrock landslides in Washington's Cascade Range are consistent with roughness values determined for bedrock landslides in the Oregon Coast Range. Our resulting landslide frequency history best matches a simulated landslide frequency history that incorporates preservation bias as well as coseismic landslide pulses at the times of geologically constrained surface rupturing earthquakes on the Boulder Creek fault in the last 4000 years. Our regional kinematic analysis demonstrates almost half the landslides in our inventory overlay hillslopes where planar sliding and/or flexural toppling are kinematically feasible. These findings together support the conclusion that surface rupturing earthquakes and bedrock orientations, specifically those conducive to flexural toppling, primarily control the spatial and temporal distributions of landslides throughout the Nooksack watershed. These findings are important for hazard assessment and planning throughout the region and suggest the evolution of the Nooksack watershed is heavily influenced by coseismic landslides.
© 2022 Abigail Catherine Underwood
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Underwood, Abigail Catherine, "Most Recent Rupture on the Boulder Creek Fault Triggered Bedrock Landsliding in the Nooksack Watershed, Whatcom County, Washington" (2022). Dissertations and Theses. Paper 6271.