Nearshore records of natural hazards of the past millennium, western Washington

Abstract

Since earthquakes cannot be predicted, assessment of earthquake and fault-related hazards relieson knowledge of fault behavior in historic and prehistoric times paired with models based on the physics of how faults work. Historic records of earthquakes in many parts of the world are limited; in these places, paleoseismology extends the earthquake record into the geologic past. In this dissertation, I used field geology and geomorphology at three sites in western Washington to identify, evaluate, and date evidence for earthquakes and lay groundwork for future paleoseismic studies. In chapter 1, submerged shorelines across the Seattle Fault Zone, a crustal fault that crosses Seattle, indicate that the fault produced earthquakes as large as M~7.5 only once in the past 11,000 years, a longer recurrence interval than used in current hazard estimates. In chapter 2, liquefaction records from the Duwamish estuary in Seattle contribute to the regional earthquake catalog and demonstrate that liquefaction incompletely records regional earthquakes. In chapter 3, I mapped and dated a low-elevation terrace at Rialto Beach on the Pacific coast of Washington, one of many such terraces along the coast, that has been hypothesized to record tectonic uplift. I found that the terrace is ~600–200 years old and that neither formation by tectonic uplift nor formation by beach progradation can be ruled out. The stratigraphic framework developed at Rialto Beach and presented here can be used to evaluate the other terraces to test the hypothesis that these terraces formed via coseismic uplift during Cascadia subduction zone earthquakes. I also found that deep-seated landslides on the slope above the Rialto Beach terrace pre-date the terrace formation, which has implications for their potential use in paleoseismic studies. Findings from all three of these studies will contribute to growing catalog of paleoseismic data in Cascadia that are used to reconstruct earthquake histories and evaluate behavior of the megathrust and crustal faults.