Hazard identification and coastal stratigraphy in Crescent Harbor and Dugualla Bay, northeast Whidbey Island

Brian Ostrom
May 2015

Problem, Hypothesis and Objectives

The Puget Sound region, in the forearc of the North American plate above the Cascadia subduction zone, is simultaneously home to several million people and vital infrastructure including ports and military bases, and to a dynamic and seismically active geologic setting (Figures 1 and 2). In the short historical record of the Puget Sound region, there have been only three earthquakes greater than magnitude 6.0. These events were deep, yet resulted in a total of 15 deaths, extensive property damage, and damage to infrastructure. Characterizing the complete seismic hazard in the Puget Lowland requires the study of the primary and secondary effects of paleoearthquakes because there have been no large, historic, shallow, crustal-fault events (or subduction-interface events). For example, a study by Johnson et. al. (2004) on the Utsalady Point fault indicates northwestern Whidbey Island primarily characterized seismic hazards associated with the Naval Air Station, the Seaplane Base, the town of Oak Harbor, and the Deception Pass bridge, which is Whidbey Island’s only land connection to the mainland. The objective of this study is refine our understanding of seismic hazards that pose a threat to northern Whidbey Island by investigating the paleoseismic history of northeastern Whidbey Island and comparing the results to Johnson et al. (2004). My study will focus on two study areas in northeastern Whidbey Island, Crescent Harbor and Dugualla Bay (Figure 3).

Whidbey Island is crosscut by two main fault zones, the Devils Mountain and the South Whidbey Island fault zones, along with the smaller Utsalady Point and Strawberry Point faults (Hayward et. al. 2006; Kelsey et. al. 2004; Johnson et. al. 2004)(Figs. 2 and 3). Any of these faults could pose a significant hazard to the residents and infrastructure of Whidbey Island, including shaking caused by an earthquake, and associated land-level change, liquefaction, and tsunamis.

In order to completely assess the seismic hazard of northern Whidbey Island, it is crucial to determine if the area was affected by the 900-930 AD Seattle Fault tsunami. If this tsunami did reach northern Whidbey Island, a tsunami from a similar rupture in the future could damage infrastructure, property and the economy. Once this study is complete, the information can be utilized by emergency management officials in Island County and by Emergency Operations officials at Naval Air Station (NAS) Whidbey, both of whom acknowledge that the hazards presented could have a devastating effect on the infrastructure and safety of the area (Island County, 2007).

Full Thesis Proposal (PDF format)