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Introduction
   

The Pacific Northwest, extending from British Columbia down the coast to northern California, is situated on the Cascadia subduction zone. This zone occurs where the Juan de Fuca oceanic tectonic plate is being pulled under the North American continental plate (Figure 1). The interaction between the plates causes large earthquakes (magnitude > 7) to occur in the region. These earthquakes can generate large tsunamis which pose a significant threat to the Pacific Northwest.

The most recent tsunami to affect the region occurred in 1964. Geomorphic signs of flooding preserved in estuaries and coastal lakes in Oregon indicate as many as 13 tsunamis triggered by large earthquakes over the past 4,600 years (Jol and Peterson, 2006). Both the 1964 (Alaska earthquake) tsunami and the 1700 AD Cascadia tsunami left extensive sand sheets on the coastal landscape. These sand sheets can be studied to better understand the hydrodynamic properties of the inundating waves and therefore better understand the risk posed by future tsunamis (Jol and Peterson, 2006).

Research previously completed at Seaside, Oregon indicated that ground penetrating radar (GPR) can be used to identify paleotsunami sand sheets, including the thickness and internal stratigraphy of the deposits (Jol and Peterson, 2006). Representative tsunami deposits, near the GPR lines of this study, have been reported from previous research in Seaside (Darienzo et al., 1993; Fiedorowicz, 1997) and Cannon Beach (Peterson et al., in progress).

This thesis presents GPR data collected to investigate the extent of paleotsunami inundation and the magnitude of wave run-up in Cannon Beach and Seaside in northern Oregon (Figure 1). The vertical and lateral extent of the tsunami deposits can be determined by analyzing and interpreting the collected datasets using radar stratigraphic analysis (Jol and Bristow, 2003). Determining the vertical and lateral extent of the tsunami deposits will allow a better understanding of the tsunami processes associated with Cascadia subduction zone earthquakes and improve tsunami hazard assessment for the coastlines of the Pacific Northwest. With an estimated 39.4 million people living in coastal counties in the Pacific region in 2003 and this number rising, an understanding of tsunami processes is critical (Crossett et al., 2004).

Figure 1. Location map of the study sites, Cannon Beach and Seaside, Oregon shown on left. The sites are situated on the Cascadia subduction zone, shown on right (Cascadia Regional Earthquake Workgroup, 2006).
       
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