Scientists from GEOMAR, the U.S. Geological Survey (USGS), and Oregon State University recently collected seismic reflection and wide-angle seismic data for the continental margin off the Oregon and Washington coasts—a subduction zone that poses a significant earthquake hazard to populated areas of the Pacific Northwest. Geologic findings indicate that great earthquakes (magnitude 8 to 9) have occurred within the Cascadia subduction zone and that an earthquake of this magnitude could some day devastate urban areas of the Pacific Northwest. One such quake may have rocked this region as recently as 300 years ago, and the debate concerning the imminence of the next one continues [Heaton and Kanamori, 1984; Atwater, 1992; Hyndman and Wang, 1993; Wang et al., 1995; Satake et al., 1996]. Potential earthquake source regions along the interplate decollement are unexplored. In fact, the geometry of the downgoing Juan de Fuca plate beneath Oregon and Washington has been poorly understood because of the paucity of shallow (<30 km) earthquakes.