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PRESTON, L.A.,, CREAGER, K.C., CROSSON, R.S., VAN WAGONER, T.M., University of Washington, Geophysics Program Box 351650, Seattle, WA 98195 United States, TREHU, A.M., Oregon State University, College of Oceanic and Atmospheric Sciences, Corvallis, OR 98331 United States, BROCHER, T.M., FISHER, M.A., U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025 United States, and SHIPS Working Group

We analyze secondary arrivals from air-gun sources of the 1998 Wet SHIPS experiment which we interpret as wide-angle reflections off the subducting Juan de Fuca plate. A reflecting surface consistent with these travel time observations is determined using a migration algorithm employing 3-D ray tracing. This reflector dips 10 degrees to the east, reaches a depth of 60 km beneath the central Puget Basin and consistently underlies, and parallels, the 5-km thick zone of relocated Wadati-Benioff seismicity. Although less robust, we also observe a weak, intermittent, reflector that lies near the top of the intraplate seismicity. These two reflectors bracket the seismicity, consistent with the lower reflector being the subducted Moho and the upper reflector being the interplate boundary. Thus, in this region, the intraplate earthquakes occur entirely within the subducted oceanic crust, consistent with the earthquakes being associated with the basalt to eclogite phase transition through dehydration embrittlement. Beyond 90 km, reflected amplitudes are typically much larger than direct arrivals indicating the importance of including slab-reflected phases in calculations of strong ground shaking from regional earthquakes, including megathrust events. Amplitudes and polarities vary coherently and dramatically over 1-2 km length scales, suggesting the Moho reflector is complicated by faults or altered by the inhomogeneous transformation of basalt to eclogite. Identification of a relationship between the level of seismic activity and the slab structure could allow us to use future structural studies to evaluate whether we should prepare for large intraslab earthquakes in areas, such as much of Oregon, in which no significant intraslab earthquakes have been observed.


Last Modified: 2011 Aug 10