Seismic Hazards of Puget Sound (SHIPS): Collaborative Research with U.S. Geological Survey, Oregon State Un., Un. of Texas at El Paso, Un. of British Columbia, Un. of Washington, Un. of Victoria, Pacific Geo- science Center Project Number: 1434HQ98GR00041 Anne M. Trehu College of Oceanic and Atmospheric Sciences Oregon State University Ocean Admin. Bld. 104 Corvallis, OR 97331-5503 541-737-2655 (voice) 541-737-2064 (fax) trehu@oce.orst.edu http://quakes.oce.orst.edu Program Elements: I and II Key words – seismology, seismotectonics, regional seismic hazards Introduction The heavily populated Puget Basin is underlain by thick sequences of Cenozoic sedimentary rocks that amplify and focus seismic energy, thus increasing ground shaking during an earthquake.  During SHIPS (Seismic Hazards Investigations of Puget Sound), controlled-source seismic data were acquired in order to model and map areas of expected strong ground shaking and to better determine the regional velocity struc- ture and tectonic framework of the Puget Sound region, including the location, configuration, and seismic properties of faults that cross this urban region.  This project represents the first phase of SHIPS.  Data were acquired during March, 1998, and include marine multichannel seismic data, expanding spread profiles, and ocean-bottom on onshore/offshore large-aperture recording of marine airgun shots.  Over 30,000 airgun array shots were recorded on a multichannel streamer, 257 Refteks, and 15 ocean-bottom seismometers.  Work at Oregon State University (OSU) has focussed on the structure beneath the Straits of Juan de Fuca and beneath the core of the Olympic Mountains.  This study provides new constraints on deformation of the subducting and overriding plates.  While this region is west of the zone of greatest earthquake activity, understanding the deformation here is critical for tying the structure beneath Puget Sound to that beneath the well-imaged continental margin and for interpreting the tectonic causes of seismicity further east. Experiment: During SHIPS98, the R/V Thompson towed a 2.4-km-long. 96-channel digital seismic streamer and a 13-gun, 79.3 liter (4838 cu. in.) airgun array through the waterways of the Puget Basin, Strait of Juan de Fuca, and Strait of Georgia (figure 1).  During the early part of the survey, before deploying the streamer, it was able to fire a 16-airgun, 110.3 liter (6730 cu. in.) array at a 40 s interval.  The airgun shots were also recorded on 273 REFTEK seismometers (figure 1) and on 15 ocean-bottom seismometers.  OSU was responsible for survey- ing, permitting, deploying and operating 32 stations on the northern Olympic Peninsula.  Most of these stations recorded clear arrivials from shots in the Straits of Juan de Fuca, Puget Sound and Hood Canal.  The data include wide-angle reflections from the base of the crust of the Juan de Fuca plate beneath the Olympic peninsula and diving waves traveling through the lower crust in this region (figures 2 and 3).