Using earthquake hypocenters obtained from the Washington regional seismograph network and analysis of Ps converted phases from teleseisms at special temporary stations located south of the Olympic Peninsula, we have developed a preliminary model of the structure of the subducted slab beneath western Washington and southern British Columbia. The slab is smoothly arched beneath Puget Sound, subducting at a dip of about 10--12¿ at the top of the arch. North and south of the arch crest the slab dips 15--20¿. Subcrustal earthquakes are localized near the top of the arch. This deformation, from planar slab geometry, is caused by the change in azimuth of the ''trench'' (or hinge line of the slab) from almost NS south of 47¿ north latitude, to approximately N30¿W north of 48¿ north. The slab appears to be easily flexed, probably due to small elastic thickness, but its geometry reflects conservation of surface area and minimization of in-plane strain. Crustal seismicity in the Puget Sound basin may result indirectly from the presence of the arch in the underlying Juan de Fuca plate. ¿ American Geophysical Union 1987