SLAB GEOMETRY OF THE CASCADIA SUBDUCTION ZONE BENEATH WASHINGTON FROM EARTHQUAKE HYPOCENTERS AND TELESEISMIC CONVERTED WAVES

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 degree at the top of the arch. North and south of the arch crest the slab dips 15-20 degree . 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 degree north latitude, to approximately N30 degree W north of 48 degree 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.