TECTONICS AND SEISMICITY OF THE ALASKAN CONTINENTAL MARGIN NEAR THE COLLISION ZONE OF THE YAKUTAT TERRANE AND THE EPICENTER OF THE 1964 GREAT ALASKA EARTHQUAKE

Deep-crustal seismic-reflection data collected in Prince William Sound reveal a north-dipping thrust fault that transects the subduction zone's upper plate. The fault's up-dip projection emerges along the strike of the Patton Bay fault, which exhibited the greatest surface offset during the 1964 Great Alaskan Earthquake (Mw=9.2). Although Plafker (1969) extended the Patton Bay fault southwest to near Kodiak Island, the thrust fault we describe does not continue southwest of Prince William Sound. This fault may have formed in response to the collision of the Yakutat terrane. Hypocenters during 1988-2004 form a diffuse cloud along the thrust fault.

Strong gravity and magnetic anomalies measured over Prince William Sound reveal Paleogene mafic and ultramafic rocks. These anomalies curve across the Sound, in general accord with the orocline expressed by Mesozoic and Paleogene rock belts in southern Alaska. However, tomographic velocities derived from earthquake and active-source first arrivals indicate a ridge of relatively high-velocity (~6.5 km/s) crustal rocks deeper than ~15 km. This ridge strikes north, does not conform to the orogenic curvature, and is not observed in potential-field data. The ridge's north strike may reflect its development after the orocline, perhaps in response to the Pliocene and younger collision of the Yakutat terrane. The lack of ridge expression in gravity and magnetic data remains an unresolved issue.

Current seismicity within the main rupture zone of the Great Alaska Earthquake is concentrated in the region where the greatest slip occurred during the 1964 earthquake. Below central Prince William Sound, mostly strike-slip faulting occurs at 10-20 km depth. Few underthrusting focal mechanisms are evident which may indicate that the plate interface is locked. Seismic reflections from rocks 15-20 km deep correlate with a regional refractor interpreted as the top of Eocene oceanic igneous rocks, which is the basement of the subducted Yakutat terrane (Brocher et al., 1994). Hypocenters tend to cluster along this basement interface. The southwestern edge of the subducted Yakutat terrane is marked in seismic-reflection data by complex faulting, and southwestward across this terrane edge, hypocentral depths increase systematically from ~15 km to ~20 km.