THE RELATION OF SEISMICITY TO UPPER AND LOWER PLATE STRUCTURE OF THE SOUTH-CENTRAL ALASKA SUBDUCTION ZONE

We have relocated over 26,000 earthquakes in south-central Alaska (eastern Prince William Sound to western Cook Inlet) occurring between 1971 and 2001. We have used first motion data to determine focal mechanisms for individual earthquakes of magnitude 2 to 5, in addition to directly inverting first motion data from clusters of earthquakes for stress field orientation. We have compared the seismicity to known upper crustal features (mapped faults, folds, terrane boundaries) and velocity models derived from seismic refraction/reflection surveys and tomographic studies. We also compared pre-1964 mainshock seismicity to the recent earthquakes. Our results indicate that a number of faults and fault cored anticlines within Cook Inlet and north of Cook Inlet are seismically active. Historic seismicity indicates these faults have produced up to magnitude 7.0 earthquakes. Seismically active crustal faults are also observed on and off shore in Prince William Sound. A concentration of seismicity located within the lower crust of North America and the upper crust of the lower plate northwest of Knik Arm (30 to 45 km depth) may represent severe deformation at the southwestern edge of the subducted Yakutat block. Focal mechanisms and the results of stress inversions for this region indicate east-west to northeast-southwest directed compression. Analysis of over 700 focal mechanisms for deeper (> 40 km) events within northern Cook Inlet region shows a rotation of maximum compressive stress direction across the suspected edge of the Yakutat block. In Prince William Sound it appears that the northern limits of some clusters of seismicity may be controlled by the location of boundaries between the Prince William, Chugach and Wrangellia terranes. Unusual clusters of seismicity located in the Tazlina Glacier region north of Prince William Sound and in the region south of Lake Tustumena on the Kenai Peninsula appear to be related to regions of higher velocity lower crust that may represent underplating or remnants of older slabs.