TOMOGRAPHIC IMAGING OF THE ALASKA SUBDUCTION ZONE

The Alaska subduction zone is characterized by the Pacific plate desce nding beneath the North American plate, causing abundant seismic activ ity in the crust and along the Wadati-Benioff zone down to a depth of approximately We have used 142,908 P wave arrival times from 12,237 sh allow- and intermediate-depth earthquakes recorded by the Alaska Earth quake Information Center jointly run by the Geophysical Institute, Uni versity of Alaska Fairbanks and U.S. Geological Survey in the period f rom January 1977 to November 1991, to investigate the three-dimensisna l (3-D) P wave velocity structure beneath central and southern Alaska. Travel times and ray paths are accurately calculated by using an effi cient 3-D ray-tracing technique. The nonlinear tomographic problem is solved by iteratively conducting linear inversions, and the velocity s tructure and hypocentral locations are simultaneously determined. We c onducted two types of inversions. One is an inversion with a laterally homogeneous starting model. The others are what we call slab inversio ns in which we introduce into the initial model the high-velocity subd ucting Pacific plate as a priori information. We found that the slab i nversions gave a seismologically more plausible result and a final roo t-mean-square travel time residual significantly smaller than that of the inversion with the homogeneous starting model. Detailed P wave tom ographic images are obtained for the crust and upper mantle down to a depth of 200 km with spatial resolutions of 30-60 km. The tomographic image of the upper crust correlates well with. the major surface geolo gical features, such as slow sedimentary basins and fast ultramafic bo dies. Prominent low-velocity anomalies exist in the crust and upper ma ntle beneath active volcanoes. In the mantle wedge the low-velocity an omalies dip toward the continental side and extend to a depth of about 150 km, which are considered to be associated with the active volcani sm in the Alaska subduction zone. The results suggest that the subduct ing Pacific plate has a thickness of 45-55 km and a P wave velocity 3- 6% higher than that of the surrounding mantle.