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Tomographic imaging of the Alaska subduction zone

The Alaska subduction zone is characterized by the Pacific plate descending beneath the North American plate, causing abundant seismic activity in the crust and along the Wadati-Benioff zone down to a depth of approximately 200 km, We have used 142,908 P wave arrival times from 12,237 shallow- and intermediate-depth earthquakes recorded by the Alaska Earthquake Information Center jointly run by the Geophysical Institute, University of Alaska Fairbanks and U.S. Geological Survey in the period from January 1977 to November 1991, to investigate the three-dimensional (3-D) P wave velocity structure beneath central and southern Alaska. Travel times and ray paths are accurately calculated by using an efficient 3-D ray-tracing technique. The nonlinear tomographic problem is solved by iteratively conducting linear inversions, and the velocity structure and hypocentral locations are simultaneously determined. We conducted two types of inversions. One is an inversion with a laterally homogeneous starting model. The others are what we call slab inversions in which we introduce into the initial model the high-velocity subducting Pacific plate as a priori information. We found that the slab inversions gave a seismologically more plausible result and a final root-mean-square travel time residual significantly smaller than that of the inversion with the homogeneous starting model. Detailed P wave tomographic 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 geological features, such as slow sedimentary basins and fast ultramafic bodies. Prominent low-velocity anomalies exist in the crust and upper mantle beneath active volcanoes. In the mantle wedge the low-velocity anomalies dip toward the continental side and extend to a depth of about 150 km, which are considered to be associated with the active volcanism in the Alaska subduction zone. The results suggest that the subducting Pacific plate has a thickness of 45–55 km and a P wave velocity 3–6% higher than that of the surrounding mantle.