High-Resolution Seismic Velocity Structure of the Alaska Subduction Zone Revealed by Double-Difference Tomography

We use catalog picks of tens of thousands of earthquakes selected from the Alaska Earthquake Information Center from 1988 to 2002 and between latitude N57^o and N65.5^o to conduct a high-resolution seismic tomography study of the Alaska Subduction Zone by double-difference tomography. Two active source data sets in the Prince William Sound (1989) and southern Alaska (1984-1985) are also included in the inversion to provide more constraints on the shallow velocity structure. There are ~150 stations used in the inversion that were operational during some or all of the time period from 1988 to 2002. The preliminary P- and S-wave velocity models show clearly the low velocities in both the crust and mantle beneath the active volcanoes. The velocity structures in and around the subducting slab are different for the Kodiak, Kenai and McKinley blocks of the subducting plate. For the Kodiak block, a low velocity zone is present within the slab right below the high velocity zone where most of the Wadati-Benioff Zone (WBZ) earthquakes are located. In comparison, for the Kenai block, the velocity is higher in the zone below the WBZ earthquakes. The McKinley block shows a more complex slab velocity pattern, with high and low velocity zones at different depths within the slab. The high-resolution velocity structure will provide additional constraint for models of the segmentation of the Alaska Subduction Zone, in addition to earthquake locations, volcanic arc geometry and composition, etc., and may provide a possible explanation for the segmentation.