Determination of Large-Scale Velocity Structure of the Crust and Upper Mantle in the Vicinity of Pavlof Volcano, Alaska

Pavlof Volcano is a 2715-m-high stratovolcano located at latitude 55.4°N and longitude 161.9°W, near the western end of the Alaska Peninsula. In this study the large-scale lateral perturbations to a vertical reference velocity structure in the vicinity of Pavlof Volcano are determined using modified versions of standard three-dimensional inversion techniques. We use as the primary data set P wave arrival times and residuals from shallow (deep) regional earthquakes located by the Shumagin Islands seismic network. Relative residuals mainly from refracted (e.g., Pn) arrivals allow us to constrain models for a laterally varying crustal velocity structure. Several strong ray path dependent patterns are seen in the residuals; the most pronounced is that rays passing through the crust beneath the volcano are delayed (positive residuals) by up to 1.0 s, while those rays traveling subparallel to and behind the volcanic axis on the retroarc (Bering Sea) side arrive as much as 0.5 s early (negative residuals). The corresponding results of a formal velocity inversion show the presence of a lowvelocity body beneath the volcanic arc that coincides geographically with Emmons Caldera (10 km SW of Pavlof). Computed velocity values are about 12–14% lower than in the surrounding crust and are comparable to values found at other volcanic areas. The exact shape and position of the low-velocity body and its suitability for geothermal power development have not yet been sufficiently determined; for such an assessment a much denser station distribution is required