Inferring crustal structure in the Aleutian island arc from a sparse wide-angle seismic data set

Compressional seismic travel times from a relatively sparse wide-angle data set hold key information on the structure of a 800 km long section of the central Aleutian arc. Since the source and receiver locations form a swath along the arc crest that is similar to50 km wide, we trace rays in 3-D for a collection of 8336 seismic refraction and reflection arrivals. We investigate variations in seismic velocity structure parallel to the Aleutian arc, assuming that our result represents average crustal structure across the arc. We explore seismic velocity models that consist of three crustal layers that exhibit smooth variations in structure in the 2-D vertical plane. We consider the influence of additional constraints and model parameterization in our search for a plausible model for Aleutian arc crust. A tomographic inversion with static corrections for island stations reduces the data variance of a 1-D starting model by 91%. Our best model has seismic velocities of 6.0-6.5 km/s in the upper crust, 6.5-7.3 km/s in the middle crust, and 7.3-7.7 km/s in the lower crust and a total crustal thickness of 35-37+/-1 km. A resolution analysis shows that features having a horizontal scale less than 20 km cannot be imaged, but at horizontal length scales of similar to50 km most model features are well resolved. The study indicates that the Aleutian island arc crust is thick compared to other island arcs and strongly stratified and that only the upper 60% of the arc crust has seismic velocities that are comparable to average seismic velocities in continental crust.