This project will apply spectral-element and adjoint methods to develop a three-dimensional seismic reference model for the crust and uppermost mantle of Alaska. Finite-frequency surface waves and body waves will be extracted from full seismic waveforms in order to provide the greatest resolution for seismic imaging. Major sedimentary basins will be incorporated within the initial three-dimensional models by using existing information such as gravity data and industry seismic data. A new catalog of seismic moment tensors will be constructed by inverting measurements of body waves and surface waves for earthquake depth and radiation pattern. Active tectonic settings exhibit deformation in the form of earthquakes, geodetic strain rate, erosion, and sedimentation. On longer time scales, these processes create a highly heterogeneous crustal composition. For example, faults may offset two distinct rock types, or a basin may open and accumulate kilometer-thick layers of sediment. On shorter time scales, these settings experience abundant earthquakes in response to stresses within the crust. By using these abundant earthquakes, seismic imaging with accurate numerical methods can reveal the extreme complexity within active tectonic settings. Spectral-element methods have been used to model wave propagation in complex geological settings, such as those with strong topographic variations and sedimentary basins. The accuracy of these wave propagation methods can be embedded in an adjoint inverse problem to improve subsurface images of Earth's crust. The primary product will be a three-dimensional reference model for Alaska. This model will describe the spatial variations in compressional wave speed, shear wave speed, and density in Alaska. The quality of the model will be highest in the crust, and it will provide opportunities for larger-scale tomographic studies of the entire subduction and collisional setting. The reference model will be useful as a framework for the upcoming GeoPRISMS focus in Alaska as well as the anticipated EarthScope Transportable Array.
