Seismic Interferometry and Data Assimilation for Lithospheric Structure and Anisotropy Across Alaska

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Alaska is composed of crustal fragments sandwiched in between old continental crusts in Asia and North America. Because of plate tectonics, Alaska continues to move relative to the stable parts of North America. Active seismicity is found across most of the state which generates hazards such as earthquakes, volcanic eruptions, and potential tsunamis in coastal areas. Alaska has been built by a wide variety of geological processes. This makes it an ideal natural laboratory. Its active southern margin is particularly complex. There, the Yakutat microplate collides with the North American plate; a result of the under-thrusting of the Pacific Plate in the Aleutian subduction zone. The recent deployment of numerous seismometers across Alaska provides an unprecedented opportunity to image the underlying geological structures. Here, the team develops new tools and models in 3D the seismic velocities in the crust and uppermost mantle to a depth of 120-150 km. A series of models are generated from the seismic data. They include an isotropic component which details the geometry, temperature and composition of the subsurface structures. The models also account for directional variations in seismic wave propagation; this provides information about past and on-going deformation processes. Outcomes of the project will improve the understanding of seismogenic processes in Alaska, including megathrust earthquakes that endanger the population centers. The project also provides partial support and training in cutting-edge seismology to several graduate students. The team will share the developed codes and analyses with the scientific community, allowing their application to other regions of the World.Here, the team measures the Rayleigh and Love wave velocities derived from ambient noise and earthquakes, accounting for their azimuthally dependence. The researchers use complementary data such as receiver functions. This helps produce more refined, higher resolution lithospheric models. Novelties include the application of three-station interferometry and the inversion for the tilted elastic tensor. All data are interpreted within the framework of a Bayesian Monte Carlo inversion. A distribution of accepted models is generated from which model uncertainties are estimated. Scientific objectives include (1) imaging the under-thrusting lithosphere along Alaska's southern margin as well as the overlying crust and mantle wedge; the goal is to search for slab tears, flat slab regions, slab edges, and thickened lithosphere. The team investigates as well (2) how crustal thickness differs beneath the Alaska and Brooks ranges and to what extent these and other ranges are compensated isostatically. They look for evidence of (3) anisotropy from corner flow around the edges of subducting lithosphere and (4) whether Yakutat lithosphere differs from that of the surrounding regions. One goal is also to (5) determine the geometry of contact between cratonic and deformed lithosphere. The seismological models will be archived and shared with the broader geoscience community via the IRIS-DMS. 3-D models and other higher-level data products are delivered to the public through dynamically updated, open access databases at University of Colorado, Boulder (transitioning to GitHub). These include dispersion maps, receiver functions, surface wave observables like H/V ratios, and research codes.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.