PROJECT SUMMARY
Overview:The Alaskan subduction system is one of the planet's archetypal subduction zones, and studies here are the basis of much first-order understanding of subduction dynamics. The Alaska Transportable Array combined with several dense portable broadband experiments (BEAAR, SALMON, MOOS, WVLF) achieves unprecedented sampling of seismic wave propagation in the Alaska subduction zone. This proposal takes advantage of these data to test fundamental hypotheses regarding subduction structure and dynamics. The proposed project integrates new seismic observations, state-of-the-art wavefield simulations, and petrologically consistent models of subduction-zone mantle structure to test these hypotheses. It focuses on three distinct corridors for which EarthScope and related projects provide unusually good sampling: (a) the Cook Inlet corridor where normal Pacific lithosphere subducts and the arc is robust; (b) the nearly amagmatic Denali corridor where the Yakutat oceanic plateau subducts and generates intermediate-depth earthquakes; and (c) the Wrangell Volcanic Field corridor where slab seismicity is nearly absent but there is very high volume volcanism. Observations of seismic attenuation provide proxies for thermal structure and melt abundance, and observations of shear-wave splitting constrain the anisotropic fabric and its variation between hot and cold parts of the mantle wedge. Parallel observations of seismicity and high-frequency phases that interact with the slab surface then allow inferences about the mantle wedge to be compared with slab dehydration. High-frequency wavefield simulations of split shear waves will assess the maximum depth of a supra-slab anisotropic slow layer, a probable signature of slab-mantle coupling depth.
Intellectual Merit:Images of the three corridors allow testing of three first-order hypotheses regarding the nature of subduction: 1) The variation between segments in proxies for slab dehydration correlates with variation in melting in the mantle wedge. Seismicity and converted-wave characteristics are indicative of dehydration, while attenuation has been shown to be sensitive to melt. Large variations are predicted among the corridors. 2) Anisotropic features within the mantle wedge, revealed by local splitting, vary among corridors and with distance from the slab edge. Hot and cold parts of the wedge should show different systematics relative to subduction direction and distance from potential slab tears. 3) The depth of slab-mantle coupling separating hot from cold wedge remains globally constant at ~80 km. Both attenuation and anisotropy sample this boundary. Testing of these hypotheses comes from integrated analysis of these datasets, including full-waveform modeling, and petrologic/mineral physics models.
Broader Impacts:This project addresses EarthScope science objectives and emphasizes interdisciplinary work at the interface between petrology, seismology, and geodynamics. The project focuses on the structure of subduction zones, from seismic structure (Vp, Vs, anisotropy, Q) to thermal and compositional structure. The leverages education and outreach opportunities through the EarthScope National Office, notably those available through the EarthScope website and social media. A focal product - possibly disseminated and archived within the EarthScope quarterly newsletter (InSights)- is a synthesis of seismological achievements in Alaska over the past 20 years. This synthesis should find a broad audience of scientists and public alike at the end of the EarthScope Transportable Array deployment in 2019. All project participants including graduate students supported at two institutions will work with the EarthScope National Office to maximize scientific outreach of the project. The project will generate improved predictions of amplitudes of seismic waves in south-central Alaska, including within the Anchorage metropolitan region; therefore the project can contribute toward seismic hazard assessments and ground motion prediction.