Carl Tape's website

Structure and dynamics of the Alaska mantle wedge

funded by the National Science Foundation
Geoff Abers, Cornell UniversityCarl Tape, University of Alaska FairbanksBrad Hacker, University of California, Santa Barbara
See project summary and motivating figures below.

OPPORTUNITY FOR GRADUATE STUDENT (OR POSTDOC)

UNIVERSITY OF ALASKA FAIRBANKS

The project will involve data analysis for shear-wave splitting, 3D wavefield simulations with anisotropic structures, and seismic imaging, in order to better understand the structure and dynamics of the mantle wedge in Alaska. Prior experience in seismological research and computer programming is desirable. Please contact me (Carl) if you have questions or are interested. (See here for more info on UAF geophysics.)

The start date is flexible and could be between now and fall 2019.

Broadband station coverage (inverted triangles) and tectonic setting of the Alaska subduction zone. The three target corridors are labeled REDOUBT, DENALI, and WRANGELL, and contain Redoubt volcano, Denali, and Wrangell volcano, respectively. The Pacific slab interface is shown by the black contours (Hayes et al. , 2012). The interpreted boundary of the subducted Yakutat crust is outlined in yellow (Eberhart-Phillips et al. , 2006). Active volcanoes are plotted as upright red triangles. Previous or active temporary experiments are colored: BEAAR (cyan), ARCTIC (red), MOOS (green), SALMON (magenta), and WVLF (yellow). Plotted white are permanent stations, Transportable Array stations, and FLATS stations.

Three target subduction profiles, shown in Figure 1, aligned at x = 0 km on Redoubt volcano (top), Denali (middle), and Wrangell (bottom). Each shows relocated seismicity (Fuis et al. , 2008) (40 km wide corridor), subduction interface of Jadamec and Billen (2010), and continental Moho (Wang and Tape, 2014) primarily based on receiver functions (e.g., Veenstra et al. , 2006). The Redoubt profile shows the station coverage for high-resolution (f < 2 Hz) wavefield simulations.

Finite element mesh for the region containing Redoubt volcano. The mesh contains 17 million elements and should provide numerically accurate synthetic seismograms up to 2 Hz.

Zoom-in on the mesh above. At this scale, you can make out the boundaries of the elements at the surface. You can also see the volcanoes Iliamna and Redoubt. The mesh contains 17 million elements and should provide numerically accurate synthetic seismograms up to 2 Hz.

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.