Imaging deep mantle structure beneath Alaska using full waveform tomography

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The Earth’s surface plates are constantly moving and are being consumed into the Earth at plate boundaries. In North America, the oceanic Pacific plate is subducting beneath the south coast of Alaska, generating powerful earthquakes and active volcanoes on Earth’s surface. But where inside the Earth does this subducted plate then go? Tracking this plate into the Earth tells us about the more recent history of plate movement on the surface over millions of years, as well as how the Earth has evolved since it formed over billions of years. To understand the present, we must investigate the past. Much like ultrasound waves are used to look at organs inside a body, seismologists use seismic waves generated by earthquakes to investigate the inner workings of the Earth. The sharpness or blurriness of the images is controlled, among others, by the pitch (or frequency) of the waves that are used. Higher frequencies improve the focus but require heavy computations in the whole Earth. Assumptions can be used to simplify the calculations, reducing the heavy computations to the part of the Earth that we want to image. This simplification means that we can improve our focus and better track the history of subduction under Alaska. The study will support the training of a graduate student and provide support for an early-career investigator. The PIs will share codes and models with the community, and will be involved in local outreach in the Bay Area.The evolution of Alaska over the past 200 Ma features multiple episodes of subduction, collision and accretion. The remnants of this long subduction history should be present down to the lowermost mantle, but past regional and global tomographic models resolve inconsistent structures, likely owing to methodological limitations and limited sampling. Our understanding of the plate tectonics history of the Northern Pacific is currently incomplete. Remaining questions include: how deep do slabs penetrate beneath Alaska, what is the slab geometry and thickness, and how does it interact with the transition zone? The primary objective of this proposal is to improve the resolution of whole mantle regional seismic images beneath Alaska using a Full Waveform Inversion method applied within a restricted region, referred to as “box” tomography. Full Waveform Inversion is required to account for the effects of multipathing and wavefront healing that otherwise mask strong and local heterogeneity, such as slabs and surrounding mantle wedges. Moreover, the spatially restricted “box” approach couples a fast 1D and slower 3D wavefield solver thus reducing computation time, which enables the team to use higher frequency regional and teleseismic body waves. Using a combination of 3-component surface wave, overtone and body waveforms, a shear velocity model will first be constructed. Increasing the maximum frequency of the computations as iterations progress, and with additional body waveforms sensitive to compressional velocity, compressional velocity images will be obtained. This analysis will significantly sharpen existing images of seismic wavespeeds and radial anisotropy, particularly at transition zone depths and the mid and lower mantle. These higher resolution tomographic images of the mantle beneath Alaska will help to (1) constrain the history of subduction and mantle dynamics in this region, and (2) compute more accurate mantle corrections for core phases observed on polar paths from the south Sandwich Islands to stations in Alaska, which present a particularly large spread of travel time anomalies, at least part of which is likely due to Alaska slab structure. The results of the work will be of interest to geodynamicists for modeling flow in the mantle beneath subduction zones, and in plate motion reconstructions, by allowing better identification of subducted slabs. It will also be of interest to geodynamicists and mineral physicists investigating the pattern and origin of inner core anisotropy. Moreover, a robust, high resolution tomographic model of Alaska and a method for providing corrections for its effects will improve the utility of the USArray stations in Alaska for other deep Earth studies.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.