Implementation of "Box Tomography" for high resolution imaging of Target Regions in the Earth's Deep Mantle

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The existence of "hotspot" volcanoes located in the middle of tectonic plates, such as Hawaii, do not fit within simple plate tectonic theory, which only explains the presence of volcanism on the borders of such plates. It has long been proposed that hotspots are the surface expression of mantle plumes, conduits of hot rock rising from the deep mantle. Such conduits - albeit broader than expected - have recently been imaged by seismic tomography, and traced down to the core-mantle boundary under major hotspots under Hawaii and Iceland, for example. However, these plume images are still blurry, making it difficult to fully understand their role in the dynamics of the mantle, while the depth at which others originate, such as under Yellowstone, are still debated. A new methodology called "full waveform tomography", which aims at utilizing all the information contained in seismic records, is increasingly gaining interest in the global seismology and exploration geophysics community, as it holds the promise of obtaining sharper images of structures in particular in the deep mantle. However, such an approach is computationally very expensive, especially at the global scale, as the seismic wave propagation needs to be computed over very large distances and has to be performed many times during successive incremental updates of the image. A promising solution is "box tomography" whereby a target region of study is defined within the Earth's deep mantle and the complete wavefield from earthquake sources near the Earth's surface to distant recording stations is computed only once at the beginning, while successive updates of the model within the target region require wavefield computations only within the limits of the target region. We will implement this methodology and apply it to two targets of geophysical significance: (1) the Yellowstone plume and (2) the root of the Iceland plume near the core-mantle boundary, which has been shown to contain a patch of extreme material properties, which may indicate the presence of partially molten rock. Broader impacts of this project include training of two graduate students in addition to providing a better understanding of mantle flow in Earth's deep mantle.The existence of mantle plumes has long been debated. Broad conduits of pronounced low shear velocity have recently been imaged tomographically, extending from the core-mantle boundary through most of the lower mantle, in the vicinity of major hotspots such as Iceland, Hawaii, or the south-Pacific superswell volcanoes. Better characterizing the details of these plumes will help improve our understanding of the organization of upwelling flow in the deep earth, and the contrasted rheology between the lower 2000 km of the mantle and the extended upper mantle transition zone. Also, there may be different types of plumes, as may be the case for Yellowstone, where a broad lower mantle plume is not imaged. To address this, full waveform tomography based on numerical computations of the seismic wavefield is a promising tool, as it has the potential of improving tomographic resolution by exploiting the information contained in entire seismograms. However, to attain the higher frequencies necessary for pushing the limits of image resolution, a formidable challenge is the increasingly heavy computations involved. Also, not all regions of the earth are adequately illuminated by the present distribution of earthquakes and receivers to warrant much higher resolution than is achieved today. A promising solution is "box tomography", in which a target volume of limited extent is embedded in a global 3D Earth model, and the imaging is restricted to that smaller volume, leaving the model fixed outside of it. In many situations, both sources and receivers are located outside of this volume, presenting challenges for connecting wavefields to reconstruct complete seismograms. We have developed a "box tomography" methodology that allows arbitrary relative locations of sources, receivers and target volume, and can be applied with any wave equation integrator. A first application to continental scale upper mantle tomography with stations inside the target volume has been developed in north America. Here we plan to finish implementing the computation of the Green's functions necessary to extrapolate the wavefield from the target region to the external stations and to apply it to two targets of geophysical significance: (1) confirming the presence of a mantle plume in the mid-lower mantle beneath the Yellowstone hotspot, and improving its resolution; (2) improving the resolution of the structure at the base of the earth's mantle, within the root of the Iceland Plume, where a large axisymmetric ultra-low-velocity zone was recently documented, and comparing it to the structure at the base of what might be the lower mantle expression of the Yellowstone plume. This project will train 2 students on state-of-the art seismological techniques.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.