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  1. SUMMARY

    Seismic tomography is a cornerstone of geophysics and has led to a number of important discoveries about the interior of the Earth. However, seismic tomography remains plagued by the large number of unknown parameters in most tomographic applications. This leads to the inverse problem being underdetermined and requiring significant non-geologically motivated smoothing in order to achieve unique answers. Although this solution is acceptable when using tomography as an explorative tool in discovery mode, it presents a significant problem to use of tomography in distinguishing between acceptable geological models or in estimating geologically relevant parameters since typically none of the geological models considered are fit by the tomographic results, even when uncertainties are accounted for. To address this challenge, when seismic tomography is to be used for geological model selection or parameter estimation purposes, we advocate that the tomography can be explicitly parametrized in terms of the geological models being tested instead of using more mathematically convenient formulations like voxels, splines or spherical harmonics. Our proposition has a number of technical difficulties associated with it, with some of the most important ones being the move from a linear to a non-linear inverse problem, the need to choose a geological parametrization that fits each specific problem and is commensurate with the expected data quality and structure, and the need to use a supporting framework to identify which model is preferred by the tomographic data. In this contribution, we introduce geological parametrization of tomography with a few simple synthetic examples applied to imaging sedimentary basins and subduction zones, and one real-world example of inferring basin and crustal properties across the continental United States. We explain the challenges in moving towards more realistic examples, and discuss the main technical difficulties and how they may be overcome. Although it may take a number of years for the scientific program suggested here to reach maturity, it is necessary to take steps in this direction if seismic tomography is to develop from a tool for discovering plausible structures to one in which distinct scientific inferences can be made regarding the presence or absence of structures and their physical characteristics.

     
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  2. Changes in plate tectonics drove degassing of carbon dioxide and global temperatures over the past 20 million years. 
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  3. Abstract

    Love wave phase velocity maps provide essential constraints on radial anisotropy and deformation in the crust and upper mantle. However, the phenomenon of overtone interference causes scatter and systematic bias in the velocity measurements and impedes efforts to image small‐scale anisotropic variations. We develop an approach for identifying Love wave measurements that are biased by overtone interference, demonstrate its efficacy with EarthScope USArray data, and determine the first earthquake‐derived Love wave phase velocity maps for the entire conterminous U.S. in the period range 35–75 s. We show that radial anisotropy in parts of the crust and most of the lithospheric mantle is necessary to reconcile these maps with Rayleigh wave phase velocities. Our results convey the impact and geographic variability of overtone interference, offer an easy‐to‐implement method to ameliorate this impact, and present high‐resolution constraints on radial anisotropy beneath North America.

     
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  4. Abstract

    Shear attenuation provides insights into the physical and chemical state of the upper mantle. Yet, observations of attenuation are infrequent in the oceans, despite recent proliferation of arrays of ocean‐bottom seismometers (OBSs). Studies of attenuation in marine environments must overcome unique challenges associated with strong oceanographic noise at the seafloor and data loss during OBS recovery in addition to untangling the competing influences of elastic focusing, local site amplification, and anelastic attenuation on surface‐wave amplitudes. We apply Helmholtz tomography to OBS data to simultaneously resolve array‐averaged Rayleigh wave attenuation and maps of site amplification at periods of 20–150 s. The approach explicitly accounts for elastic focusing and defocusing due to lateral velocity heterogeneity using wavefield curvature. We validate the approach using realistic wavefield simulations at the NoMelt Experiment and Juan de Fuca (JdF) plate, which represent endmember open‐ocean and coastline‐adjacent environments, respectively. Focusing corrections are successfully recovered at both OBS arrays, including at periods <35 s at JdF where coastline effects result in strong multipathing. When applied to real data, our observations of Rayleigh wave attenuation at NoMelt and JdF revise previous estimates. At NoMelt, we observe a low attenuation lithospheric layer (> 1,500) overlying a highly attenuating asthenospheric layer (∼ 50 to 70). At JdF, we find a broad peak in attenuation (∼ 50 to 60) centered at a depth of 100–130 km. We also report strong local site amplification at the JdF Ridge (>10% at 31 s period), which can be used to refine models of crust and shallow mantle structure.

     
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  5. Abstract

    While variations in crustal structure beneath the Denali fault in Alaska are well‐documented, the existence of fault‐correlated structures throughout the entire thickness of the continental lithosphere is not. A new model of shear‐wave velocity structure obtained through joint inversion of surface wave and converted body wave data shows a northward increase in lithospheric thickness and velocity occurring across the Denali fault system. In northern Alaska, a dramatic increase in lithospheric thickness at the southern margin of the Arctic‐Alaska terrane lies in the vicinity of the Kobuk fault system. These correlations support the view that transpressive deformation tends to localize at the margins of thicker, higher‐strength lithosphere.

     
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  7. Abstract

    The rate of ocean‐crust production exerts control over sea level, mantle heat loss, and climate. Different strategies to account for incomplete seafloor preservation have led to differing conclusions about how much production rates have changed since the Cretaceous, if at all. We construct a new global synthesis of crust production along 18 mid‐ocean ridges for the past 19 Myr at high temporal resolution. We find that the global production rate during 6–5 Ma was only 69%–75% of the 16–15 Ma interval. The reduction in crust production is mostly due to slower seafloor spreading along almost all ridge systems. While the total ridge length has varied little since 19 Ma, some fast‐spreading ridges have grown shorter and slow‐spreading ridges grown longer, amplifying the spreading‐rate changes. Our production curves represent a new data set for investigating the forces driving plate motions and the role of tectonic degassing on climate.

     
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  9. Abstract

    Seismic models show that the shallow mantle beneath the contiguous United States is characterized by low velocities west of the Rocky Mountain Front, high velocities beneath the North American craton, and moderate wave speeds along the eastern seaboard. However, numerous questions remain, including the distribution of temperature and partial melt in the mantle that may explain the slower velocities in the west; the origin and depth extent of the western edge of the cratonic lithosphere; and explanations for slower velocities observed along the eastern seaboard. To investigate these questions, we have measured teleseismic Rayleigh wave phase and amplitude at USArray stations for periods of 25–180 s and calculated phase velocity using the Helmholtz tomography method. Notably, the new long‐period maps represent an opportunity to resolve mantle structure at sublithospheric depths and show differences relative to shorter periods in both the distribution of slow velocities in the west and the extent of fast velocities in the central United States. The maps are then used as a basis for comparing six recent 3‐D shear velocity models of the contiguous United States, from which we predict phase velocities. While predictions from surface‐wave models agree best with our observations at short and intermediate periods, the longest‐period maps are most similar in amplitude to body‐wave models.

     
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