Seismic Velocity Structure Beneath the Eastern United States and Northern Mississippi Embayment

The unprecedented high-quality seismic data from the EarthScope Transportable Array, the New Madrid Cooperative Seismic Network, and the Northern Embayment Lithosphere EarthScope Flex Array experiment provide us a great opportunity to investigate and compare the subsurface structure beneath the eastern U.S. and the northern Mississippi embayment with high resolution. The Mississippi embayment located in the south part of the eastern U.S. has undergone three episodes of major plate disruption and an enigmatic thermal event in the Cretaceous that modified the lithospheric structure beneath it. The northern part of the embayment hosts the New Madrid seismic zone, which was struck by three M7 or greater intraplate earthquakes in 1811-1812. Previous geophysical surveys suggest significant crustal heterogeneity and substantial velocity variations in the upper mantle beneath the seismic zone. However, the seismic structure differences between the northern embayment and the broader eastern U.S. region are poorly constrained, which undoubtedly exerts significant control on the lithospheric strain accumulation and release through earthquakes. Our goal is to determine seismic structure in both regions and compare the differences in terms of seismic speed variations. We construct subsurface seismic images using multiple types of complementary geophysical observations beneath both the eastern U.S. region and the northern embayment with a higher lateral resolution in the embayment. Specifically, we invert spatially smoothed P-wave receiver functions together with Rayleigh-wave group and phase velocities, and high-resolution gravity observations to construct robust estimates of the subsurface 3D seismic structure using a linearized inversion. Preliminary results include many first-order patterns of eastern US structure, including thick sedimentary cover beneath the coast plains, Mississippi Embayment, and Appalachian and midwestern basins, thicker crust beneath the Appalachians compared with the coastal regions, and relatively fast upper mantle with a few exceptions such as a low shear speed beneath New England and southeast Canada. In the northern Mississippi Embayment we image relative high shear speeds in the lower crust underlain by a relatively slow mantle.