Simultaneous Inversion of Receiver Functions, Surface-wave Dispersion, and Gravity Observations for Lithospheric Structure Beneath the Central and Eastern United States

Abstract:

In contrast with the western United States (US), the eastern US has been less a focus for detailed passive seismological imaging of the crust and upper mantle. However, in the last few years, the EarthScope-USArray Transportable Array has substantially increased the available high-quality observations that sample the “stable” regions of North America with a roughly uniform ~70km spacing. The result is an unprecedented opportunity to illuminate the subsurface and to explore how upper crustal phenomena relate to deeper crustal and upper mantle structure and processes, which likely exert some control on the regional tectonics. But even with such a fine network, tightly constraining the subsurface 3D geologic variations is a challenge. We combine the complementary sensitivities P-wave receiver functions, surface-wave dispersion, and wavenumber-filtered gravity variations to constrain lateral variations in shear-wave speed beneath the eastern US and southeastern Canada. The receiver-function wavefield is interpolated and smoothed to equalize the lateral sensitivity of the receiver functions and the surface-wave dispersion and to simplify the receiver functions. Combining information from adjacent stations through interpolation suppresses poorly sampled and difficult to interpret back-azimuthal variations and allows the stable extraction of the key features in the receiver-function wavefield. We use a hybrid 3D, multi-objective inversion that provides more constraints than individual inversions, and generally produces a more robust estimate of the subsurface structure. The subsurface model is parameterized with shear-wave speed (the primary sensitivity of the surface-wave dispersion and P-wave receiver functions) within one-degree cells that are modeled using 1D seismic calculations. We relate P-wave speed and density variations to shear-speed using velocity ratios and density-velocity relations and we constrain the 3D variations to be laterally and vertically smooth. Preliminary results are dominated by thick sedimentary accumulations along the Mississippi Embayment. Within the continental interior and the ancient Appalachians the crust is thicker and faster. We present more details on the inversion approach and discuss our 3D model in light of the tectonic history of the study area.