Crustal and uppermost mantle structure beneath the United States

Abstract

This paper presents a new model of the shear velocity structure of the crust and uppermost mantle beneath the contiguous U.S. The model is based on more than a decade of USArray Transportable Array (TA) data across the U.S. and derives from a joint Bayesian Monte Carlo inversion of Rayleigh wave group and phase speeds determined from ambient noise and earthquakes, receiver functions, and Rayleigh wave ellipticity (H/V) measurements. Within the Bayesian inverse theoretic framework, a prior distribution of models is posited and a posterior distribution is inferred beneath all of the more than 1800 TA stations across the U.S. The resulting mean and standard deviation of the mean of the posterior distribution at each station summarize the inversion results, which are then interpolated onto a regular 0.25°×0.25° grid across the U.S. to define the final 3‐D model. We present arguments that show that the standard deviation of the posterior distribution overestimates the effect of nonsystematic errors in the final model by a factor of 4–5 and identify uncertainties in density and mantle Q as primary potential sources of remaining systematic error in the final model. The model presents a great many newly resolved structural features across the U.S. that require further analysis and dedicated explication. We highlight here low‐velocity anomalies in the upper mantle that underlie the Appalachians with centers of anomalies in northern Georgia, western Virginia, and, most prominently, New England.

Quantitative Relationships Between Basalt Geochemistry, Shear Wave Velocity, and Asthenospheric Temperature Beneath Western North America

“…In all four models, the slowest shear wave velocities are observed beneath the Snake River Plain. (g, h) Same for WUSA16 (Shen & Ritzwoller, 2016). In general, V s between 60 and 100 km is slower than that of the WUS reference model (Pollitz, 2008).…”

“…(e, f ) Same for DNA13 (Porritt et al, 2014). (g, h) Same for WUSA16 (Shen & Ritzwoller, 2016). reflect analytical errors and geographic averaging together with systematic errors associated with the depth and thickness of the spinel-garnet transition zone.…”

“…The chosen models are PM2012, SL2013NA, DNA13, and WUSA16 that were developed by Priestley and McKenzie (2013) (Pollitz, 2008). Dashed lines = isentropic curves labeled according to potential temperature, T p ; horizontal line = maximum crustal thickness of 50 km (Shen & Ritzwoller, 2016); solid diagonal line = solidus for dry mantle peridotite (Katz et al, 2003). Colored scheme as before.…”

“…Dashed lines = isentropic curves labeled according to potential temperature, T p ; horizontal line = maximum crustal thickness of 50 km (Shen & Ritzwoller, 2016); solid diagonal line = solidus for dry mantle peridotite (Katz et al, 2003). (g, h) Same for WUSA16 (Shen & Ritzwoller, 2016 (2013) constructed a lower resolution global model. (e, f ) Same for DNA13 (Porritt et al, 2014).…”

The inversion of spectral ratio H/V in a layered system using the diffuse field assumption (DFA)

“…In another recent paper on the subject, Shen and Ritzwoller (2016) successfully constructed a new model of the shear velocity structure of the crust and uppermost mantle beneath the contiguous U.S. using data from more than a decade of USArray recordings. They performed a joint inversion of Rayleigh wave group and phase speeds determined from ambient noise and earthquakes, receiver functions, and measurements of Rayleigh wave ellipticity.…”