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The stress field in the eastern United States is commonly considered to be broadly uniform and due to homogeneous far‐field forces; however, modern and geologic stress indicators in this region show substantial heterogeneity. Using CitcomS to model stresses based on simple input density, temperature, and viscosity fields, we show that local isostasy is key in explaining the intraplate stress field in the southeastern United States. Crustal thickness variations appear to be most important in reproducing observations, although we slightly better match the observed stress field by including variable crustal viscosity informed by magnetotelluric imaging. Our results demonstrate that local gravitational body forces can substantially reorient far‐field stresses and thereby influence patterns of intraplate seismicity. We also show that variable crustal viscosity encourages a steepening of isostatic topography in the southeastern United States; this observation suggests that a sharp boundary in crustal strength may be important in explaining the apparently long‐lived Appalachian topographic escarpment. Plain Language Summary: The origin of stresses within tectonic plates, and hence the ultimate driving force of earthquakes far from plate boundaries, is a contentious topic. Historically, the geoscience community has viewed intraplate stress patterns as being simply the result of forces applied along plate edges. However, this paradigm does not explain why earthquakes far from plate boundaries cluster into discrete seismic zones, nor does it explain the diverse mechanisms of intraplate earthquakes. Here, we consider the southeastern United States, which is thousands of kilometers from any plate boundary, but which nevertheless experiences major earthquakes, such as the 2011 Virginia earthquake that shook the nation's capital. Using numerical models, we show that forces arising from variable thickness of the Earth's crust can largely explain the stress and earthquake patterns observed in this region. Another curious feature of the southeastern United States is the steep topography of the Blue Ridge Escarpment in the Appalachian Mountains; despite having been subjected to episodic erosion for over a hundred million years, the southeastern edge of the range is surprisingly sharp. With our models, we show that weak zones within the crust beneath the Appalachians may help to explain how that steep escarpment has survived over long timescales. Key Points: Stress indicators in the southeastern United States show substantial heterogeneityGravitational body forces from variable crustal thickness are important in explaining many features of observed stress patternsCrustal weak zones beneath the Appalachian Mountains may help maintain a sharp passive margin escarpment (the Blue Ridge Escarpment) [ABSTRACT FROM AUTHOR]

Long period (10-20,000 s) magnetotelluric (MT) data are being acquired across the continental USA on a quasi-regular grid of ~70 km spacing as an electromagnetic component of the National Science Foundation EarthScope/USArray Program. These data are sensitive to fluids, melts, and other orogenic indicators, and thus provide a valuable complement to other components of EarthScope. We present and interpret results of 3-D MT data inversion from 325 sites acquired from 2006-2011 to provide a regional scale view of electrical resistivity from the middle crust to nearly the mantle transition zone, covering an area from NW Washington to NW Colorado. Beneath the active extensional subprovinces in the south-central region, on average we see a resistive upper crust, and then extensive areas of low resistivity in the lower crust and uppermost mantle. Further below, much of the upper half of the upper mantle appears moderately resistive, then subsequently the lower upper mantle becomes moderately conductive. This column suggests a dynamic process of moderately hydrated and fertile deeper upper mantle upwelling during extension, intersection of that material with the damp solidus causing dehydration and melting, and upward exodus of generated mafic melts to pond and exsolve saline fluids near Moho levels. Lithosphere here is very thin. To the east and northeast, thick sections of resistive lithosphere are imaged under the Wyoming and Medicine Hat Cratons. These are punctuated with numerous electrically conductive sutures presumably containing graphitic or sulfide-bearing meta-sediments deeply underthrust and emplaced during ancient collisions. Below Cascadia, the subducting Juan de Fuca and Gorda lithosphere appears highly resistive. Suspected oceanic lithosphere relicts in the central NW part of the model domain also are resistive, including the accreted "Siletzia" terrane beneath the Coast Ranges and Columbia Embayment, and the seismically fast "slab curtain" beneath eastern Idaho interpreted by others as stranded Farallon plate. Upwelling of deep fluid or melt in the Cascade volcanic arc region manifests as conductive features at several scales. These include quasi-horizontal conductive patches under the arc and fore-arc, likely denoting fluids evolved via breakdown of hydrous minerals in the current down-going slab. In the backarc, low resistivities concentrate in "plumes" connecting into a deeper aesthenospheric layer to the east, consistent with subduction-driven upwelling of hot, hydrated or melted, aesthenospheric mantle. Low resistivities <10 Ωm deep beneath the stable cratons suggest higher levels of hydration there, and/or influence of poorly resolved structures outside the array. [ABSTRACT FROM AUTHOR]

A linearized baroclinic, spectral-in-time tidal inverse model has been developed for assimilation of surface currents from coast-based high-frequency (HF) radars. Representer functions obtained as a part of the generalized inverse solution show that for superinertial flows information from the surface velocity measurements propagates to depth along wave characteristics, allowing internal tidal flows to be mapped throughout the water column. Application of the inverse model to a 38 km × 57 km domain off the mid-Oregon coast, where data from two HF radar systems are available, provides a uniquely detailed picture of spatial and temporal variability of the M[sub 2] internal tide in a coastal environment. Most baroclinic signal contained in the data comes from outside the computational domain, and so data assimilation (DA) is used to restore baroclinic currents at the open boundary (OB). Experiments with synthetic data demonstrate that the choice of the error covariance for the OB condition affects model performance. A covariance consistent with assumed dynamics is obtained by nesting, using representers computed in a larger domain. Harmonic analysis of currents from HF radars and an acoustic Doppler profiler (ADP) mooring off Oregon for May–July 1998 reveals substantial intermittence of the internal tide, both in amplitude and phase. Assimilation of the surface current measurements captures the temporal variability and improves the ADP/solution rms difference. Despite significant temporal variability, persistent features are found for the studied period; for instance, the dominant direction of baroclinic wave phase and energy propagation is always from the northwest. At the surface, baroclinic surface tidal currents (deviations from the depth-averaged current) can be 10 cm s[sup –1] , 2 times as large as the depth-averaged current. Barotropic-to-baroclinic energy conversion is generally weak within the model domain over the shelf but reaches 5 mW m[sup –2] at times over the slopes of Stonewall Bank. [ABSTRACT FROM AUTHOR]

Examines the stability of magnetutelluric method on electromagnetic fields at the surface of the Earth in California. Evidence of small-frequency independent variations on impedance amplitude; Estimation of transfer function of electric and magnetic field time-series.