The aftershocks of the 23 August 2011 M w 5.7 Mineral, Virginia, earthquake were recorded by 36 temporary stations installed by several institutions. We located 3960 aftershocks from 25 August 2011 through 31 December 2011. A subset of 1666 aftershocks resolves details of the hypocenter distribution. We determined 393 focal mechanism solutions. Aftershocks near the mainshock define a previously recognized tabular cluster with orientation similar to a mainshock nodal plane; other aftershocks occurred 10-20 km to the northeast. A large percentage of the aftershocks occurred in regions of positive Coulomb static stress change, and ∼80% of the focal mechanism nodal planes were brought closer to failure. However, the aftershock distribution near the mainshock appears to have been influenced strongly by rupture directivity. Aftershocks at depths less than 4 km exhibit reverse mechanisms with north-northwest-trending nodal planes. Most focal mechanisms at depths greater than 6 km are similar to the mainshock, with north-northeast-trending nodal planes. A concentration of aftershocks in the 4-6 km depth range near the mainshock are mostly of reverse type but display a 90°range of nodal-plane trend. Those events appear to outline the periphery of mainshock rupture, where positive Coulomb stress transfer is largest. The focal mechanisms of aftershocks at depths less than 4 km and those greater than 6 km, along with the mainshock, point to the possibility of a depthdependent stress field prior to the occurrence of the mainshock.Analysis of earthquake occurrence using a new magnitude scale (M D ) indicates a Gutenberg-Richer law b-value of 0.864 and an Omori law p-value of 1.085, indicative of a typical aftershock sequence.Online Material: Catalogs of aftershock location, magnitude, and focal mechanisms.

[1] A variety of models for mantle flow beneath southeastern North America have been proposed, including those that invoke westward driven return flow from the sinking Farallon slab, small-scale convective downwelling at the edge of the continental root, or the upward advective transport of volatiles from the deep slab through the upper mantle. We use shear wave splitting observations and receiver function analysis at broadband seismic stations in the southeastern United States to test several of these proposed mantle flow geometries. Near the coast, stations exhibit well-resolved null (no splitting) behavior for SKS phases over a range of back azimuths, consistent with either isotropic upper mantle or with a vertical axis of anisotropic symmetry. Farther inland we identify splitting with mainly NE-SW fast directions, consistent with asthenospheric shear due to absolute plate motion (APM), lithospheric anisotropy aligned with Appalachian tectonic structure, or a combination of these. Phase-weighted stacking of individual receiver functions allows us to place constraints on the timing of arrivals from the 410 and 660 km discontinuities and on average transition zone thickness beneath individual stations. At most stations we find transition zone thicknesses that are consistent with the global average (∼240 km), with two stations showing evidence for a slightly thickened transition zone (∼250 km). Our results are relevant for testing different models for mantle dynamics beneath the southeastern United States, but due to the sparse station coverage, we are unable to uniquely constrain the pattern of mantle flow beneath the region. Our SKS splitting observations support a model in which mantle flow is primarily vertical (either upwelling or downwelling) beneath the southeastern edge of the North American continent, in contrast to the likely horizontal, APM-driven flow beneath the continental interior. However, our receiver function analysis does not provide unequivocal support either for widespread hydration of the transition zone or for widespread thickening due to the downwelling of relatively cold mantle material. We expect that the necessary data to constrain such models more tightly can be obtained from the operation of denser seismic networks, including the Transportable Array and Flexible Array components of USArray.

The Atlantic and Gulf Coastal Plain in the southern and southeastern United States contains extensive Cretaceous and Cenozoic sedimentary sequences of variable thickness. We investigated the difference in response of sites in the Coastal Plain relative to sites outside that region using Fourier spectral ratios from 17 regional earthquakes occurring in 2010–2018 recorded by the Earthscope transportable array and other stations. We used mean coda and Lg spectra for sites outside the Coastal Plain as a reference. We found that Coastal Plain sites experience amplification of low‐frequency ground motions and attenuation at high‐frequencies relative to average site conditions outside the Coastal Plain. The spectral ratios at high frequencies gave estimates of the difference between kappa at Coastal Plain sites and the reference condition. Differential kappa values determined from the coda are correlated with the thickness of the sediment section and agree with previous estimates determined from Lg waves. Averaged estimates of kappa reach ∼120  ms at Gulf coast stations overlying ∼12  km of sediments. Relations between Lg spectral ratio amplitudes versus sediment thickness in successive frequency bins exhibit consistent patterns, which were modeled using piecewise linear functions at frequencies ranging from 0.1 to 2.8 Hz. For sediment thickness greater than ∼0.5  km, the spectral amplitude ratio at frequencies higher than approximately ∼3  Hz is controlled by the value of kappa. The peak frequency and maximum relative amplification at frequencies less than ∼1.0  Hz depend on sediment thickness. At 0.1 Hz, the mean Fourier amplitude ratio (Coastal Plain/reference) is about 2.7 for sediment of 12 km thickness. Analysis of residuals between observed and predicted ground motions suggests that incorporating the amplification and attenuation as functions of sediment thickness may improve ground‐motion prediction models for the Coastal Plain region.

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The disaggregation of probabilistic seismic hazard calculations based on elastic input energy may prove useful for the identification of scenario events because input energy is a convenient single-parameter descriptor of motion duration and amplitude. To investigate this application, regression models are derived for the absolute input energy equivalent velocity, Vea, and the elastic pseudo-relative velocity response, PSV, in the frequency range 0.5 to 10 Hz. Disaggregation of a general seismic hazard model using Vea indicates that the modal magnitudes for the higher frequency oscillators tend to be larger, and vary less with oscillator frequency, than those derived using PSV. Larger magnitude earthquakes contribute more to seismic hazard if Vea is used. The dependence of Vea and PSV upon site classification is virtually identical, and Vea can be predicted with slightly less uncertainty as a function of magnitude, distance and site classification.