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S U M M A R YWe utilize two-and-three-quarter years of vertical-component recordings made by the Transportable Array (TA) component of Earthscope to constrain three-dimensional (3-D) seismic shear wave velocity structure in the upper 200 km of the western United States. Single-taper spectral estimation is used to compile measurements of complex spectral amplitudes from 44 317 seismograms generated by 123 teleseismic events. In the first step employed to determine the Rayleigh-wave phase-velocity structure, we implement a new tomographic method, which is simpler and more robust than scattering-based methods (e.g. multi-plane surface wave tomography). The TA is effectively implemented as a large number of local arrays by defining a horizontal Gaussian smoothing distance that weights observations near a given target point. The complex spectral-amplitude measurements are interpreted with the spherical Helmholtz equation using local observations about a succession of target points, resulting in Rayleigh-wave phase-velocity maps at periods over the range of 18-125 s. The derived maps depend on the form of local fits to the Helmholtz equation, which generally involve the nonplane-wave solutions of Friederich et al. In a second step, the phase-velocity maps are used to derive 3-D shear velocity structure. The 3-D velocity images confirm details witnessed in prior body-wave and surface-wave studies and reveal new structures, including a deep (>100 km deep) high-velocity lineament, of width ∼200 km, stretching from the southern Great Valley to northern Utah that may be a relic of plate subduction or, alternatively, either a remnant of the Mojave Precambrian Province or a mantle downwelling. Mantle seismic velocity is highly correlated with heat flow, Holocene volcanism, elastic plate thickness and seismicity. This suggests that shallow mantle structure provides the heat source for associated magmatism, as well as thinning of the thermal lithosphere, leading to relatively high stress concentration. Our images also confirm the presence of high-velocity mantle at 100 km depth beneath areas of suspected mantle delamination (southern Sierra Nevada; Grande Ronde uplift), low velocity mantle underlying active rift zones, and high velocity mantle associated with the subducting Juan de Fuca plate. Structure established during the Proterozoic appears to exert a lasting influence on subsequent volcanism and tectonism up to the Present.

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Seismic waveform modeling of boundary interaction phases is used to determine the discontinuity structure of the crust in the Subandean and foreland basin regions overlying the zone of flat subduction beneath east‐central Peru. The data analyzed are from intermediate‐depth earthquakes (110 to 155 km) recorded on an array of three‐component short‐period (1 Hz) digital seismographs deployed in the epicentral region. Full use is made of both P‐to‐S and S‐to‐P converted phases in the modeling. Results from the determination of crustal structure in the Subandean and foreland basin region of east‐central Peru confirm the presence of vast depositional basins comprised of low velocity sediments up to at least 8 km thick which flank the Andean orogen to the east and correlate with a substantially thickened crust atop the Brazilian shield. Crustal thickness in the foreland basin varies from about 35 km or less where sedimentary cover is minimal to 44 km in regions of maximum sedimentary deposition. There is some evidence that the crust thins slightly on the western side of the foreland basin (in the gap between basin and Subandean fold and thrust belt), but it rapidly thickens to 45–50 km beneath the Subandes proper, and to more than 50 km in the southern part of the Subandean belt. The results are consistent with, but do not require, a thick‐skinned model of foreland crustal deformation similar to that found for the block faulted terrane in Argentina above the zone of flat subduction there. At least some basin formation appears to be due to block faulting, where faults may penetrate into the mantle.

S U M M A R YThe damage zone of a major fault can act as a low-velocity seismic waveguide. The fault-zone guided waves provide a potential method to constrain the in situ physical properties of the fault zone (FZ) at depth. Recently, there has been debate over the depth extent of observed fault waveguides and whether fault properties at seismogenic depth can be constrained by guided waves (GWs). To address these questions, elastic finite-difference synthetic seismograms were generated for fault-zone models that include an increase in seismic velocity with depth both inside and outside the FZ. Previous synthetic studies for a homogeneous fault showed that earthquakes off of the fault do not generate GWs unless the waveguide is restricted to a few kilometres depth. In contrast, earthquakes both inside and outside of a depth-varying fault waveguide generate strong GWs within the near-surface portion of the FZ. This is because the frequency-dependent trapping efficiency of the waveguide changes with depth. The nearsurface fault structure efficiently guides waves at lower frequencies than the deeper FZ. The low-frequency waves that are guided at the surface are not efficiently guided at greater depth, and therefore, travel as body waves. Fault structure at seismogenic depth requires the analysis of data at higher frequencies than the GWs that dominate at the surface and have been the subject of most previous investigations.