The western United States has a rich and complicated recent tectonic history [e.g., Humphreys and Coblentz, 2007], which over the past ∼20 Ma has encompassed the rollback and steepening of the Farallon slab, the northward migration of the Mendocino Triple Junction, extension in the Basin and Range, the outpourings of the Steens/Columbia River flood basalts (S/CRB), and likely episodes of localized lithospheric delamination [Zandt et al., 2004; West et al., 2009]. Even in this complex tectonic setting, the High Lava Plains (HLP) of Oregon and the eastern Snake River Plain (SRP) of Idaho stand out as distinctive features. The HLP represents a bimodal volcanic province that over the past ∼12 Ma has exhibited both age-progressive rhyolitic volcanism [Jordan et al., 2004, Ford et al., 2013] and widespread basaltic volcanism [Till et al., 2013] with no obvious spatiotemporal pattern. The age-progressive trend in the HLP rhyolites is oblique to the absolute plate motion of the North American plate, in contrast to the Yellowstone/Snake River Plain (Y/SRP) trend to the east. The spatiotemporal trend in rhyolitic volcanism beneath the SRP is roughly parallel to the absolute motion of the North American plate [Pierce and Morgan, 1992] and also corresponds to the preexisting lithospheric structure of the western Idaho shear zone [Tikoff et al., 2008]. Many different models have been proposed to explain the origin and evolution of the S/CRB, HLP, and SRP volcanic trends [e.g., Hooper et al., 2007]. These models variously invoke a deep mantle plume [Armstrong et al., 1975; Richards et al., 1989; Camp and Ross, 2004; Smith et al., 2009; Obrebski et al., 2010; Kincaid et al., 2013], the rollback, steepening, tearing, and/or fragmentation of the Juan de Fuca slab at depth [Carlson and Hart, 1987; James et al., 2011; Liu and Stegman, 2012; Long et al., 2012], lithospheric delamination [Hales et al., 2005; Camp and Hanan, 2008; Darold and Humphreys, 2013], and/or preexisiting lithospheric structures [eCross and Pilger, 1978; Tikoff et al., 2008] as playing a role in generating volcanism.
 The recent interdisciplinary High Lava Plains Project (www.dtm.ciw.edu/research/HLP), along with the EarthScope USArray Transportable Array (TA), has provided a wealth of new geophysical data in the region. In particular, recent studies using body wave [Roth et al., 2008; Schmandt and Humphreys, 2010; James et al., 2011; Obrebski et al., 2011], surface wave [Warren et al., 2008; Wagner et al., 2010, 2012a], and ambient noise [Gao et al., 2011; Hanson-Hedgcock et al., 2012] tomography, receiver function analysis [Eagar et al., 2010, 2011; Schmandt et al., 2012], and SKS splitting measurements [Long et al., 2009] have yielded new insight into the structure of the crust and upper mantle beneath the HLP and SRP. One of the most striking geophysical characteristics of the HLP is the strong seismic anisotropy in the upper mantle beneath it. SKS splitting measurements for the western United States [Zandt and Humphreys, 2008; Liu, 2009; Eakin et al., 2010; Long et al., 2012] reveal strong splitting with large (>2 s) delay times beneath the HLP, and models that incorporate surface wave data also generally show strong upper mantle anisotropy in this region [e.g., Yuan and Romanowicz, 2010; Lin et al., 2011; Lin and Ritzwoller, 2011]. A study of SKS splitting beneath eastern Oregon that incorporated data from the first phase of the HLP experiment showed that stations in southeastern Oregon exhibit average delay times up to ∼2.7 s [Long et al., 2009], much larger than the ∼1 s that is typical for continental regions [Silver, 1996].
 One of the most striking geophysical characteristics of the SRP is the zone of very low upper mantle velocities that is ubiquitous beneath the SRP volcanic track. This low-velocity region takes the form of a roughly linear (in map view) feature that extends to depths of ∼150–200 km [e.g., Stachnik et al., 2008; Schmandt and Humphreys, 2010; James et al., 2011; Obrebski et al., 2011], deepening slightly beneath Yellowstone [Wagner et al., 2010]. The origin of this uppermost mantle structure continues to be debated, but recent electromagnetic imaging has suggested that the zone of particularly low wavespeeds coincides with relatively high conductivities [Kelbert et al., 2012], consistent with the presence of partial melt. Interestingly, unlike the HLP, the SRP has not been associated with particularly strong upper mantle anisotropy in past studies. Specifically, Schutt et al.  measured SKS splitting for a linear transect deployed perpendicular to the strike of the SRP and found a pronounced decrease in splitting delay times above the SRP proper. Later work by Waite et al. , which focused on a region to the east of the Schutt et al.  transect, did not identify a decrease in δt values beneath the SRP and instead found delay times of ∼ 1 s, comparable to the surrounding region.
 Seismic anisotropy is an important observable for understanding mantle processes because of the link between mantle deformation and the resulting anisotropy [e.g., Long and Becker, 2010]. In the upper mantle, seismic anisotropy is usually attributed to the lattice-preferred orientation (LPO) of anisotropic minerals, primarily olivine [e.g., Karato et al., 2008]. If the geometrical relationship between strain and the resulting anisotropy is known or assumed, then measurements of seismic anisotropy can be used to infer the pattern of mantle flow and deformation, giving seismologists one of the most powerful tools available to probe mantle dynamics with seismologic observations. While the geometry of mantle anisotropy is very often used to infer mantle flow patterns, the strength of anisotropy is more difficult to constrain observationally and thus more difficult to exploit to gain insight into mantle deformation. In particular, the poor depth resolution of SKS phases means that estimates of anisotropy strength from splitting measurements trade off directly with estimates of the thickness of the anisotropic layer. While surface wave measurements can provide depth constraints, it is a challenge to properly constrain the magnitudes of anisotropic anomalies in a regularized tomographic inversion of surface wave dispersion data. Although it is difficult to constrain, the strength of anisotropy is potentially important as it is related to upper mantle conditions during deformation, such as the amount of strain, olivine fabric type, mineralogy, and/or partial melt fraction [Karato et al., 2008].
 Here we take advantage of the data sets provided by the HLP broadband seismic experiment and the USArray Transportable Array (TA) and apply SKS splitting analysis and surface wave inversion to understand the lateral and depth variations in the strength of anisotropy beneath the Pacific Northwest, with a particular focus on the HLP and SRP. The very dense station spacing of the HLP deployment [Carlson et al., 2005] allows us to use SKS splitting to obtain robust constraints on small-scale lateral variations in splitting delay times, δt, beneath the HLP proper. In combination with surface wave inversions that yield information about the depth extent of the anisotropy, we are able to obtain a robust picture of lateral and depth variations in upper mantle anisotropy beneath the PNW region. Beneath the HLP, we find evidence for pronounced lateral gradients in anisotropic strength, with a region of particularly strong uppermost mantle anisotropy that coincides spatially with Holocene basalt volcanism. Given the relatively simple flow field beneath the region inferred from previous studies of anisotropy [Long et al., 2012], these variations may indicate lateral variations in the strength and/or geometry of olivine fabric, or in the amount or alignment of partial melt. Beneath the SRP, we find evidence for a layer of weak to moderate anisotropy with a strikingly different geometry than the surrounding regions of the upper mantle overlying a more strongly anisotropic layer more closely oriented to APM. This region coincides spatially with the region of extremely low wavespeeds inferred from seismic tomography [e.g., Wagner et al., 2010, 2012a; James et al., 2011] and high conductivity inferred from magnetotelluric (MT) measurements [Kelbert et al., 2012]. We discuss several plausible scenarios to explain the distinctive anisotropy we infer beneath both the HLP and SRP.