Seismic anisotropy provides key information to map the trajectories of mantle flow and understand the evolution of our planet. While the presence of anisotropy in the uppermost mantle is well-established, the existence and nature of anisotropy in the transition zone and uppermost lower mantle are still debated. Here we use 3-D global seismic tomography images based on a large data set sensitive to this region to show the ubiquitous presence of anisotropy in the lower mantle beneath subduction zones. Whereas above the 660-km seismic discontinuity slabs are associated with faster SV anomalies up to about 3%, in the lower mantle faster SH anomalies of about 2% persist near slabs down to about 1,000-1,200 km. These observations are consistent with 3-D 1 numerical models of deformation from subducting slabs and the associated lattice preferred orientation of bridgmanite produced in the dislocation creep regime in areas subjected to high stresses. This study provides evidence that dislocation creep may be active in the Earth's lower mantle, providing new constraints on the debated nature of deformation in this key but inaccessible component of the deep Earth.The Earth's upper and lower mantle have quite distinct physical properties, with the characteristics of material exchange between them being a long-debated issue. Progress in global seismic tomography in the 1990s 1,2 showed that the upper and lower mantle interact mainly via subducting slabs and mantle plumes, albeit subject to the presence of strong resistance along the upper-lower mantle boundary at ∼660 km depth. More recently, enhanced tomography images showed that amongst the slabs that penetrate into the lower mantle, many of them stagnate down to about ∼1,000 km depth 3 . Conversely, mantle plumes rising from the deep lower mantle seem to deflect laterally when they reach this region 4 . However, the uppermost lower mantle, located at depths of ∼660-1,000 km, remains an enigmatic part of the Earth. It has been suggested that compositional layering 5,6 or a viscosity increase 7,8 may cause flow stagnation in this region, but its rheology and role in mantle convection are poorly understood.The stagnation of subducting slabs at ∼660 km depth and their penetration into the lower mantle lead to intense strain and deformation around the slabs, which in turn can align mineral aggregates. Since the most abundant lower mantle mineral (bridgmanite) is anisotropic, observable seismic anisotropy should develop when considering a dislocation creep deformation mechanism 9,10,11 . However, apart from the D" region in the lowermost mantle 12 , the presence of seismic anisotropy in the lower mantle is uncertain and debated 13,14,15 , with most previous seismological models suggesting that the bulk of the uppermost lower mantle is radially isotropic in shear wavespeed 16 . In order to resolve this paradox, it has been proposed that the dominant

We build SWUS-amp, a three-dimensional shear-wave speed model of the uppermost mantle of the western U.S. using Rayleigh wave amplification measurements in the period range of 35-125 s from teleseismic earthquakes. This represents the first-ever attempt to invert for velocity structures using Rayleigh wave amplification data alone. We use over 350,000 Rayleigh wave amplitude measurements, which are inverted using a Monte Carlo technique including uncertainty quantification. Being a local seismic observable, Rayleigh wave amplification is little affected by path-averaged effects and in principle has stronger depth resolution than classical seismic observables, such as surface wave dispersion data. SWUS-amp confirms shallow mantle heterogeneities found in previous models. In the top 100 km of the mantle, we observe low-velocity anomalies associated with Yellowstone and the Basin & Range province, as well as a fast-velocity anomaly underneath the Colorado

Recent seismic studies indicate the presence of seismic anisotropy near subducted slabs in the transition zone and uppermost lower mantle (mid‐mantle). In this study, we investigate the origin of radial anisotropy in the mid‐mantle using 3‐D geodynamic subduction models combined with mantle fabric simulations. These calculations are compared with seismic tomography images to constrain the range of possible causes of the observed anisotropy. We consider three subduction scenarios: (i) slab stagnation at the bottom of the transition zone; (ii) slab trapped in the uppermost lower mantle; and (iii) slab penetration into the deep lower mantle. For each scenario, we consider a range of parameters, including several slip systems of bridgmanite and its grain‐boundary mobility. Modeling of lattice‐preferred orientation shows that the upper transition zone is characterized by fast‐SV radial anisotropy anomalies up to −1.5%. For the stagnating and trapped slab scenarios, the uppermost lower mantle is characterized by two fast‐SH radial anisotropy anomalies of ∼+2% beneath the slab's tip and hinge. On the other hand, the penetrating slab is associated with fast‐SH radial anisotropy anomalies of up to ∼+1.3% down to a depth of 2,000 km. Four possible easy slip systems of bridgmanite lead to a good consistency between the mantle modeling and the seismic tomography images: [100](010), [010](100), [001](100), and <110>false{true1¯10false}. The anisotropy anomalies obtained from shape‐preferred orientation calculations do not fit seismic tomography images in the mid‐mantle as well as lattice‐preferred orientation calculations, especially for slabs penetrating into the deep lower mantle.

&lt;p&gt;The seismic imaging of radial anisotropy can be used as a proxy for the direction of mantle flow. Previous studies have imaged radial anisotropy throughout the mantle on a global scale and are starting to show some consistent features. However, the interpretation of existing models is hindered by the lack of uncertainties provided from the employed inversion method. To address this, we build a new global radially anisotropic model of the Earth&amp;#8217;s upper mantle which consists of two main stages. Firstly, we build global Rayleigh and Love wave phase and group velocity maps using ~47 million measurements, including fundamental mode and up to 5&lt;sup&gt;th&lt;/sup&gt; overtone measurements, and compute their associated uncertainties. Weights according to similar paths and data uncertainties are employed in the inversions. We construct a total of 310 2D maps, at periods between T16-375 s, expanded in spherical harmonics up to degree lmax=60 and observe many relevant small-scale structures, such as e.g. the curvature of the Tibetan plateau at T~40s (fundamental mode). As expected, uncertainties are higher in regions of poor data coverage (e.g., southern hemisphere and oceans). Then, we invert for 1D profiles of radial anisotropy using two Monte Carlo based inversion methods: the Neighbourhood Algorithm (NA) and the Reversible-Jump Markov Chain Monte Carlo algorithm (RJMCMC). The NA has been widely used for seismic inversion, as it efficiently explores the parameter space. However, the advantage of the RJMCMC is that in addition to constraining e.g. radial anisotropy, it can also constrain e.g. layer thickness. We compare the 1D profiles from both methods, and their associated uncertainties, which will lead to a new global 3D model of radial anisotropy.&lt;/p&gt;

Surface wave amplification measurements have narrower depth sensitivity when compared to more traditional seismic observables such as surface wave dispersion measurements. In particular, Love wave amplification measurements have the advantage of strong sensitivity to the crust. For the first time, we explore the potential of Love wave amplification measurements to image crustal velocity in the western U.S. The effects of overtone interference, radial anisotropy and Moho depth are all explored. Consequently, we present SWUS‐crust, a three‐dimensional shear‐wave velocity model of crustal structure in the western U.S. We use Rayleigh wave amplification measurements in the period range of 38‐114 s, along with Love wave amplification measurements in the period range of 38‐62 s. We jointly invert over 6,400 multi‐frequency measurements using the Monte‐Carlo based Neighbourhood Algorithm, which allows for uncertainty quantification. SWUS‐crust confirms several features observed in previous models, such as high‐velocity anomalies beneath the Columbia basin and low‐velocity anomalies beneath the Basin and Range province. Certain features are sharpened in our model, such as the northern border of the High‐Lava Plains in southern Oregon in the middle crust.