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Subducting the buoyant crustal material of an aseismic oceanic ridge has been regarded as a dominant contributor to flat slab subduction. However, normal‐dip subduction is also observed in some cases where ridges are subducting. In this study, we compare the subduction of two ridges on the Nazca Plate: Nazca Ridge (flat slab) and Iquique Ridge (normal‐dip slab). Anisotropy determined by shear wave splitting observation suggests that the low‐velocity anomalies found beneath the ridges are mapping anisotropic structure into isotropic velocities. After a tomographic inversion incorporating anisotropy models for both ridges, we find that the low‐velocity anomalies found beneath the Nazca Ridge are not anisotropic and therefore likely represent warm mantle, and those beneath the Iquique Ridge are caused by anisotropy. We conclude that subslab mantle buoyancy has a larger impact on the subduction angle than the crustal material of the ridge. Plain Language Summary: Understanding the subduction process and how the mantle flows is pivotal in understanding the planetary evolution of Earth. Yet, several subduction characteristics remain unsolved, and the angle of plate subduction, which is often categorized into <30° (shallow), ∼30–35° (normal), >35° (steep), is one of those. Subduction of an oceanic ridge on a plate has been proposed as a cause of a shallow subduction angle since the thick crust of the ridge is less dense than the surrounding mantle. However, normal angles have been observed in some cases where oceanic ridges are subducting. In this study, we compare the Nazca Ridge and the Iquique Ridge, on the Nazca Plate subducting beneath South America. The subduction angles of the Nazca and Iquique Ridges are shallow and normal, respectively. When we incorporate directional variations in seismic wave velocities, which are produced by mantle flow, in seismic tomographic imaging, we find that the subducting oceanic ridge may not be a primary factor producing shallow angle subduction. Instead, the warm mantle beneath the Nazca Ridge may provide the buoyancy to support the Nazca Plate. Comparably, since the mantle beneath the Iquique Ridge is not warm, the subduction angle would stay normal. Key Points: Despite having different subduction angles, subslab low‐velocity anomalies are found beneath both Nazca Ridge and Iquique RidgeWe introduced hypothetical seismic anisotropy in tomographic inversion to explore the origin of the subslab low‐velocity anomaliesThe low‐velocity anomalies beneath the Nazca Ridge and Iquique Ridge may come from buoyant warm mantle and anisotropy, respectively [ABSTRACT FROM AUTHOR]

• We present 3D seismic attenuation (Q) model of the Nazca Plate subduction zone. • Our model shows clear high- Q slab and low- Q continental plate structures. • Our model for the Brazilian Shield shows high- Q at 40–80 km depth in the backarc. • Along-arc slab Q variations indicate different slab hydration states. • Relatively high- Q at the mantle wedge corner indicates cold thermal state. Seismic attenuation provides key constraints on understanding the structure and dynamics of subduction-zone system. In this study, we provide the first three-dimensional P and S -wave attenuation models beneath the Nazca-South America subduction zone by inverting t * data from 397 local shallow and intermediate-depth earthquakes. The study area covers the southern part of the Peruvian flat-slab region (11°–15 °S) (where the subducting Nazca Ridge is migrating), and the Central Andean Plateau (∼13°–18 °S) (associated with the subducting Nazca Plate) and the Brazilian mantle lithosphere. We obtain five major features in our attenuation model with seismicity including (1) well-defined along-strike and along-dip Nazca slab structure in terms of low attenuation, (2) high attenuation associated with the passage of Nazca Ridge and present location of the oceanic ridges, (3) high attenuation in the backarc mantle wedge and continental crust, (4) clear image of Brazilian Shield in terms of low attenuation, underthrusting to the west, and (5) high attenuation related to the slab dehydration beneath the volcanic arc at 100 and 200 km depths. In particular, prominent low- Q zones in the continental crust and mantle wedge beneath the active volcanic arc reflect source zones of arc magmatism caused by fluids from the slab dehydration. The observed along-arc differences in slab Q estimates can be attributed to different hydration states between the flat and normal-dip slabs due to the subduction of the Nazca Ridge. [ABSTRACT FROM AUTHOR]