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Abstract Shear wave splitting (SWS) patterns at subduction zones are often interpreted by complex mantle flow above or below the slab. However, our recent previous work shows dipping anisotropic slabs can explain observed patterns in Japan. Here, we extend this analysis to the Alaska subduction zone, using 2,567 high‐quality teleseismic SWS measurements from 195 broadband stations. As was found in Japan, the observed SWS patterns in Alaska depend on earthquake backazimuth. The fast‐S polarization directions are either trench parallel or perpendicular in southeastern Alaska and form a prominent circular pattern in central Alaska. We found that a dipping anisotropic slab following the Slab 2.0 geometry, with 30% shear anisotropy, and exhibiting tilted transverse isotropy with a symmetry axis normal to the slab interface, predicts both the fast‐S polarizations and delay times (δt = 1.0–1.5 s). This suggests that intra‐slab anisotropy can be the primary control on SWS, without requiring complex mantle flow. 

Abstract Complex shear wave splitting (SWS) patterns in subduction zones are often interpreted geodynamically as resulting from complex mantle flow; however, this may not always be necessary. We analyzed 7,093 high‐quality SWS measurements from teleseismic S waves recorded by Hi‐net stations across the Ryukyu arc in Japan. Our findings show a systematic rotation of the fast S polarization from trench‐parallel to trench‐perpendicular depending on the earthquake backazimuth. For the same earthquake, the measured splitting patterns also vary spatially across the southwest Japan. Using full‐wave seismic modeling, we showed that a dipping slab with ∼30% shear anisotropy of the tilted transverse isotropy (TTI) type, with a symmetry axis perpendicular to the slab interface, can predict the observed delay times and polarization rotation. Our results highlight the importance of considering dipping anisotropic slabs in interpreting SWS at subduction zones.