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Abstract To investigate the effects of a slab edge and varying slab geometry on the mantle flow systems beneath south central Alaska, a total of 971 pairs of teleseismic shear wave (SKS, SKKS, and PKS) and 65 pairs of local S wave splitting parameters (fast orientations and splitting times) are measured using data from the USArray and other networks. The Pacific‐Yakutat slab edge separates two regions with different characteristics of the splitting measurements. The area to the west of the slab edge has greater splitting times and mostly trench parallel fast orientations, and the area to the east is dominated by smaller splitting times and spatially varying fast orientations. The spatial distribution of the splitting parameters and results of anisotropy layering and depth analyses can be explained by a model involving three flow systems. The sub‐slab flow initially entraining with the shallow‐dipping Yakutat slab deflects to a trench‐parallel direction due to slab retreat and an increase in slab dip, and flows northeastward toward the slab edge, where it splits into two branches. The first branch enters the mantle wedge as a toroidal flow and flows southwestward along the slab, and the second branch continues approximately eastward. The flowlines of the toroidal and continued flow systems are approximately orthogonal to each other in the vicinity of the slab edge, producing the observed small splitting times and spatially varying fast orientations. 

Abstract To discern spatial and explore possible existence of temporal variations of upper crustal anisotropy in an ∼15 km section of the San Jacinto Fault Zone (SJFZ) that is composed of the Buck Ridge and Clark faults in southern California, we conduct a systematic shear wave splitting investigation using local S‐wave data recorded by three broadband seismic stations located near the surface expression of the SJFZ. An automatic data selection and splitting measurement procedure is first applied, and the resulting splitting measurements are then manually screened to ensure reliability of the results. Strong spatial variations in crustal anisotropy are revealed by 1,694 pairs of splitting parameters (fast polarization orientation and splitting delay time), as reflected by the dependence of the resulting splitting parameters on the location and geometry of the raypaths. For raypaths traveling through the fault zones, the fast orientations are dominantly WNW‐ESE which is parallel to the faults and may be attributed to fluid‐filled fractures in the fault zones. For non‐fault‐zone crossing raypaths, the fast orientations are dominantly N–S which are consistent with the orientation of the regional maximum compressive stress. A three‐dimensional model of upper crustal anisotropy is constructed based on the observations. An increase in the raypath length normalized splitting times is observed after the 03/11/2013 M4.7 earthquake, which is probably attributable to changes in the spatial distribution of earthquakes before and after the M4.7 earthquake rather than reflecting temporal changes of upper crustal anisotropy.