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1. Local- S shear wave splitting along the length of the Alaska–Aleutian subduction zone

   https://doi.org/10.1093/gji/ggae107  

   Lynner, Colton; Toro-Acosta, Cherilyn; Paulson, Eve; Birkey, Andrew (April 2024, Geophysical Journal International)

   SUMMARY The Alaska–Aleutian subduction zone represents an ideal location to study dynamics within a mantle wedge. The subduction system spans several thousand kilometres, is characterized by a slab edge, and has ample seismicity. Additionally, the majority of islands along the arc house broad-band seismic instruments. We examine shear wave splitting of local-S phases originating along the length of the subduction zone. We have dense measurement spacing in two regions, the central Aleutians and beneath Alaska. Beneath Alaska, we observe a rotation in fast splitting directions near the edge of the subducting slab. Fast directions change from roughly trench perpendicular away from the slab edge to trench parallel near the boundary. This is indicative of toroidal flow around the edge of the subducting Alaska slab. In the central Aleutians, local-S splitting is primarily oriented parallel to, or oblique to, the strike of the trench. The local-S measurements, however, exhibit a depth dependence where deeper events show more consistently trench-parallel directions indicating prevalent trench-parallel mantle flow. Our local-S shear wave splitting results suggest trench-parallel orientation are likely present along much of the subduction zone excited by the slab edge, but that additional complexities exist along strike. 

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2. Analysis of Shear Wave Splitting Patterns in Alaska: Evidence for Strong Intra‐Slab Anisotropy

   https://doi.org/10.1029/2025GL116411  

   Appini, Sharmila; Zheng, Yingcai; Wu, Jonny; Mooney, Walter D (October 2025, Geophysical Research Letters)

   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. 

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3. 3-D synthetic modelling and observations of anisotropy effects on SS precursors: implications for mantle deformation in the transition zone

   https://doi.org/10.1093/gji/ggab529  

   Huang, Quancheng; Schmerr, Nicholas C; Beghein, Caroline; Waszek, Lauren; Maguire, Ross R (January 2022, Geophysical Journal International)

   SUMMARY The Earth's mantle transition zone (MTZ) plays a key role in the thermal and compositional interactions between the upper and lower mantle. Seismic anisotropy provides useful information about mantle deformation and dynamics across the MTZ. However, seismic anisotropy in the MTZ is difficult to constrain from surface wave or shear wave splitting measurements. Here, we investigate the sensitivity to anisotropy of a body wave method, SS precursors, through 3-D synthetic modelling and apply it to real data. Our study shows that the SS precursors can distinguish the anisotropy originating from three depths: shallow upper mantle (80–220 km), deep upper mantle above 410 km, and MTZ (410–660 km). Synthetic resolution tests indicate that SS precursors can resolve $$\ge $$3 per cent azimuthal anisotropy where data have an average signal-to-noise ratio (SNR = 7) and sufficient azimuthal coverage. To investigate regional sensitivity, we apply the stacking and inversion methods to two densely sampled areas: the Japan subduction zone and a central Pacific region around the Hawaiian hotspot. We find evidence for significant VS anisotropy (15.3 ± 9.2 per cent) with a trench-perpendicular fast direction (93° ± 5°) in the MTZ near the Japan subduction zone. We attribute the azimuthal anisotropy to the grain-scale shape-preferred orientation of basaltic materials induced by the shear deformation within the subducting slab beneath NE China. In the central Pacific study region, there is a non-detection of MTZ anisotropy, although modelling suggests the data coverage should allow us to resolve at least 3 per cent anisotropy. Therefore, the Hawaiian mantle plume has not produced detectable azimuthal anisotropy in the MTZ. 

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4. Splitting constraints in regions with sparse coverage: uniformly processed PS measurements from a massive global data set

   https://doi.org/10.1093/gji/ggaf522  

   Wolf, Jonathan; Romanowicz, Barbara A; West, John D (January 2026, Geophysical Journal International)

   SUMMARY Shear wave splitting, caused by seismic anisotropy, provides insights into convective processes in the mantle. While upper mantle anisotropy beneath the continents is well-resolved with high lateral resolution, its characterization beneath the oceans remains limited due to the paucity of seismic stations. In this study, we leverage the sensitivity of P waves that convert to S upon reflection at the surface (PS) to infer seismic anisotropy near their bounce point. We measure PS splitting automatically from a global data set that includes all earthquakes with magnitudes $$\ge 5.9$$ from January 1995 to present, collected from 25 datacenters, totaling approximately 5800 events and 18 million three-component seismograms. After careful quality control, we obtain 889 PS splitting measurements from this data set, mostly for regions without other splitting constraints. These regions include the South China Sea, the Philippine Sea, the western Pacific Ocean, the western Atlantic Ocean near the Caribbean and the South Atlantic Ocean. Where independent SKS constraints are available, these are in general agreement with our PS results. Comparisons of PS fast directions to azimuthal seismic anisotropy derived from surface waves and the direction of absolute plate motion show a dominant contribution of asthenospheric deformation to the measurements. This agreement is particularly pronounced beneath the oceans, away from subduction zones, for instance, across much of the Pacific basin. In contrast, splitting patterns near subduction zones are generally more complex: fast directions are most often trench-parallel or oblique, and only rarely trench-perpendicular, suggesting that a trench-parallel component of shear deformation is commonly generated in subduction settings, as commonly inferred from SKS splitting. We make all measurements publicly available as a data product, along with detailed metadata to enable future work. 

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5. Trench motion-controlled slab morphology and stress variations: Implications for the isolated 2015 Bonin Islands deep earthquake

   https://doi.org/10.1002/2017GL073989  

   Yang, T. (October 2017, Geophysical research letters)

   null (Ed.)

   Abstract The subducted old and cold Pacific Plate beneath the young Philippine Sea Plate at the Izu‐Bonin trench over the Cenozoic hosts regional deep earthquakes. We investigate slab morphology and stress regimes under different trench motion histories with mantle convection models. Viscosity, temperature, and deviatoric stress are inherently heterogeneous within the slab, which we link to the occurrence of isolated earthquakes. Models expand on previous suggestions that observed slab morphology variations along the Izu‐Bonin subduction zone, exhibited as shallow slab dip angles in the north and steeper dip angles in the south, are mainly due to variations in the rate of trench retreat from the north (where it is fast) to the south (where it is slow). Geodynamic models consistent with the regional plate tectonics, including oceanic plate age, plate convergence rate, and trench motion history, reproduce the seismologically observed principal stress direction and slab morphology. We suggest that the isolated ~680 km deep, 30 May 2015 Mw 7.9 Bonin Islands earthquake, which lies east of the well‐defined Benioff zone and has its principal compressional stress direction oriented toward the tip of the previously defined Benioff zone, can be explained by Pacific slab buckling in response to the slow trench retreat. 

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