The Development of Seismic Anisotropy Below South-central Alaska: Evidence From Shear Wave Splitting

South-central Alaska features many subduction zone characteristics: backarc, forearc and volcanic arc, and with recently deployed the Transportable Array, it is an ideal place to study subduction zone anisotropy. In [1] we present shear-wave splitting analysis of 157 local earthquakes of mb>=3.0 that occurred between 2014 and 2019, yielding 210 high quality measurements at 23 stations. Splitting delay times (dt) are generally small (dt~0.3s), increasing with distance from the trench. Arc parallel fast directions, phi, are only seen in the forearc, but rotate to arc perpendicular phi in the backarc. When reviewed in light of recent tomographic models of subduction zone seismic structure [2], the forearc local-earthquake signal likely originates from anisotropic serpentinite in fractures atop the subducting Pacific plate, with possible additional signal coming from fractures in the North American crust. Mantle wedge corner flow, potentially with additional arc-perpendicular anisotropy in the subducting slab, explains backarc anisotropy. Intriguingly, our local earthquake splitting results do not parallel SKS splitting measurements [3] in the same region. This could simply be due to the data sets being sensitive to different parts of mantle wedge. Our local S-wave splitting results may be sensitive to a relatively small-scale shallow 2-D corner flow, while the SKS data sets are sensitive to deeper 3-D flow [3]; additional arc-perpendicular anisotropy in the subducting slab may contribute to both. However, the Cook inlet segment, where most of the local anisotropic signal comes from, is quite some distance from the slab edge, and geodynamic modelling suggests that toroidal flow may not dominate this far south [4]. [1] Karlowska, E., Bastow, I., Rondenay, S., Martin-Short, R. & Allen, R., 2021. The development of seismic anisotropy below south-central Alaska ... GJI, 225(1), 548-554. [2] MartinShort, R., Allen, R., Bastow, I.D., Porritt, R.W. and Miller, M.S., 2018. Seismic imaging of the Alaska subduction zone ... G3, 19(11), 4541-4560. [3] Venereau, C., MartinShort, R., Bastow, I., Allen, R., & Kounoudis, R., 2019. The role of variable slab dip ... G3, 20(5), 2433-2448. [4] Jadamec, M. & Billen, M., 2010. Reconciling surface plate motions ... Nature, 465(7296), pp.338-341.