Seismic anisotropy and heterogeneity in the Alaska subduction zone

We determined P- and S-wave tomography and P-wave anisotropic structure of the Alaska subduction zone using 259 283 P- and 73 817 S-wave arrival times from 7268 local shallow and intermediate-depth earthquakes recorded by more than 400 seismic stations. The results show strong velocity heterogeneities in the crust and upper mantle. Low-velocity anomalies are revealed in the mantle wedge with significant along-arc variations under the active volcanoes. In the mantle wedge, the low-velocity zone extends down to 100-150km depth under the backarc. The results indicate that H ₂O and fluids brought downwards by the subducting Pacific slab are released to the mantle wedge by dehydration and they are subsequently transported to the surface by the upwelling flow in the mantle wedge. Significant P-wave anisotropic anomalies are revealed under Alaska. The predominant fast velocity direction (FVD) is trench-parallel in the shallow part of the mantle wedge (<90km depth) and in the subslab mantle, whereas the FVD is trench-normal within the subducting Pacific slab. The trench-parallel FVDs in the mantle wedge and subslab mantle may be caused by 3-D mantle flow that is induced by the complex geometry and strong curvature of the Pacific slab under Alaska. The flat and oblique subduction of the Pacific slab may play a key role in forming the trench-parallel FVD under the slab. The trench-normal FVD in the subducting Pacific slab may reflect the original fossil anisotropy when the Pacific Plate was produced at the mid-ocean ridge.