Slab tearing along a subducted oceanic plate joint beneath the Alaska Peninsula

Along-trench variations in various properties of subducted slabs have been inferred in subduction zones globally, yet the causes of these variations remain poorly understood. Along the Alaska Peninsula, the variation of slab (de)hydration and fluids shallower than 50-km depth cannot explain a seismicity gap below 150 km nor the abrupt changes in volcano density and arc orientation around Aniakchak volcano. Here we investigate the cause of these observations using seismic full-wave ambient noise tomography. Our 3D shear-wave velocity model reveals multiple high-velocity slab segments below about 50 km, with a much-lower-velocity segment broadly below Aniakchak. This low-velocity segment coincides with the inferred location of a subducted oceanic plate joint, where two sets of plate fabrics intersect, and with slab-normal fast directions in the asthenosphere indicated by seismic anisotropy. We infer that the subducted oceanic joint facilitated slab weakening and tearing, which has developed into a slab window below about 150 km, explaining the seismicity gap and slab-orthogonal mantle flow. Hence we suggest that oceanic plate joints may have an important role as potential weak zones in slabs and thus potentially influence subduction dynamics and seismicity.