High-resolution Seismic Imaging Of Slab Morphology Beneath Izu-Bonin

Seismic tomography provides unique constraints on the morphology,the deformation,and(indirectly)the rheology of subducting slabs.We use teleseismic double-difference P-wave tomography to image with unprecedented clarity the structural complexity of the Izu-Bonin slab.The abundant seismicity allows us to resolve a plunging tear that splits the slab in the mantle transition zone at 26.5-28.50°N.North of the tear,we image a zone of reduced P-wavespeed at 420 km depth,which we interpret as the result of viscous dissipation within an incipient shear zone.To the south of the tear,the slab overturns and lies recumbent at the base of the mantle transition zone.The localised tearing,shearing and buckling indicate that the Izu-Bonin slab has a strongly non-Newtonian rheology which remains highly-viscous throughout the upper mantle and transition zone.Our results also reveal that the 680 km deep 2015 Bonin earthquake(Mw 7.9)occurred at the northernmost edge of the overturning slab,20-30 km from the slab-mantle wedge interface.Relatively low temperatures,high thermal gradients and high stresses may all have facilitated this large isolated earthquake.