ISOLATED DEEP EARTHQUAKES AND THE FATE OF SUBDUCTION IN THE MANTLE

The locations of earthquakes with depth in the mantle provide direct e vidence of the trajectory and state of stress of subducting lithospher e. The occurrence over the last decade of isolated large deep earthqua kes outside the main Wadati-Benioff zones in the Kurile, Izu-Bonin, an d Chile deep seismic areas (in addition to the observed flat distribut ions of deep seismicity beneath the Banda Sea and Fiji Plateau areas a nd the large number of deep earthquakes located away from the main sla b in the Tonga arc) supports recent tomographic and other seismologica l studies, leading to a reconsideration of slabs as simple planar stru ctures sinking undeformed into the lower mantle. The focal geometries of these isolated earthquakes, which often exhibit near vertical compr essional stresses, are distinct from adjacent events in the main Wadat i-Benioff zone. They are also different in orientation from events in the deflecting toe of the slab and from the stress orientation expecte d if the deflected slab acts as a stress guide. These isolated mantle earthquakes are interpreted to take place in subducted lithosphere tha t has deflected to a horizontal posture at the base of the upper mantl e. Conversely, the deep seismicity in the adjacent Wadati-Benioff zone is associated with the deflection of the subducting lithosphere.