Isolated deep earthquakes and the fate of subduction in the mantle

Abstract

The locations of earthquakes with depth in the mantle provide direct evidence of the trajectory and state of stress of subducting lithosphere. The occurrence over the last decade of isolated large deep earthquakes outside the main Wadati‐Benioff zones in the Kurile, Izu‐Bonin, and Chile deep seismic areas (in addition to the observed flat distributions of deep seismicity beneath the Banda Sea and Fiji Plateau areas and the large number of deep earthquakes located away from the main slab in the Tonga arc) supports recent tomographic and other seismological studies, leading to a reconsideration of slabs as simple planar structures sinking undeformed into the lower mantle. The focal geometries of these isolated earthquakes, which often exhibit near vertical compressional stresses, are distinct from adjacent events in the main Wadati‐Benioff zone. They are also different in orientation from events in the deflecting toe of the slab and from the stress orientation expected if the deflected slab acts as a stress guide. These isolated mantle earthquakes are interpreted to take place in subducted lithosphere that has deflected to a horizontal posture at the base of the upper mantle. Conversely, the deep seismicity in the adjacent Wadati‐Benioff zone is associated with the deflection of the subducting lithosphere.