Aftershocks on the Planar Rupture Surface of the Deep-Focus Mw 7.9 Bonin Islands Earthquake

Abstract

The largest deep-focus earthquakes typically occur in isolated settings, making their aftershock sequences crucial for understanding the interaction between these deep events, the subducting slabs, and their surroundings. The exceptionally deep Mw 7.9 Bonin Islands earthquake, which occurred at a depth of 680 km in a region previously thought to be aseismic, provides a rare opportunity to investigate the behavior and mechanism of deep earthquakes. Using an array-based beamforming technique, we flag numerous false detections in previous work, detect additional aftershocks following the Bonin Islands earthquake, and relocate using backprojection. The aftershocks are located outside the mainshock rupture zone and are aligned along a plane that extends from the mainshock rupture, suggesting a deformation migration across a pre-existing zone of weakness. These observations are compatible with the hypothesis of a thin metastable olivine wedge within the lower part of the Izu–Bonin slab, which may play a role in the deep earthquake mechanism. We reject a previous claim of deeper aftershock activity within the lower mantle. Our results further the understanding of the structure of the mantle and the physical mechanisms driving deep-focus earthquakes.