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Abstract Deep‐focus earthquakes provide insight into how subducting slabs deform over a range of spatial and temporal scales as they descend into the mantle. This study uses a 4D source imaging approach to determine centroid locations of the 2015 Mw 7.9 Bonin Islands deep‐focus earthquake and its aftershock sequence. Imaged sources of the mainshock show a complex rupture, but one that is compatible with a sub‐horizontal rupture plane. Previously undetected early aftershocks are imaged down to depths of approximately 750 km and represent the first reported earthquakes that initiate in the lower mantle. These events and a previously reported group of shallower distal aftershocks occur at the lower and upper boundaries of an imaged slab segment that deforms as it penetrates into the lower mantle. We hypothesize that mainshock failure allowed gravitational settling of the slab segment to occur which produced the distal aftershock sequences. 

Abstract Supershear earthquakes, propagating faster than the Earth's shear wave velocity, can generate strong ground motion at distances far from the ruptured fault. Despite the hazards associated with these earthquakes, the exact fault properties that govern their occurrence are not well constrained. Although early studies associated supershear ruptures with simple fault geometries, recent dynamic rupture models have revealed a supershear transition mechanism over complex fault geometries such as fault stepovers. Here we present the first observation of a supershear transition on a fault stepover system during the 2017 M_w7.7 Komandorsky Islands earthquake. Using a high‐resolution back‐projection technique, we find that the earthquake's rupture velocity accelerates from 2.1 to 5.0 km/s between two offset faults, demonstrating the viability of a new supershear transition mechanism occurring in nature. Given the fault complexity of the Earth's transform plate boundaries, this result may improve our understanding of supershear rupture processes and their associated hazards.