This research utilizes a seismic array processing method known as back-projection to constrain the spatiotemporal distribution of seismic sources during earthquakes. This approach is used to understand the mechanisms that initiate and propagate deep-focus earthquakes that have focal depths below 300 km. The top figure below from Kiser et al. (2021) shows that source image and associated record section from a distal aftershock of the 2015 Mw 7.9 Bonin Islands earthquake that occurred at an anomalous depth ~750 km. This is the first observation of an earthquake that originated in the lower mantle. The bottom figure below from Kehoe et al. (2024) shows the strikes and dips of rupture propagation from moderate-sized deep-focus earthquakes in the Izu-Bonin subduction zone. These results show a moment-dependent change in the rupture characteristics of these events at around moment magnitude 6.5 which is inferred to represent a change in the source mechanisms between the larger and smaller events.
