Source Complexity of the 2015 Mw 7.9 Bonin Earthquake

The 30 May 2015 Mw 7.9 Bonin earthquake, one of the largest and deepest earthquakes ever recorded by modern seismology, provides a unique opportunity to study the source process and physical mechanisms of deep-focus earthquakes. We develop a novel back projection technique that allows source imaging in full three-dimensional space with a high-depth resolution. Our results indicate an initial SW-NE bilateral source propagation followed by a northwest source extension. The multiple-source inversion reveals a two-step source process with propagating directions nearly perpendicular to each other, consistent with the 3-D back projection result. The spatial distribution and focal mechanisms of the subevents cannot be modeled by a single planar rupture, which may display a curved rupture plane or subevents crossing multiple fault interfaces. The complex source process can be best explained by stress or structure heterogeneity within the deep slab.