Slab morphology at the source region of the 2015 Mw 7.9 Bonin earthquake imaged by full waveform inversion

Abstract:

The mechanism of the very deep 2015 Mw 7.9 Bonin earthquake (about 680 km deep) is highly debated partly due to the controversial slab morphology imaged near the source. We obtain significantly improved slab images near this earthquake source region based on full waveform inversion using a large waveform dataset recorded in East Asia. Both P and S wave speed images in the trench normal direction indicate a well-defined vertical slab deflected to the west at the bottom of the transition zone which hosts abundant deep-focus earthquakes at depths of 350–550 km where significant slab necking is observed. This vertically sinking slab has undergone substantial folding and piling and the more ancient slab segment at the bottom of the pile has penetrated to the top of the lower mantle while bending eastward. The 2015 Bonin earthquake occurred where the slab bends in the lower mantle with noticeable slab necking. Additionally, a prominent slab segment at the bottom of the mantle transition zone is imaged above and to the east of the 2015 Bonin earthquake main shock, spatially correlated with the recently imaged foreshock and aftershock locations, interpreted as a horizontally detached Pacific slab from the south due to the recent trench-advancing induced backward bending and tearing. The spatial relationship between the imaged 3-D slab morphology and the 2015 Bonin earthquake sequences suggest the significant slab deformation and necking controls the distribution of the deep-focus seismicity.