Seismic Observations Indicating That the 2015 Ogasawara (Bonin) Earthquake Ruptured Beneath the 660 km Discontinuity

The termination of deep earthquakes at a depth of similar to 700 km is a key feature for understanding the physical mechanism of deep earthquakes. The similar to 680 km deep 30 May 2015, Ogasawara (Bonin) earthquake (M-w 7.9) and its aftershocks were recorded by seismic stations at distances from 7 degrees to 19 degrees. Synthetic seismograms indicate that the P waveforms depend on whether the earthquake is located above or below the 660 km discontinuity. In this study, I show that broadband recordings indicate that the 2015 earthquake may have occurred below the 660 km velocity discontinuity. Recordings of the P wave from the strongest aftershock lack evidence for wave triplication expected when a subhorizontal discontinuity underlies the hypocenter. Theoretical waveforms computed with a 660 km discontinuity above the aftershock and mainshock match the observed waveforms more accurately. These observations may indicate earthquake ruptures due to mantle minerals other than olivine or strong deformation of the 660 km phase transition.