LAUSR

The termination of deep earthquakes at a depth of ~700 km is a key feature for understanding the physical mechanism of deep earthquakes. The ~680 km deep May 30, 2015, Ogasawara (Bonin) earthquake (Mw7.9) and its aftershocks were recorded by seismic stations at distances from 7° to 19°. 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.