Transition zone structures beneath eastern China derived from triplicated waveforms

Fine structure of the 660 discontinuity is essential to understanding the Earth evolution and its dynamics. Both receiver function images and triplication waveforms have provided some evidence for a double 660 discontinuity. However, due to lack of resolution of these methods, definitive finding of the double feature is still under question. In this study, we use seismic data from over 1000 broadband stations, archived by the Data Management Center of State Earthquake Information Services in China, to probe the transition zone structures in details. The permanent array is denser than USArray and records regional earthquakes at triplication distances from western Pacific, which provide excellent ray path coverage of the upper-mantle structures beneath eastern China. In this study, we analyze waveforms from three deep earthquakes in the transition zone beneath the Bonin Island, northeastern China, and Russia. These earthquakes mainly sample the transition-zone structures, which are related to the subducted Pacific Plate beneath eastern China. The CD branch of the S-wave triplication from the Bonin earthquake has relative longer period, while the AB branch shows two distinct arrivals with enhanced high-frequency energy. This double-AB feature appears to be associated with a double 660 discontinuity and disappear at stations in the south and north, which suggest that the subducted Pacific slab may be present in the transition zone beneath the Yellow Sea effecting this phase transition. Preliminary modeling of the waveforms indicates a ~2% velocity increase at 660 km and another ~6% increase at about 680 km. The double 660 feature may be indicative of Al content of more than 4%.