Two-dimensional/three-dimensional waveform modeling of subducting slab and transition zone beneath Northeast Asia

Subduction plays a fundamental role in dynamics of the mantle convection and in material circulations of the Earth's interior. Slabs have been imaged to subduct near horizontally in the transition zone (TZ) beneath the Northeast Asia in seismic tomography. Triplication waveform modeling is an effective tool to study the detailed seismic structure in the TZ. However, TZ triplication modeling has traditionally relied on 1-D models. In this study, we use the spectral element method to explore influences of 2-D/3-D slab structure on TZ triplication waveforms and to model, for the first time, the slab and TZ structures beneath the Northeast Asia. Synthetic tests suggest that, for a subduction zone earthquake, slab structure can have important influences on TZ triplication waveforms and that, even in a narrow azimuth range, the effects from 2-D/3-D slab structure on the wave propagation can lead to erroneous conclusions with 1-D modeling. Our data are high-quality triplicated SH waveforms (at distances of 10°-32°) from a deep event (below 410 km discontinuity) in the Pacific subducting slab. Our 2-D/3-D waveform modeling results suggest that a simple model of the subducting slab (+5% high-velocity anomaly and ∼100 km thick down to 560 km) but normal below 560 km can match most of the observed waveforms remarkably well. The bottom of the TZ of the sampling region (north of the Yellow Sea) contains a patch of very slow anomaly. The results indicate that subhorizontal slab above the 660 discontinuity is not everywhere beneath Northeast Asia and the subducting slab is not everywhere continuous to the bottom of the TZ. Compared with the traditional 1-D modeling, our new 2-D/3-D approach provides better fits to the data and allows us to constrain the slab geometry and to separate TZ structure from slab structure Key Points SEM synthetics are used to model 2-D/3-D effects of slab and TZ A simple slab (+5% high and ∼100 km thick to 560 km) is preferred The bottom of the TZ contains a patch of very slow anomaly