Lateral variations in SH velocity structure of the transition zone beneath Korea and adjacent regions

Using seismic profiles of triplicated waveforms, we show significant lateral variations in the SH velocity (Vs) structure of the transition zone (TZ) beneath Korea and adjacent regions. Beneath Sakhalin, we detected a high Vs anomaly (similar to 2%) limited to middle regions of the TZ (mid-TZ), and a large Vs jump across the 660-km discontinuity. A similar jump in Vs also occurs beneath the northern portion of the North China Craton (NCC). Beneath Korea, a high Vs anomaly (similar to 2%) in the lower TZ is inferred, accompanied by a relatively small Vs jump across the 660-km discontinuity, which is depressed by about similar to 15-20 km. The deep structure under the eastern part of northeast China (NEC) also includes a slight Vs anomaly (similar to 1%) in the lower TZ but does not exhibit significant depression of the 660-km discontinuity. Compared with previous study, our observations reveal strong regional variations of the TZ structure on a relatively short scale. These variations most likely reflect the geometrical distribution of the subducting northwest Pacific plate. Our results suggest that the subducting slab dips across the mid-TZ under Sakhalin, and becomes flattened atop of the 660-km discontinuity beneath Korea, while only the tip of the slab reaches the lower TZ beneath the NEC. The TZ beneath the NCC does not show evidence of the slab stagnation.