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Imaging the structure of the 660-km discontinuity is crucial to understanding the thermal and compositional states of the mantle transition zone. Here we study triplication data sampling beneath southeastern China and observe strong arrivals with rays turning at top of the lower mantle at an epicentral distance of 10 degrees. Such strong arrivals indicate that the S velocity at the discontinuity increases by similar to 8.2 +/- 0.5% over less than 10 km. This sharp 660-km discontinuity suggests that harzburgite enrichment exists at the bottom of the 660-km discontinuity and the temperature near 660 km does not exceed 2000 K. Through comparisons with 1-D models for other regions, we find that the seismic structure in the mantle transition zone below our study area is similar to that beneath oceanic regions. Thus, we suggest that the mantle near the 660-km discontinuity beneath southeastern China is locally not affected by slab or plume.

Plain Language Summary The characteristics (depth, undulation, transition thickness, and velocity jump) of the 660-km discontinuity are critical for studying detailed phase transitions within the mantle transition zone and the dynamic processes that link the upper and lower mantle, such as cold subduction and hot plumes. However, regional studies of the transition thickness and the velocity jump are challenging due to limited data. In this study, we fit the waveforms of data with rays traveling above and beneath the 660-km discontinuity and identify a 660-km discontinuity with a S velocity jump of similar to 8.2% within 10 km beneath southeastern China. Moreover, we suggest that the temperature approximately 660 km does not exceed 2000 K and that the mantle near the 660-km discontinuity beneath southeastern China is acting as "normal" mantle, sharing similar physical properties to the mantle under oceanic regions.