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We examine P-wave velocity structure at the base of the mantle beneath the western Pacific, near the western edge of the Pacific Large-Low Shear Velocity Province (LLSVP), using high-quality seismograms provided by a large-scale mobile broad-band seismic observation in north eastern China (the NECESSArray project). Forward modelling using the reflectivity method is conducted to explain the variation of P-wave traveltimes as a function of epicentral distance near the core shadow zone. Additionally, PcP–P traveltimes are examined to enlarge the survey area. As a result, a rapid variation of P-wave velocity is detected at the base of the mantle. Regions of thin (20–50km thick) and low velocity (−2 to−5percent) layers at the base of the mantle are intersected by an 80-km-thick region with a high velocity (+2percent). A slightly fast region exists at the north west of the region with the thin low-velocity layer.These layers are typically separated by several hundred kilometres and would be difficult to explain by thermal effects alone. These observations suggest that very complicated thermochemical reactions occur near the edge of the Pacific LLSVP.
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