Rapid lateral variation of P-wave velocity at the base of the mantle near the edge of the Large-Low Shear Velocity Province beneath the western Pacific

Abstract

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 northeastern 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 (20-50 km thick) and low velocity (-2 to -5 per cent) layers at the base of the mantle are intersected by an 80-km-thick region with a high velocity (+2 per cent). A slightly fast region exists at the northwest 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. & copy; 2014 The Authors 2014. Published by Oxford University Press on behalf of The Royal Astronomical Society.