Detection of a Thick and Weak Low‐Velocity Layer atop the Mantle Transition Zone beneath the Northeastern South China Sea from Triplicated P‐Wave Waveform Modeling

Triplicated P waveforms related to the 410‐km discontinuity from five intermediate‐depth earthquakes in the central Philippines are clearly recorded by the Chinese Digital Seismic Network, but some branches of the S‐wave triplications are obscure. Matching the observed P‐wave triplications with synthetics through a grid‐search technique, we obtain the best‐fit 1D P‐wave velocity model near the 410‐km discontinuity beneath the northeastern South China Sea. In such a model, a low‐velocity layer (LVL) is found to reside atop the mantle transition zone, and it is characterized by a thickness of 92.5±11.5 km and a P‐wave velocity decrement of 1.5%±0.1% compared with the IASP91 model. The relatively thick and weak LVL is possibly a response of a small amount of remnant hydrous partial melts after plume‐like upwelling.