固体地球物理研究生学术报告

Previous studies have documented that there exists two prominent Large Low Shear Velocity Provinces (LLSVPs) at the base of the mantle beneath Africa and Pacific Ocean. The geometry of the Africa LLSVP was constrained well due to the fine coverage of raypaths of events and stations, including its height, boundary and velocity perturbation. The Pacific LLSVP, however, is regrettably not. Recordings of a series of events in adjacent Tonga-Fiji area at new deployed stations in Alaska are used to map the geometry of LLSVP and ULVZ beneath the mid-Pacific. The height of LLSVP may increase from south (~ 720 km) to north (~ 920 km) and its S-wave velocity perturbation decrease gradually from -1% to -4% on the top while -4% evenly in the bottom 300 km through corrected SH arrivals analysis. The boundary of LLSVP, however, is not finely constrained. Through the ScS-S results and modelling of seismic waveforms (S, ScS, Sdiff), our preferred constrained ULVZ model consists of a dome which is ~360km wide, 30 km high and contains S-wave velocity reductions of 10% which could be compared with previous studies, the boundary of ULVZ region could be precisely constrained although there are many trade-offs between its height and perturbation. We also find dubious slab grave around 2300 km depth, providing a good explanation for our model in geodynamics.