Upper mantle and transition zone structure beneath Leizhou-Hainan region: Seismic evidence for a lower-mantle origin of the Hainan plume

The origin of the widespread volcanism at the Leizhou-Hainan (Leiqiong) region in the Southern China remains obscure. We take advantage of the highly active seismicity and dense seismic networks surrounding this region to investigate its upper mantle and Mantle Transition Zone (MTZ) structure. Over 5000 P-wave waveforms whose raypaths bottom at depths around the MTZ are collected, and traveltimes of their first arrivals are hand-picked. By matching the traveltime curve variation over the epicentral distance range from 10° to 35°, we first construct a 1-D upper mantle and MTZ velocity structure for the region. This initial model is then refined by forward modeling, in which the observed triplicated waveforms from selected earthquakes are compared with the synthetic seismograms with varying velocity structure. In our preferred model for Leiqiong, the P-wave velocities deeper than 200 km at the upper mantle are 0.8-1.2% lower than the IASP91, and 0.6% slower in the MTZ, while the top and bottom boundaries of the MTZ depresses 12 km and slightly uplifted, respectively, compared to the global averages. This model provides independent constraints on the structure beneath Leiqiong, suggesting a thermal anomaly within the MTZ and a lower mantle origin for the volcanism seen in this region.