The seismic evidence of velocity variation for Changbaishan volcanism in Northeast China

Changbaishan (CBS) is the largest intraplate volcano in Northeast China, located far to the west of the Pacific Plate subduction trench. Two end-member models for CBS volcanism differ significantly in the detailed structures of the mantle transition zone (MTZ). The source of the CBS volcanism is still debatable. Diagrams of P- and S-wave velocity variations down to 800 km depth were made by using joint P and S inversion for relative traveltime residuals measured from waveform data recorded by temporary and permanent seismic stations. A broad low-velocity (low-V) zone in the upper mantle was highlighted directly beneath the CBS. Two prominent low-V branches connected to the broad low-V zone: one east of the CBS extending down to the top of the MTZ, the other west of the CBS ranging down to the base of the MTZ. This indicates two deep sources feeding the CBS volcano. A weak high-V zone in the MTZ was also highlighted directly beneath the CBS volcano, possibly the pond of slab fragments. The eastern low-V branch may be related to slab dehydration while the western low-V branch outlines the upwelling conduit in the MTZ. Thus, our proposed dynamic model for the CBS volcanism suggests that the CBS volcano has two deep sources: one from dehydration of the flat-lying Pacific slab and the other from upwelling, likely emerging from the top of the lower mantle.