The mantle transition zone (MTZ) plays an important role in deep dynamics of the subducted slab and mantle convection in the subduction zone. Based on triplicated waveforms of three deep-focus earthquakes in northern Kuril recorded by the China National Seismic Network (CNSN), we constructed both P and SH velocity structures of the MTZ beneath the northwest Asia edge, using improved grid-searched waveform modelings. we confirmed a high-velocity anomaly (HVA, average 3.3% δV_p and 2.3% δV_s) with a thickness of 130-138 km in lower MTZ. Furthermore, the HVA hosts a top interface with positive velocity contrasts (4.3% δV_p, 3.2% δV_s), while the 660-km discontinuity (660) shows small velocity contrasts (3.6% δV_p, 5.1% δV_s) and slight depressions (0-10 km), suggesting the existence of the stagnating Pacific slab. We infer that the high V_p/V_s ratio (~1.85) implies the water content (~0.41 wt%) preserved inside the stagnant slab, evidencing the water transportation into lower MTZ. The weakened slab in the MTZ easily piles and buckles, inducing the slab thickening. The long-term slab stagnation may alter the localized material and heat exchange mode between the MTZ and lower mantle. Our results can provide more insight into deep subduction dynamics in the northwestern Pacific.
 

Northeast Asia edge,MTZ velocity structure,Stagnant Pacific slab,Grid-searched waveform modeling,CNSN waveforms