Distinct Velocity variations around the Base of the Upper Mantle beneath Northeastern Asia

Both the global and regional P wave tomographic studies have revealed low-velocity anomalies beneath the high velocity descending slabs in the mantle transition zone and uppermost lower mantle under a number of subduction zones. The limited resolution at large depths and possible trade-off between the high and low velocities, however, makes it difficult to substantiate this feature and evaluate accurately the vertical extent of the low-velocity structure. Using broadband waveform modeling on the triplicated phases near the 660-km discontinuity for three deep events, we constrained both the P and SH wave velocity structures around the base of the upper mantle in northeastern Asia. For the two events beneath the south Kurile, the rays traveled through the lowermost transition zone and uppermost lower mantle under the descending Pacific slab. Our preferred models consistently suggest normal-to-lower P and significantly low SH wave velocities above and below the 660-km discontinuity extending to about 760-km depth compared with the global IASP91 model, corroborating previous observations for a slow structure underneath the slab. In contrast, both high P and SH wave velocity anomalies are shown in our preferred model for the region beneath the Japan back arc, likely reflecting the structural feature of a slab stagnant above the 660-km discontinuity. The velocity jumps across the 660-km discontinuity were found to be on average 4.5% and 7% for P and S waves under the south Kurile, and 3% and 6% under the Japan back arc. The relatively small velocity contrasts in the latter are also consistent with the stagnant slab scenario in which the amplitude of the discontinuity is reduced by the high velocity of the slab. In addition, our synthetic modeling indicates that an apparent uplift or depression of the discontinuity (>10 km) could not expain the data reasonably well, suggesting that temperature variations might be small in both regions. While the limited temperature effect may be sufficient to induce the high velocities of the slab beneath the Japan back arc, the low velocity anomalies, especially the apparently different P and SH velocity gradients around the 660-km discontinuity beneath the south Kurile are likely indicative of lateral chemical heterogeneities existing at the upper and lower mantle boundary. Given the complex morphology of the subducting Pacific slab in this region, we speculate that the distinct velocity structure retrieved here may represent mantle materials that are trapped at around the base of the upper mantle between the surrounding subducting Pacific slabs, and are probably different from the slabs and ordinary mantle both thermally and chemically.