Velocity gradients below the 410‐km discontinuity beneath eastern Asia and its implication on water content distribution

The velocity gradient below the 410-km discontinuity provides clues for the water content gradient in the mantle transition zone. Here we report an abnormal X phase refracted below the 410, providing a unique opportunity to constrain the velocity gradient at that depth. We observe the X phase sampling the MTZ beneath south and southwest China close to the subducted Paleo-Pacific slab, compared to the typical triplication waveforms sampling northeast Asia, the modern Pacific subduction system. The P velocity gradient below the 410 is 0.0028 s(-1) or lower beneath south and southwest China, indicating a uniform water content; while it is 0.0037 s(-1) or higher beneath northeast Asia, suggesting a water content decreasing from the 410-km downward. Our research provides seismic evidence for water infiltrating the MTZ from a hydrous layer atop the 410 and then widely diffused into the MTZ after long-term subduction.