Velocity Gradients Below the 410‐km Discontinuity Beneath Eastern Asia and Its Implication on Water Content Distribution

Abstract

AbstractThe 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.