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.

Automated summary

This study investigates the water content gradient in the mantle transition zone through the analysis of X phase reflections. The results show that in the southern and southwestern regions of China, the P wave velocity gradient is low, indicating a uniform water content. In contrast, in the northeastern region of Asia, the P wave velocity gradient is high, suggesting a decreasing water content from the 410-km depth. This provides seismic evidence for water infiltrating into the mantle transition zone from a hydrous layer above the 410 and diffusing widely after long-term subduction.