Sharpness of the 410-km discontinuity from the P410s and P2p410s seismic phases

Sharpness of the 410-km boundary is of interest because it is sensitive to water content in the transition zone (TZ). We evaluate the width of the 410-km discontinuity with a new seismic method. Our estimates are inferred from the amplitude ratio of the P2p410s and P410s seismic phases that are detected in P-wave receiver functions. We applied this method to seismic recordings from arrays of broad-band stations deployed in central Fennoscandia, southern Africa and southern China. The obtained estimates of width of the 410-km discontinuity range from 10 to 22 km and always exceed the width of 7 km which is expected for anhydrous conditions. The enlarged width may be interpreted in terms of hydrous conditions, but we have found only one region (the eastern Yangtze Craton in China) where the broad 410-km discontinuity, as expected, is accompanied by a broad TZ. Water in the TZ may be a kind of a global phenomenon, but evidence of the enlarged width of the TZ may be missing in most of our data because the reference seismic model is affected by water, as well.