According to its discoverers, Jesse Lawrence (from Scripps Institute of Oceanography) and Michael Wysession (from Washington University), the Beijing Anomaly is evidence for large amounts of water contained within the mantle.
[1] The extent of the anomaly below a large portion of Northeast Asia would require the existence of contributing factors which would facilitate the distribution of water within the mantle itself.
[1] It has therefore been suggested that the downgoing oceanic lithosphere can carry with it large amounts of water into deeper regions of the mantle, directly below the continental margins (to depths reaching far beyond 1400 km).
The seismic attenuation anomalies in Northeast China are potentially explained by water having been extracted from the cold, deep-seated oceanic lithosphere and “flooding” the overlying lower mantle wedge at depth, giving rise to zones of high elasticity (and potentially fluid-rich) which would cause levels of seismic attenuation that were observed tomographically.
[2] Recent research has been able to better constrain the degree to which subducting oceanic mantle is hydrated using measurements of P-wave attenuation from localized earthquakes near the Wadati–Benioff zone.