Decadal change of seismic structure in the Earth's lowermost mantle.

Studying temporal changes of seismic property could provide a direct mean to monitor dynamic processes in the Earth's lowermost mantle, yet no related seismic observations were discovered. Here, we report temporal changes of seismic data between a pair of nearly co-located earthquakes occurring on 2000/07/07 and 2009/12/17 and infer two types of decadal-scale structural changes in the lowermost mantle: (1) a 10s km-scale shrinkage or movement of an ultra-low velocity zone near the core-mantle boundary, and (2) opposing shear wave anisotropy changes in the top and bottom portions of the lowermost ~300 km mantle. These findings suggest that ultra-low velocity zones are deformed as partially molten materials or moved by vigorous localized material flows and the region of the lowermost mantle possesses a separate layer circulation. This report demonstrates the capability of seismic monitoring of deep mantle dynamics and calls for new geodynamical models to account for these previously unrecognized features.