Seismic evidence for an 850 km thick low-velocity structure in the Earth's lowermost mantle beneath Kamchatka

We detect an 850 km thick low-velocity structure in the Earth's lowermost mantle beneath Kamchatka surrounded by and overlying a 210 km thick high-velocity D″ structure. The velocity structure is constrained by modeling the observed anomalously broadened waveforms for seismic shear waves sampling the lowermost mantle recorded at large distances from 90° to 100°. Waveform modeling analyses reveal that the low-velocity anomaly has a stem with a diameter of about 550-km in the lowermost 210-km of the mantle and a cap with a diameter of about 1600-km. The low-velocity structure of the cap decreases from 0% at the top to -1.5% at about 400-km above the core-mantle boundary (CMB) and to -1.2% at 210-km above the CMB. We suggest that the geometrical and velocity features of the low-velocity anomaly indicate that it may represent a localized mantle plume undetected before in the lower mantle.