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 degrees to 100 degrees. Waveform modeling analyses reveal that the low-velocity anomaly has a stem with a diameter of about 550km in the lowermost 210km of the mantle and a cap with a diameter of about 1600km. The low-velocity structure of the cap decreases from 0% at the top to -1.5% at about 400km above the core-mantle boundary (CMB) and to -1.2% at 210km 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.