We have constrained a small-scale, dome-shaped low-velocity structure near the core-mantle boundary (CMB) of Earth beneath Perm (the Perm anomaly) using travel-time analysis and three-dimensional (3-D) forward waveform modeling of seismic data sampling of the mantle. The best-fitting dome-shaped model centers at 60.0°E, 50.5°N, and has a height of 400 km and a radius that increases from 200 km at the top to 450 km at the CMB. Its velocity reduction varies from 0% at the top to –3.0% at 240km above the CMB to –3.5% at the CMB. A surrounding 240-km-thick high-velocity D'' structure has also been detected. The Perm anomaly may represent a stable small-scale chemical pile in the lowermost mantle, although the hypothesis of a developing mantle plume cannot be ruled out.
A method for reconstructing crustal velocity structure using the optimization of stacking receiver function amplitude in the depth domain, named common conversion amplitude (CCA) inversion, is presented. The conversion amplitude in the depth domain, which represents the impedance change in the medium, is obtained by assigning the receiver function amplitude to the corresponding conversion position where the P-to-S conversion occurred. Utilizing the conversion amplitude variation with depth as an optimization objective, imposing reliable prior constraints on the structural model frame and velocity range, and adopting a stepwise search inversion technique, this method efficiently weakens the tendency of easily falling into the local extremum in conventional receiver function inversion. Synthetic tests show that the CCA inversion can reconstruct complex crustal velocity structures well and is especially suitable for revealing crustal evolution by estimating diverse velocity distributions. Its performance in reconstructing crustal structure is superior to that of the conventional receiver function imaging method.