Review of seismic velocity anomalies in the lower mantle

The lower mantle accounts for more than 50% of the total volume of the Earth, and influences the evolution of the Earth. Early studies showed that the lower mantle is relatively homogeneous, but since the 1970s, seismic tomography revealed the velocity structure of the deep Earth and found many velocity anomalies in the lower mantle. The accumulation of new array data and the progress of computer technology in the present century have enabled us to further constrain the seismic structures of these anomalous zones. Since these structures could be closely related to subduction plates and mantle plumes, it is of great significance to understand the fine structure of these wave velocity anomalies to study the reconstruction of ancient plates and the mantle dynamics. This study focuses on summarizing the methods and results in revealing the lower mantle anomalies using seismic data in the past 30 years, especially the distribution and characteristics of different types of velocity anomalous zones worldwide. African and Pacific LLSVP are the two most important LLSVPs in the lower mantle. Their lateral and vertical extents span approximately a thousand and more than a thousand kilometers, respectively. The present study found sharp velocity contrasts along the boundary of the LLSVP, which argues for chemical anomalies in the LLSVP. The ULVZ is located at the bottom of the lower mantle, with extremely low S-wave velocities that are 20%~40% smaller than the average. The lateral extent of the ULVZ is generally smaller than a thousand kilometers, but some ULVZ exceed that extent; the heights range from ten kilometers to dozens of kilometers. Partial melting, particularly related to chemical anomalies, is thought to be the most plausible explanation for ULVZ. D" discontinuities were also observed in many regions, especially where high-velocity subducted slabs were found. The slabs reduce the temperature near the core-mantle boundary, which yields stable pPv and D" discontinuities. In addition, the seismic anisotropy of the lower mantle is also a focus because it reflects the flow conditions of the mantle. The existing results show that several wave velocity anomaly structures (LLSVP, ULVZ, D", anisotropy) in the lower mantle may be closely related and connected to the subducting slabs in space, and the difference in mantle temperature and chemical composition may be an important factor that produces these anomalies. Finally, this study also summarizes the results of the velocity structure of the lower mantle in China and looks forward to the future research direction of the velocity anomaly and anisotropy of the lower mantle in China.