Development and application of uppermost mantle Pn tomography

The ray path of Pn waves is concentrated in the limited depth range of the uppermost mantle, which has unique advantages in ray transverse coverage density. Therefore, the Pn phase is the dominant phase for studying the velocity and anisotropic structure of the uppermost mantle. The lateral variation of Pn wave velocity reflects the temperature and composition difference of the uppermost mantle, and Pn anisotropy can reflect the movement of the mantle material and deformation characteristics. The high accuracy of Pn wave velocity and anisotropy structure in the uppermost mantle can provide key information about the lithospheric structure, plate movement, and deep thermal material migration process. With development and improvement, Pn tomography has become a mature technology to study the structure of the uppermost mantle and has been applied to obtain structural information such as crustal thickness, upper mantle velocity, and anisotropy on a global scale. This method characterizes the lateral heterogeneity of the global upper mantle structure and provides further understanding of the Earth's internal structure and plate subduction, continental collision deformation, volcanic activities, and other dynamic processes. With increasing global seismic stations and observation data, a large amount of high-quality Pn data provide favorable conditions for the study of the structure of the uppermost mantle. This paper reviews the development of the Pn tomography method and its applications in the world.