The P-wave velocity and azimuthal anisotropy structure of southeastern margin of the Tibetan Plateau from adjoint-state traveltime tomography

The southeastern margin of the Tibetan Plateau is a crucial region to understand the mechanisms of plateau uplift and deformation. This region is seismically active and has experienced multiple large earthquakes, resulting in significant human and economic losses. Constructing velocity and anisotropic tomography models is crucial for understanding the seismogenic mechanism and deep structural deformation in this area. In this study, we extract high-quality P-wave first-arrival data from the earthquake catalog of the China Earthquake Administration and use them to construct both common-receiver and common-source differential traveltime datasets. We then apply a novel adjoint-state traveltime tomography approach to obtain new P-wave velocity and azimuthal anisotropy models for the region. This method eliminates the need for ray tracing, thereby reducing the potential bias from the ray theory and ray tracing. A comparison between our results and previous imaging models and shear-wave splitting measurements reveals several new details. The results indicate weak anisotropy in the shallow depth of the central Chuandian block. Two low-velocity anomalies are identified beneath the Songpan-Ganzi and Lijiang-Xiaojinhe fault zones, as well as beneath the Xiaojiang Fault. The distinct anisotropic characteristics of these two low-velocity anomalies suggest different tectonic contexts: Beneath the Songpan-Ganzi and Lijiang-Xiaojinhe fault zones, the azimuthal anisotropy aligns north-south and northeast-southwest, while beneath the Xiaojiang Fault, it aligns northwest-southeast. In addition, the anisotropy of the upper mantle in the southern part of the study area has a significant east-west feature. The earthquake relocation results reveal intensified seismic activity in regions with significant velocity contrasts and near fault zones. Segmental seismic activity is observed along some major fault zones, and seismicity is also more pronounced in fault intersection areas. The new imaging results provide new perspectives and insights for understanding the seismogenic mechanisms and regional tectonic deformation in the region.