Crustal velocity structure and composition of Bayan Har block and surrounding areas

The Bayan Har block, one of China's most seismically active regions, has experienced multiple major earthquakes (≥ M 7.0) in recent years. It is a key area for investigating the interactions between the Qinghai-Xizang (Qingzang) Plateau and adjacent blocks, plateau uplift, and strong earthquake mechanisms. P-wave velocity and crustal composition provide key constraints on the properties of distinct tectonic units and their evolutionary modification processes. Based on the results of 8 Deep Seismic Sounding (DSS) profiles completed in the Bayan Har block and surrounding areas over the past 20 years, We constructed one-dimensional P-wave velocity models for the crust of Bayan Har block, Qilian fold belt, Qinling fold belt, Alxa block, Ordos block and Sichuan basin. Furthermore, crustal composition models for different tectonic units were established based on these results. The results reveal that the crustal thickness of the Bayan Har block gradually decreases towards the NNE, NE, and SE directions, while the average crustal velocity increases correspondingly. The felsic layer in the crust accounts for more than half of the total crustal thickness. The mafic content within the crust of different tectonic units exhibits notable variations, which may reflect that the Bayan Har block, Qilian fold belt, and Qinling fold belt have experienced more intensive lithospheric evolution processes compared to Ordos basin and Sichuan basin. The seismicity distribution in this region is significantly controlled by crustal velocity and composition heterogeneity across the Bayan Har block and adjacent areas, which demonstrates that earthquakes within and around the Bayan Har block exhibit both high frequency and larger magnitudes. These seismic characteristics primarily result from intense crustal stress accumulation and release during the outward expansion of the Qingzang Plateau.