Insights into Seismogenic Mechanism in the Qilian Fold Belt on the Northeast Margin of the Qinghai–Tibet Plateau

Since 2016, the Qilian fold belt (QFB) on the northeastern margin of the Qinghai–Tibet Plateau has experienced frequent earthquakes. Investigating the seismogenic structures and environments associated with the moderate-to-strong earthquakes is crucial for understanding the earthquake generation mechanisms and assessing future earthquake risks in this region. In this study, we collected phase observation reports and regional broadband waveforms for the study area from the Chinese Seismic Network, and the high-precision earthquake location and 3-D velocity structure were inverted using the double-difference tomography method. In addition, we used the full-waveform inversion method to determine the source mechanism of the MS6.2 Jishishan earthquake and 24 other earthquakes (M ≥ 4.0) in this area. Our results show that the Jishishan mainshock was dominated by thrust slip ruptures on the fault at the southern margin of Lajishan, with aftershocks mostly occurring along NNW-striking faults. The focal mechanisms of the MS6.0 Delingha and MS6.9 Menyuan earthquakes indicate right-lateral and left-lateral strike–slip faulting, respectively. The spatial distribution and source properties of these earthquakes were directly influenced by the significant difference in the deformation in the eastern and western segments of the QFB. Our analysis of the three-dimensional P- and S-wave velocity structures and hypocentral distribution beneath the QFB reveals that the strong seismic activity was closely related to the distribution of the main velocity anomalies, both low- and high-velocity anomalies. Most of the moderate-to-strong earthquakes (MS > 4) occurred in the transition zones between the low- and high-velocity anomalies, and four major earthquakes (MS ≥ 6.0) occurred in areas with obvious high-speed anomalies. We suggest that the lateral variations in the velocity structure controlled the spatial location and rupture mechanism of the moderate-to-strong earthquakes and that the boundaries of these velocity anomalies may have the potential for future seismic activity.