The deep tectonic background of the M_S6.9 Menyuan earthquake on January 8，2022 in Qinghai Province

On January 8, 2022, the Menyuan M_S6.9 earthquake occurred on the Lenglongling fault in Qinghai Province. Combining the data of crustal thickness, velocity structure and anisotropy, we discussed the deep structural characteristics of the Menyuan earthquake, and revealed that the location of the Menyuan earthquake is closely related to the structural variations in the crust. The results show that, Menyuan M_S6.9 earthquake occurred in the area where both the crustal thickness and v_P/v_S ratio showed rapid spatial changes. Roughly in the depth range of 10−20 km, the hypocenter is located in the vertical transition zone from shallow to deep where the P-wave velocity changed from high to low, and also the S-wave velocity and Poisson’s ratio distribution show obvious lateral changes and there is a clear low-velocity zone beneath the hypocenter; the variation of azimuthal anisotropy of phase velocity on both sides of the Lenglongling fault is relatively obvious. The epicenter of the M_S5.2 aftershock on January 12 was close to that of the 2016 M_S6.4 earthquake, revealing that the Menyuan M_S6.9 earthquake and its aftershocks caused a relatively sufficient rupture of the Lenglongling fault. Between the two Menyuan earthquakes and its adjacent areas, it is difficult to accumulate more energy in a short time, so it is unlikely that a larger earthquake will occur in a short time. The continuous northward expansion of the northeastern margin of the Qinghai-Tibet Plateau leads to surface uplifting and crustal thickening, which is the main tectonic cause of frequent strong earthquakes in this region.