The Menyuan, Qinghai M S 6.9 Earthquake on January 8, 2022: A Strike-slip Cascading Rupture Event

Investigating the extension of aftershocks and the rupture mechanism can effectively constrain the geometric structure of a seismogenic fault and provide a scientific explanation for the earthquake rupture model and the seismogenic structure. On January 8, 2022, an MS6.9 earthquake occurred in Menyuan County, Qinghai Province. The epicenter was located at the intersection of the Tuolaishan and Lenglongling faults in the Qilian-Haiyuan fault zone. The precise relocation of the earthquake sequence shows that the mainshock appeared to be an asymmetric bilateral rupture. The fault plane had a steep dip angle and was inclined toward the SW. The Menyuan earthquake sequence was characterized by two obvious belts with NWW and SSE orientations, exhibiting clear segmentation and non-uniform spatial characteristics. The focal mechanism solutions of the mainshock and partially mediated aftershocks with MS ≥ 4.0 indicate that the source type of the four early aftershocks was consistent with that of the mainshock, predominantly strike-slip faulting, while the two large later aftershocks located at the eastern tip of the aftershock belt were mainly thrust-type. This reveals that the terminus of the mainshock rupture was arrested by the bifurcation fault of the Lenglongling fault. The release of energy from recent accumulated compressional tectonic stress led to the variation in the types of earthquakes. The regional stress inversion indicates that the focal region is undergoing compression in the NE direction, consistent with the northeast convergence of the characteristics of the stress field of the northeast margin of the Qinghai-Tibet Plateau. Integrated with the regional geotectonic environment, we infer that the seismogenic fault of the mainshock was associated with the Tuolaishan fault. The earthquake rupture propagated simultaneously toward the connected Tuolaishan and Lenglongling faults, generating a typical strike-slip cascading rupture event, and the energy release of the mainshock was concentrated at the intersecting fault. The Menyuan earthquake revealed the complexity and heterogeneity of the fracture at the intersection of the faults under a strike-slip regime. In addition, it also revealed that the occurrence of large-scale strike-slip earthquakes in the Qilian-Haiyuan fault zone accommodates the compressive force from the northeastern margin of the Qinghai-Tibet Plateau.