Analysis of seismogenic structure of Madoi, Qinghai Ms7. 4 earthquake on May 22, 2021

High-precise relocation of aftershock sequence and detailed focal mechanism inversion could reveal the rupture properties of larger earthquakes, seismogenic structure and regional stress field, which provides the scientific evidence for decision-making regarding post-earthquake emergency management. In this study, the double-difference earthquake location algorithm was employed to delicately describe the spatial and temporary characteristics of Madoi, Qinghai n7.4 earthquake sequence within 9 days after the earthquake, a total of 1055 earthquakes were determined with greatly improved location accuracy. The focal mechanism of moderate aftershocks with M-s >>-4. 0 were inverted from the local and region waveforms recorded by the Qinghai, Tibet, Sichuan and Gansu regional seismic stations with the method of full-wave moment tensor inversion, and then the regional stress field of focal region was estimated by fault plane solutions of these aftershocks. The accuracy of relocation of results show the Madoi sequence lied between the Madoi-Gande fault and the southern margin of Gande fault, which appeared to be a steep seismogenic zone and an asymmetrical bilateral rupture in spatial-temporal evolution. Clear bifurcation geometry features were recognized at the both ends of surface trace of aftershocks, indicating the mainshock occurred in the complex tectonic system and triggered its branch faults. Most of moderate aftershocks, 12 out of 15 earthquakes, were of strike-slip faulting following the tectonic area, 3 thrust-type earthquakes mainly concentrated in the eastern tip of the main tectonic line which probably indicates the segmented character of the fault. The stress field inversion suggests the focal region is undergoing the compression in the EW direction, and being compatible with the dominant stress orientation in Tibetan Plateau. We deduced the Madoi mainshock ruptured on a left-lateral strike-slip fault with NWW-striking in the Bayan Har block, and it activated faults near the eastern and western tip of seismogenic fault and involved multiple fault branches. The heterogeneity of rupture plane controls the variability of spatial and temporal distribution of aftershocks.