关键词: 玛多7.4级地震, Burgers流变模型, 同震库仑应力, 黏弹性库仑应力

Abstract:

The seismogenic fault of the Maduo M_S7.4 earthquake in Qinghai Province on May 22, 2021 is not on the conventionally north boundary of the Bayan Har Block, but a secondary fault named Kunlunshankou-Jiangcuo Fault inside the Bayan Har Block which is nearly parallel to the East Kunlun Fault, with a distance of about 70km. As a result, the study on the stress effect of the Maduo earthquake on surrounding faults is urgent, especially on the main boundary faults of the Bayan Har Block, such as the East Kunlun Fault. In this paper, the lithospheric structure of the study area is stratified by using the USTClitho1.0 results of the unified seismic velocity model of the lithosphere in Chinese mainland. The co-seismic slip model of the Maduo earthquake is inversed by the results of InSAR deformation field and precise aftershock location. The model reveals that the coseismic slip of this earthquake is mainly sinistral strike-slip, the fault strike is 276 degrees, the dip angle is 80 degrees, the average rake angle is 4 degrees, the maximum slip is about 5.1m, and the main slip area is mainly concentrated on the depth of 0~15km. By considering the Burgers rheological model which is more consistent with the actual deformation process of lithosphere, the paper calculates the co-seismic Coulomb stresses and viscoelastic Coulomb stresses in the source area and peripheral faults induced by the Maduo earthquake by using PSGRN/PSCMP program.
The results show that, besides the fracture surface of the seismogenic fault, there are three positive co-seismic Coulomb stress change areas on the west and east ends of the seismogenic fault, of which the stress loading area on the west end is oriented toward the northwest of the seismogenic fault, and the other two stress loading areas on the east end are toward the north and east of the seismogenic fault. The positive section of co-seismic Coulomb stress change of the peripheral faults is consistent with the distribution of the source area. The co-seismic Coulomb stress change induced by Maduo earthquake is bigger than 0.01MPa on the near source section of East Kunlun Fault, the east section of Kunzhong Fault, the northwest segment of Gande-Nanyuan Fault and the middle segment of Wudaoliang-Changshagongma Fault. The maximum co-seismic Coulomb stress changes at the depth of 12.5km reach 0.165MPa, 0.022MPa, 0.102MPa and 0.012MPa, respectively, which proves that the Maduo M_S7.4 earthquake has a strong seismic triggering effect on the above faults. By comparison, the impact of Maduo M_S7.4 on co-seismic Coulomb stress change is also positive in the middle section of Longriba Fault, the south section of Xianshuihe Fault and the north section of Longmenshan Fault, but the magnitude is relatively smaller(less than 0.01MPa), in which the co-seismic Coulomb stress change in the middle section of Longriba Fault increases by thousands of Pa, while the co-seismic Coulomb stress change in the south section of Xianshuihe Fault and the north section of Longmenshan Fault increases by only tens to hundreds of Pa.
For the fault sections with co-seismic Coulomb stress change bigger than 0.01MPa mentioned above, their viscoelastic Coulomb stress changes during 50 years are calculated. The results show that the viscoelastic relaxation of lithosphere after the Maduo earthquake further increases the viscoelastic Coulomb stress changes on the above faults, especially the East Kunlun Fault, where the cumulative Coulomb stress will be increased by 0.038MPa after 50 years. The seismic triggering effect of Maduo earthquake on the above faults will continue to increase over time and more attention should be paid to the seismic risk of the above faults in the future.

Key words: Maduo M_S7.4 earthquake, Burgers rheological model, co-seismic Coulomb stress, viscoelastic Coulomb stress