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

摘要：

2021年5月22日青海玛多 M_S7.4 地震的发震断层并不是传统意义上的巴颜喀拉块体北边界, 而是发生在巴颜喀拉块体内部一条与东昆仑断裂带主断裂近平行的次级断层上, 针对玛多地震对周边断层尤其是巴颜喀拉块体主边界造成的应力影响亟需开展研究工作。文中利用中国大陆岩石圈统一地震速度模型USTClitho1.0结果完成了研究区的岩石圈结构分层设置, 结合InSAR形变场及余震精定位结果反演得到玛多地震的同震滑动模型, 通过考虑更符合岩石圈实际变形过程的Burgers流变模型, 利用PSGRN/PSCMP程序计算得到玛多地震引起的震源区及周边断层的同震及震后黏弹性库仑应力变化。研究结果表明, 玛多 M_S7.4 地震震源区同震库仑应力加载区除沿发震断层破裂面分布外, 在发震断层的西端、 东端分别有3处库仑应力变化正值区, 其中西端朝向发震断层的NW方向, 东端存在2处库仑应力加载区, 分别朝向发震断层的北部以及东部区域, 周边断层同震库仑应力变化正值段的分布与震源区库仑应力加载区的分布较为一致; 玛多地震引起的东昆仑断裂带近震源区段、 昆中断裂东段、 甘德南缘断裂西北段、 五道梁-长沙贡玛断裂中段的同震库仑应力变化均>0.01MPa, 且震后岩石圈黏弹性松弛作用使得上述断层的黏弹性库仑应力进一步增加, 未来应重点关注上述断层段的地震危险性。

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