关键词: 川滇地区, 活动块体边界断裂, 滑动速率, 应力分布

Abstract:

Previous studies have shown that M≥8 earthquakes and more than 80% M≥7 earthquakes occurred in the boundary zones of active blocks. Therefore, studies on the slip rate and stress distribution of the boundary faults can provide the basis for assessing the risk of strong earthquake. It also can help us understand the regional tectonic deformation, motion and dynamic process. Based on current cognition of the division of active block and fault system in the Sichuan-Yunnan region, we build a two-dimensional finite-element contact model, which includes ten small blocks and the primary block boundary faults, such as East Kunlun Fault, Minjiang Fault, Huya Fault, Xianshuihe-Xiaojiang Fault and Red River Fault. Slip rate and stress distribution of the primary block boundary faults are obtained by using long-term GPS observation data from 1991 to 2015 and “block-loading” method. This loading method can reflect interaction between the block and the boundary. Compared with the direct loading of GPS results, it can avoid local distortion caused by the large single-point error. Comparing GPS observation results with simulation results, the residual error less than 1mm accounts for 66%, and the error less than 2mm accounts for 86%. The direction angle residual error less than 5° accounts for~56%, and that less than 10° accounts for 82%, which means that simulation results of this study are reasonable. In addition, by collecting the relevant information on seismic activity and focal mechanism solutions in the Sichuan-Yunnan region, and combining with the simulation results, we discuss the relationship between slip rate distribution, transfer and stress transformation in large left-lateral strike-slip fault zones, the tectonic mechanism with normal fault type, as well as the probable cause of the seismic discrepancy between the northern and southern segments of the Red River Fault. The main conclusions are as follows:
(1)As the strike of the left-lateral strike-slip East Kunlun and Xianshuihe-Xiaojiang fault zones turns sharply from NW to near north-south-direction, the strike-slip component is partially absorbed by the fault-bend parts and then converted into strain accumulation, resulting in high stress distribution in the fault-bend areas. Among them, the area from the easternmost end of East Kunlun Fault to Huya Fault absorbs a strike-slip rate of~0.15mm/a. The accumulative rates of compressional stress are 3 711.7Pa and 699.3Pa, respectively. And the area from southeastern end of Xianshuihe Fault to Anninghe and Daliangshan Faults absorbs a strike-slip rate of~1mm/a. The accumulative rates of compressional stress are 3 051.7 Pa and 2 844.6 Pa, respectively.
(2)Affected by the left-lateral shear of Xiaojiang Fault, the south-central segment of the Red River Fault is dominated by right-lateral strike-slip with weak compression. The right-lateral strike-slip rate is 1.20~2.68mm/a. The right-lateral strike-slip rate of north segment of Red River Fault is 0.71~1.54mm/a. This indicates that right-lateral strike-slip in the northern segment of Red River Fault is caused by traction of the south-central segment. The Red River Fault constitutes a right-lateral shear deformation zone arranged in right-step en echelon pattern with the Jinsha River Fault and Deqin-Zhongdian Fault. In the vicinity of Deqin-Zhongdian Fault, the Yulong snow mountain eastern piedmont fault, the southern segment of the Lijiang-Xiaojinhe Fault and the Ninglang-Yongsheng-Binchuan Fault, form a tectonic pull-apart zone. The normal focal mechanisms are predominantly distributed within this zone. This deformation pattern is not consistent with imbricated thrust conversion-limited extrusion model, which suggests that the current movement mode of Jinsha River and Lijiang-Xiaojinhe fault zones and their effect on regional deformation may have changed.
(3)The north segment of the Red River Fault appears to be slightly tensional, while the south segment is weakly compressional. According to Coulomb's criterion, the shear stress required for fault rupture in the northern section should be lower than that in the southern section. As a result, the north section is more likely to rupture and the seismic activity is significantly stronger than that of the south-central part.

Key words: Sichuan-Yunnan region, active block boundary faults, slip rate, stress distribution