Abstract:

Based on the focal mechanism solutions of 2 600 M_L≥3.0 earthquakes in Sichuan and Yunnan area from January 2000 to March 2017, the focal mechanism quantitative classification and stress field inversion are carried out for the sub blocks and fault zones with relatively dense focal mechanisms. Using the focal mechanism solutions of 727 M_L≥4.0 earthquakes from January 1970 to March 2017, the regional stress tensor damping method is used to inverse the spatial distribution of principal compressive stress in Sichuan and Yunnan area before and after Wenchuan M_S8.0 and Lushan M_S7.0 earthquakes, and the temporal and spatial evolution characteristics of current stress field are discussed.
The focal mechanisms are distributed mainly in Longmenshan fault zone, Xianshuihe-Anninghe-Zemuhe-Xiaojiang fault zone, Mabian-Yanjin fault zone, Lijiang-Xiaojinhe fault zone, the central Yunnan block, the west Yunnan block and the southwest Yunnan block in Sichuan and Yunnan area. The focal mechanism is mainly strike slip type in Sichuan and Yunnan area, but there are local differences. The Longmenshan fault zone is dominated by thrust type earthquakes, while in the Mabian-Yanjin fault zone, there are relatively more strike slip and thrust type earthquakes. The types of earthquakes in Sichuan Basin are complex, and there is no obvious dominant type. In general, the focal mechanisms of the Longmenshan fault zone and Sichuan Basin earthquakes are affected by strong earthquake and other factors, and the focal mechanism types have good inheritance in Sichuan and Yunnan area.
The stress field in Sichuan and Yunnan area has obvious subarea characteristics, and it rotates clockwise from north to south. The compressive stress in Longmenshan fault zone and Sichuan Basin shows nearly EW direction. It shows NWW direction in the eastern boundary of Sichuan and Yunnan rhombic block and NNW direction in the inner part of rhombic, while it shows NNE direction in the western and southern Yunnan blocks. The principal compressive stress in Sichuan is more complex than that in Yunnan. The principal compressive stress direction in Sichuan experiences EW-NW-EW rotation from west to east, the dip angle is steep in the west and slow in the east, and the stress regime also experiences the transition from normal faulting to strike-slip to thrust. The principal compressive stress direction in Yunnan is NNE in the west and NNW in the east, forming an inverted “V” shape in space, the stress regime is mainly strike-slip and the dip angle is horizontal.
Before and after the Wenchuan M_S8.0 and Lushan M_S7.0 strong earthquakes, the stress field in the Longmenshan fault zone changed greatly, followed by the Sichuan Basin and its surrounding areas, and there was no obvious change in other areas of Sichuan and Yunnan. The stress field in the Longmenshan fault zone experienced a complete transformation process from basic stress field to variable stress field to basic stress field.

Key words: Sichuan and Yunnan area, focal mechanism solution, regional stress field, damped inversion

摘要：

基于川滇地区2000年1月—2017年3月2 600次M_L≥3.0地震的震源机制解, 文中对震源机制相对密集的次级地块和断裂带进行了震源机制量化分类和整体应力场反演, 利用1970年1月—2017年3月727次M_L≥4.0地震震源机制解, 采用区域应力张量阻尼方法反演了汶川8.0级和芦山7.0级强震前后川滇地区的主压应力空间分布, 探讨了川滇地区现今应力场的时空演化特征。川滇地区的震源机制总体以走滑型为主, 但龙门山断裂带、 四川盆地及其边缘的马边-盐津断裂带的机制类型存在局部差异; 川滇地区的应力场存在明显的分区特征, 自北向南总体呈现顺时针旋转, 四川地区的主压应力方向从西到东经历了EW—NW—EW的转变, 云南地区的主压应力方向在西部地区呈NNE向, 在东部地区则表现为NNW向, 空间上形成倒“V”形; 汶川8.0级和芦山7.0级强震前后, 应力场在龙门山断裂带变化较大, 四川盆地及其周缘变化次之, 其他区域未发生明显变化, 其中震中所在的龙门山断裂带应力场经历了基本应力场—变化应力场—基本应力场的一次完整转变过程。

关键词: 川滇地区, 震源机制解, 区域应力场, 阻尼反演