Abstract: Analysis of stress state of faults is helpful to understand crustal mechanical properties and seismicity. In the paper, we invert the horizontal crustal stress field in the southeastern Tibetan plateau using focal mechanism solutions of small and medium-size earthquakes, and apply them to estimate the stability of regional major faults.
Firstly, we collect focal mechanism solutions of small and medium-sized earthquakes in the southeastern Tibetan plateau. The dataset includes more than 1 000 focal mechanism solutions in the past twenty years. Magnitudes of these earthquakes vary from M3.0 to M6.0. Most of the focal mechanism solutions were determined using waveform inversion technique. Although most of focal mechanism solutions in the southeastern Tibetan plateau are strike-slip faulting, their spatial pattern is different in sub-regions. Normal faulting earthquakes mainly occurred in the western Sichuan region, reverse faulting earthquakes mainly occurred in the boundary zone between the Tibetan plateau and the South China craton, and strike-slip faulting earthquakes mainly occurred in the central and southern Yunnan region.
Next, we settle on a mesh with grid spacing of 0.5° in longitude and latitude in the region and invert the horizontal crustal stress field at each grid point. Spatial variation of the maximum principal stress axis in the southeastern Tibetan plateau shows a clockwise rotation around the eastern Himalaya syntax. The azimuth of maximum compressional stress axis is about 88.1° in the western Sichuan region, about 124.6° in the South China craton, and about 21.6° in the western and southern Yunnan region. The azimuth of regional maximum compressional stress is nearly parallel to the direction of terrain elevation gradient, and that of the minimum compressional stress is nearly parallel to the tangential direction of the topographic elevation contours. The spatial pattern reflects the control role of gravity spreading of the Tibetan plateau on the regional horizontal stress field.
Finally, we analyzed regional fault stability based on these collected focal mechanism solutions. The fault instability parameter (I) is defined based on the Mohr-Coulomb criterion and indicates the degree of fault approximating to rupture. The instability parameters on fourteen major faults in the southeastern Tibetan plateau were calculated. Our results show that the stability of the Lianfeng-Zhaotong Fault is the lowest before 2014 in the region, which indicates the fault zone is close to rupture at that time. Our results provide a new useful tool to assess regional seismic potential using dense focal mechanism solutions.

Key words: southeastern Tibetan plateau, focal mechanism, crustal stress field, inversion, fault stability

摘要： 对断层应力状态进行分析有助于理解断层力学性质及其上发生的地震活动。文中搜集了青藏高原东南边缘地区的中小地震震源机制结果，反演了区域地壳水平应力场，并进一步基于摩尔-库仑准则分析了区域主要断裂带的稳定性。结果表明：青藏高原东南缘地区地壳水平应力场主要表现为剪切型应力状态，龙门山地区表现为压缩型应力状态，而川西高原地区主要表现为拉张型应力状态；区域地壳最大主压应力方位近平行于地形高程梯度方向，最大主张应力的方位近平行于地形高程等值线的切线方向，反映了青藏高原重力扩展作用对区域水平应力场的控制作用；基于应力的断裂带稳定性分析结果表明，青藏高原东南缘地区的14条主要断裂带中，莲峰-昭通断裂在2014年前后稳定性最低，应力状态最接近于破裂状态。

关键词: 青藏高原东南缘地区, 震源机制解, 地壳应力场, 反演, 断层稳定性