Source mechanism of small-moderate earth- quakes and tectonic stress field in Yunnan Province

In the paper, source mechanisms of 33 small-moderate earthquakes occurred in Yunnan are determined by modeling of regional waveforms from Yunnan digital seismic network. The result shows that most earthquakes occurred within or near the Chuandian rhombic block have strike-slip mechanism. The orientations of maximum compressive stresses obtained from source mechanism are changed from NNW-SSN to NS in the areas from north to south of the block, and tensile stresses are mainly in ENE-WSW or NE-SE. In the eastern Tibetan Plateau, the orientations of maximum compressive stress radiate toward outside from the plateau, and the tensile stress orientations mostly parallel to arc structures. Near 28N the orientations of both maximum compressive stress and tensile stress changed greatly, and the boundary seems to correspond to the southwestern extended line of Longmenshan fault. Outside of the Chuandian rhombic block, the orientations of P and T axes are some different from those within the block. The comparison shows that the source mechanism of small-moderate events presented in the paper is consistence with that of moderate-strong earthquakes determined by Harvard University, which means the source mechanism of small-moderate events can be used to study the tectonic stress field in this region.

In this paper, source mechanisms of 33 small-to-moderate earthquakes occurred in Yunnan are determined by modeling of regional waveforms from Yunnan digital seismic network. The result shows that most earthquakes occurred within or near the boundary of the Sichuan-Yunnan rhombic block have strike-slip mechanisms. The orientations of maximum compressional stresses obtained from source mechanisms are mainly in NNW-SSE in north of the block, and turns to NS in the south. The tensile stresses are mainly in ENE-WSW or NE-SW. In the eastern Tibetan Plateau, the orientations of P axes radiate toward outside from the plateau, and the T axes mostly parallel to arc structures. Near in 28°N, the orientations of both P and T axes changed greatly, and the boundary seems related to the southwest extension line of Longmen Shan fault. Outside of the Sichuan-Yunnan rhombic block, the orientations of P and T axes are some different from those within the block. The comparison shows that the source mechanism of small-to-moderate events presented in this paper is consistence with that of strong earthquakes determined by Harvard University, which means the source mechanism of small-to-moderate events can be used to study the tectonic stress field in this region.