Crustal stress field in Yunnan: implication for crust-mantle coupling

We applied the gCAP algorithm to determine 239 focal mechanism solutions \(\left( {3.0 \le M_{\text{W}} \le 6.0} \right)\) with records of dense ChinArray stations deployed in Yunnan, and then inverted 686 focal mechanisms (including 447 previous results) for the regional crustal stress field with a damped linear inversion. The results indicate dominantly strike-slip environment in Yunnan as both the maximum (σ ₁) and minimum (σ ₃) principal stress axes are sub-horizontal. We further calculated the horizontal stress orientations (i.e., maximum and minimum horizontal compressive stress axes: S _H and S _h, respectively) accordingly and found an abrupt change near ~26°N. To the north, S _H aligns NW-SE to nearly E-W while S _h aligns nearly N-S. In contrast, to the south, both S _H and S _h rotate laterally and show dominantly fan-shaped patterns. The minimum horizontal stress (i.e., maximum strain axis) S _h rotates from NW-SE to the west of Tengchong volcano gradually to nearly E-W in west Yunnan, and further to NE-SW in the South China block in the east. The crustal strain field is consistent with the upper mantle strain field indicated by shear-wave splitting observations in Yunnan but not in other regions. Therefore, the crust and upper mantle in Yunnan are coupled and suffering vertically coherent pure-shear deformation in the lithosphere.