Focal mechanism solutions and tectonic stress field characteristics of the 2017 M_S6.9 Milin earthquake sequence

The 2017 M_S6.9 Milin earthquake occurred in the core of the Eastern Himalayan Syntaxis (EHS), at the collision front between the Indian and Eurasian plates, where tectonic deformation is highly complex and strong earthquakes occur frequently. This study focused on the Milin earthquake sequence and used waveform data recorded by regional temporary and permanent stations deployed in the EHS from 2015 to 2024. We obtained 36 focal mechanism solutions for earthquakes with M_L≥3.0 using the Cut-and-Paste and Hardebeck and Shearer methods and analyzed the tectonic stress field characteristics of the source region using the joint iterative linear inversion method. The results show that focal mechanisms in the study area were dominated by thrust events and accompanied by strike-slip and normal events, indicating that regional deformation is primarily controlled by thrust faulting under the persistent northeastward compression of the Indian Plate, with local adjustment by strike-slip and extensional stresses. (2) Stress-field inversion results indicated that the maximum principal stress axis (σ₁) has an NE-SW orientation with a low plunge, and the stress shape ratio (R) is 0.78, indicating that the study area is dominated by horizontal compressive stress, with the principal stress direction consistent with the convergence direction of the Indian Plate. (3) The persistent northeastward compression of the Indian Plate controls the thrust-faulting tectonic regime that caused the Milin earthquake sequence, whereas the coordinated interaction and local heterogeneity of the shallow fault system result in diverse focal mechanisms and distinct deformation styles between the shallow and deeper crust. In summary, the results of this study provide further insight into the seismogenic mechanism of strong earthquakes in continental collision zones.