Subsurface structure and spatial segmentation of the Longmen Shan fault zone at the eastern margin of Tibetan Plateau: Evidence from focal mechanism solutions and stress field inversion

As the eastern margin of the Tibetan Plateau, the Longmen Shan fault zone (LMSFZ) is characterized by a complex structural style and strong seismicity, such as the 2008 Wenchuan Ms8.0 and 2013 Lushan Ms7.0 earthquakes. In order to better understand the subsurface geometry of the LMSFZ we inverted 391 focal mechanism solutions (FMS) of earthquakes (M ≥ 3.5) that occurred between 2009 and 2016 in the Longmen Shan region using seismic waveforms. Subsequently, we calculated the stress field based on the FMSs. The results show that the LMSFZ is dominated by a thrust faulting stress regime with subhorizontal NW-SE-trending maximum principal stress σ1 and subvertical minimum principal stress σ3, indicating a lateral compression due to the southeastward movement of the Bayan Har block. Changes in fault dips at ~ 10 km depth are inferred as the result of a shallow level detachment layer, which may also promote shortening of the upper crust and uplift of the Longmen Shan. Based on differences in subsurface fault geometry, stress field and geomorphological evidence, we divide the LMSFZ into two segments by Beichuan County: the southwestern segment has a listric fault geometry, while the northeastern segment is dominated by subvertical transpressional strike-slip faults. According to the FMSs results, we suggest that the Lixian fault is a tear fault accommodating different thrusting rates in the eastern margin of the Bayan Har block. The normal faulting earthquakes in the Lixian area are caused by local extension resulting from the sinistral shear movement of Lixian fault in a strike-slip faulting stress regime. It is suggested that under the continuous southeastward movement of the Bayan Har block and the obstruction of the rigid Sichuan basin, stress is accumulating along the southwestern segment of the LMSFZ, to which accordingly a high seismic hazard is ascribed and which should be carefully monitored. [Display omitted] •391 Focal mechanism solutions in the Longmen Shan fault zone are presented.•A shallow detachment layer at ~10 km depth is revealed by changes in fault dips.•Normal faulting earthquakes in the Lixian area are caused by local extension.•The Longmen Shan fault zone should be divided into two segments.•Seismic hazard in the southwestern segment is potentially high.