Structural geometry and active faulting along the southern segment of the Longmenshan thrust belt: Insights into the seismotectonics of the 1327 Tianquan and 2013 Lushan earthquakes (eastern Tibetan Plateau, China)

Abstract

The 2008 Mw 7.9 Wenchuan and 2013 Mw 6.7 Lushan earthquakes ruptured the central and southern segments of the Longmenshan thrust belt on the eastern Tibetan Plateau, respectively. The ∼M 7 Tianquan earthquake in 1327 is also thought to have occurred on the southern segment with effects spatially overlapping those of the Lushan earthquake, but the deformation style and seismogenic mechanism of the segment remain poorly understood. Satellite image interpretation, field investigations, and trenching reveal that the Shuangshi-Dachuan fault (SDF) on the southern segment displaces landforms, dips southeast, and is dominated by thrust motion with a left-lateral strike-slip component. The latest and penultimate events along the fault occurred in 1085–1457 CE and 1215 BCE-104 CE, respectively. Some ancient landslides exits along the fault, and one of them formed close to 703 ± 17 yr BP. The ages of the latest event and of the landslide agree well with the occurrence of the Tianquan earthquake, indicating that the earthquake was most likely generated by the SDF. Magnetotelluric imaging and the aftershock belt of the Lushan earthquake show that the Longmen Shan forms a wedge-shaped high-resistivity body protruding eastward. The SDF corresponds well to the east-dipping upper electrical boundary of the wedge-shaped body and is interpreted as a shallow backthrust; the seismogenic fault of the Lushan earthquake corresponds to the west-dipping lower boundary and is interpreted as a blind thrust fault. The left-lateral slip component of the SDF may be a response to the lateral extension of the weak lower crust.