Source parameters and seismogenic structure of the 2017 Mw 6.5 Mainling earthquake in the Eastern Himalayan Syntaxis (Tibet, China)

Abstract

On November 18, 2017, a moderate to strong earthquake occurred at Mainling County, Nyingchi City, Tibet, which was the first moderate earthquake in the Eastern Himalayan Syntaxis (EHS) region in the past five decades. To gain a better understanding of the rupture mechanism and seismogenic structure of this earthquake, the rupture process through a joint inversion of far-field body wave data and Interferometric Synthetic Aperture Radar (InSAR) data was present in this study. In addition, the three-dimensional (3D) crustal density structure of the EHS area was inverted using the gravity data. According to the joint inversion results of the rupture process, the Mainling earthquake lasted approximately 18 s and released energy of 8.32 × 1018 N m (Mw 6.5). The rupture was dominated by the thrust movement. The maximum slip was approximately 0.83 m, and the slip distribution was mainly concentrated at depths of 5–15 km in two patches. According to the results of gravity inversion, the density structure has good constraints on the source depth. The Mainling earthquake’s epicenter was located near the junction of high and low density areas. Its rupture extended over nearly 40 km in the ESE direction along the fault strike. In addition, the density structure also clearly shows a uniform density anomaly in this region, which may be beneficial to the release of seismic energy. The clockwise deformation of the erosion effect in the EHS superimposed over the uplifting and thrusting motion of the Namche Barwa (NB) area, and the Northward Indentation effect of the Indian block. All of these processes have strengthened folding deformation and the accumulated of stress and an abnormally high-density concentration within the NB area. The sudden release of accumulated stress led to the occurrence of the Mainling earthquake. These are the seismogenic structure of the NB zone. The regional structure and material characteristics indicates that the Dongjiu segment of the Dongjiu-Mainling Shear Zones (DMSZ) lies in the density anomaly gradient belt, with high density, low velocity, and high resistance media on its northeastern side, which benefits the stress accumulation. The earthquake activity and risks of this segment merits further attention.