Seismic Sequences of the 2017 Mainling Mw 6.5 Earthquake: Imaging the Seismogenic Fault by Aftershock Analysis

Abstract

In the southeastern part of Tibet, an earthquake with a local magnitude of 6.9 occurred in the prefecture of Mainling on 18 November 2017. The mainshock and more than 900 aftershocks were recorded by a local seismic network comprising seven three-component seismic stations. In this study, both HypoDD location of aftershocks and focal mechanism inversion of moderate events were performed in order to accurately identify the pattern of active faults. The result reveals that the mainshock has a thrust source mechanism located at a depth of 14 km beneath the NE flank of the Namcha Barwa–Gyala Peri (NB-GP) massif. The aftershock sequences are caused mainly by two determined faults, one of which is the seismogenic fault stretching with a SE–NW trend parallel to the GP ridge and with a high NE-oriented dipping angle, and the other is activated by the mainshock and displays features of a SSE-NNW trend and SW-dipping, inferring the adjustment of stress in the focal area. The source parameters of the mainshock and the selected aftershocks show the reverse property of the seismogenic fault and its adjunct fault, thus inferring the backlash and uplift of the NB-GP massif, especially GP, for adjusting the uneven extrusion from the eastern Himalayan syntaxis to the adjacent Lhasa block. Furthermore, it is deduced that the rupture energy of the mainshock and aftershocks was limited by the surrounding rigid rock mass with high seismic velocity, such as the Lhasa block in the north and east, and Namcha Barwa complex in the south, and other aftershocks appearing at the NW top of GP and the SE side of Yarlung Tsangpo Big Bend reflect the strong squeezing effect of the NB-GP massif to its northeastern geological mass.