Coseismic and Postseismic Fault Kinematics of the July 22, 2020, Nima (Tibet) Ms6.6 Earthquake: Implications of the Forming Mechanism of the Active N‐S‐Trending Grabens in Qiangtang, Tibet

Abstract

The N‐S‐trending grabens, the most prominent active structures in the Tibetan plateau, contain important information about the evolution and deformation mechanism of the plateau. The Ms6.6 earthquake that occurred in Nima, Qiangtang on July 22, 2020, provides an opportunity to investigate the kinematics of the grabens. Here, we use field survey and interferometric synthetic aperture radar to study the structural style, coseismic and postseismic deformation, fault slip, and stress changes of the Nima earthquake. The N‐S‐trending Qomo Ri graben (QRG) and the normal faults on the east and west sides are kinematically correlated with the NE‐trending sinistral Jiangaizangbu River and West Yibug Caka faults. The QRG is characterized by an extensional strike‐slip duplex and negative flower structure. The maximum coseismic deformation of the Nima earthquake is 29.4 cm in the line‐of‐sight direction observed by Sentinel‐1 ascending images. The coseismic slip model demonstrates that the NE20–30°‐striking fault caused the Nima earthquake has a maximum slip of 1.39 m and a main slip area at 4–12 km depth. The seismological fault may transit from a moderately dipping normal fault near the surface to a steeper strike‐slip fault at depth. Obvious postseismic deformation was observed in the stress‐enhancement area. The afterslip model shows that the mainshock fault is correlated with parallel normal faults to its west. We infer that the QRG was formed by local E‐W‐trending extension at the joints of the left‐stepping sinistral fault system, and the active normal faulting in the Qiangtang block favors stress localization.

Coseismic and early postseismic deformation of the 2020 Nima Mw 6.4 earthquake, central Tibet, from InSAR and GNSS observations

“…The study area is located in central Tibet with an average altitude of greater than 4,300 m, where only a few GNSS stations are available. The known coseismic or postseismic slip models of the Nima earthquake are inverted using only the Sentinel-1 C-band SAR interferometry images (Yang et al, 2021a;Ji et al, 2021;Gao et al, 2022). Three continuous GNSS stations near the deformation center (Figure 1) is deployed in 2018, under a collaboration between Tianjin University (led by Jing Liu-Zeng) and Institute of Geology, China Earthquake Administration.…”

“…There is no obvious surface rupture observed, which makes it difficult to directly ascertain the seismogenic fault. Previous InSAR research suggested different results on the seismogenic fault for the 2020 Nima earthquake, such as the east-dip WYF with a constant strike (Yang et al, 2021a;Ji et al, 2021; or an unmapped east-dip branch fault with variable strikes and dips (Gao et al, 2022), instead of the west-dip EYF determined by geology (Liu et al, 2021). To determine the geometry parameters of the seismogenic fault, we check multiple alternative slip models (Table 1).…”

“…InSAR is an ideal tool to study the 2020 Nima earthquake processing in this extremely remote part of Tibet. Several studies from InSAR data have suggested that the Nima earthquake occurred on the WYF with a constant-strike angle (Yang et al, 2021a;Ji et al, 2021; or a previously unmapped branch with a variablestrike angle (Gao et al, 2022), instead of the EYF (Liu et al, 2021). Based on Sentinel-1 C-band Synthetic Aperture Radar (SAR) datasets after the mainshock, Yang et al suggested that the postseismic deformation was dominated by aseismic slip with nearly pure normal faulting (Yang et al, 2021a).…”

“…Gao et al thought the postseismic deformation was caused by slow slip on a parallel branch to the west of the seismogenic fault. (Gao et al, 2022). Therefore, we inverted jointly continuous GNSS observations and Sentinel-1 datasets to determine the seismogenic fault, the coseismic and postseismic slip distributions associated with the 2020 Nima event.…”

“…It starts in Dongcuo Basin in the south, extends NEE~NNE to the pull-apart Yibu Chaka basin, and then connects with the sinistral Jiangai-Zangbu fault on the north side of the basin (Taylor et al, 2003). The Yibu-Chaka basin is dissected by a series of NS~NNE-striking normal faults into a symmetric horst-graben system, characterized by a negative flower structure (Gao et al, 2022). The largest graben is the Yibu-Chaka graben in the central of the basin, which connects two opposite dipping sets of domino normal fault blocks and is bounded by two primary normal faults (the west and east Yibu-Chaka faults, abbreviated as the WYF and EYF) .…”