Impact of buckling on short- and long-term fold growth through combined seismic imaging and InSAR analysis: a case study from the Qaidam Basin, northern Tibetan Plateau

How folds grow in response to instantaneous earthquakes and long-term tectonic loading is essential for understanding the upper-crustal deformation mechanism and assessing seismic hazards, but usually impeded by limited knowledge of underground deformation. We herein attempt to address this issue through a case study of the 28 March 2019 Mangya earthquake in the Shizigou anticline, western Qaidam Basin (northern Tibetan Plateau) based on multiple datasets. Three-dimensional reflection seismic imaging reveals complicated underground structures, including a shallow thrust, a deeply-buried reverse fault, and an intervening salt-bearing weak layer. The InSAR geodesy reveals co-seismic surface folding with concomitant uplift and subsidence zones in the northeastern limb rather than the core of the anticline. Elastic inversion suggests a likely underground co-seismic slip surface at depth of ~2.1 km. They together indicate that Mangya earthquake was caused by neither the shallow thrust nor the deep reverse fault; instead, it likely resulted from buckling of strata with contrasting viscosity in the limb of the Shizigou anticline in response to bed-parallel contraction. The buckling process not only leads to complicated seismic behaviors of the Shizigou region that features lack of large earthquakes on the underlying faults compared with the nearby regions, but also modulates the long-term geometric growth of the Shizigou anticline. Our findings highlight the importance of buckling process in fold growth over different time scales, and the requirement of a clear illumination of subsurface structures to precisely understand the mechanism of fold growth and related earthquakes in fold-thrust belts.