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.
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The original SAR images of Sentinel-1 data are freely available data from European Space Agency (https://scihub.copernicus.eu/), and the SRTM and Landsat data are available from the U.S. Geological Survey (https://earthexplorer.usgs.gov/). Pre-stack migrated seismic profiles in depth domain were used to depict the underground structure and its along-strike variation of the Shizigou anticline . These data have a spatial resolution of ~50 m and a maximum probing depth of ~10 km, deep enough to capture the deformation of the entire Cenozoic successions. The co-seismic surface deformation of the Mangya earthquake was extracted from Interferometric analysis of two Synthetic Aperture Radar (SAR) images acquired a couple of days before and after the earthquake from Sentinel-1A. The details of these images, including the acquisition date, perpendicular baseline, temporal baseline and orbit types, are listed in Table 2. DEM data from the Shuttle Radar Topography Mission (SRTM) was used to remove the topographic phase component (Farr et al., 2007). The adaptive filtering method was employed to reduce the phase noise and enhance the signal-to-noise ratio (Goldstein and Werner, 1998). The minimum-cost flow algorithm (Costantini, 1998) was used to unwrap the InSAR interferograms, which are subsequently processed by orbit refinement and re-flattening (Figure 3). LOS (light of sight) displacement distributions of the two tracks were then obtained by conversion of phase to displacement . Detailed description of the inversion of seismogenic structure are included in Supporting Information Text S1.