Complex surface displacements of the Nanyu landslide in Zhouqu, China revealed by multi-platform InSAR observations

The Zhouqu County of Gansu Province in northwestern China is a geological hazard-prone area with frequent landslide and debris flow events. As a recent example, the Jiangdingya landslide, a smaller landslide within the larger ancient Nanyu landslide, was reactivated and partially collapsed to block the Bailong River near the Nanyu Township on 12 July 2018. Previous studies have shown that heavy rainfall is the major triggering factor of the failure event, but the spatio-temporal pattern of surface displacements of the landslide remains unknown due to a lack of in-situ measurements on the slope. In this study, we retrieved the surface displacements of the Nanyu landslide before the slope failure event using time-series Synthetic Aperture Radar Interferometry (InSAR) analysis of Sentinel-1 and ALOS-2 PALSAR-2 data. Our results show that the ancient Nanyu landslide has been in an unstable state prior to the failure event. To investigate the spatial patterns in displacement, we inverted a quasi three-dimensional (3D) surface displacement field using multi-orbit InSAR measurements combined with the surface parallel flow model, which showed a significant spatial heterogeneity in displacement patterns across the ancient landslide. In particular, a sign of local reactivation was observed on the upper slopes, which might be related to recent road construction activities. Furthermore, the detectability of unstable slopes near the Nanyu landslide with Sentinel-1 and ALOS-2 PALSAR-2 observations was analyzed to deepen our knowledge on the optimal choice of SAR data for geohazards detection and monitoring.

Automated summary

The Zhouqu County in Gansu Province, China, is prone to geological hazards such as landslides and debris flows. In this study, the surface displacements of the Nanyu landslide before a failure event were analyzed using time-series Synthetic Aperture Radar Interferometry. The results revealed that the ancient Nanyu landslide was unstable prior to the failure event. Additionally, the detectability of unstable slopes near the Nanyu landslide was analyzed using Sentinel-1 and ALOS-2 PALSAR-2 observations.