Investigating earthquake legacy effect on hillslope deformation using InSAR-derived time series

Mountainous landscapes affected by strong earthquakes typically exhibit higher landslide susceptibility in post-seismic periods compared to pre-seismic conditions. This concept is referred to as the earthquake legacy effect, which needs to be better understood to develop an accurate post-seismic landslide hazard assessment. The earthquake legacy effect is mainly assessed by monitoring either rapid landslide occurrences or slow-moving landslides over time. However, landslide inventories provide discrete information both in spatial and temporal domains, whereas monitoring only selected slow-moving landslides discards the response of hillslopes exposed to a given earthquake. Therefore, to provide a more comprehensive understanding of the concept, this research focuses on post-seismic hillslope evolution by examining the deformation time series generated from the Interferometric Synthetic Aperture Radar technique over the area affected by the 2017 M-w 6.4 Nyingchi earthquake, China. We also analyse factors controlling these InSAR-derived hillslope deformations. Our results show a high coherence between hydrologic conditions (i.e., precipitation and terrestrial water storage) and surface deformation in pre- and post-seismic periods. The earthquake disturbs this strong correlation for a while (similar to 2 years) right after the seismic tremor, and then, a seasonal deformation pattern depending on hydrologic conditions appears again. Our findings show that the average post-seismic hillslope deformation is still higher than its pre-seismic counterpart approximately four and a half years after the earthquake. These findings trigger further research questions regarding whether hillslopes could fully recover after a major earthquake or gain a new level of hillslope susceptibility caused by intense ground shaking.