Determining the extent and runout path of large earthflow through multi-temporal InSAR and topographic analysis

Precipitation regime fluctuations induced by climate change have a profound effect on the stability of earthflows in alpine and gorge regions. Traditional geomorphological mapping of areas at risk of earthflows requires high-resolution optical imagery and digital terrain datasets, which are often unavailable in these remote regions. This study demonstrates that the integration of synthetic aperture radar interferometry (InSAR) observations with topographic analysis enables effective delineation and identification of earthflow runout paths, using a large earthflow in southern Gansu Province, China, as a representative case study. The topographic wetness index (TWI) is shown to be an effective indicator for highlighting characteristic spatial signatures, thereby facilitating the accurate identification of the source zone, main track, depositional area, and runout path of a reactivated earthflow. Monitoring of precursory displacements reveals that continuous, moderate precipitation plays a critical role in the reactivation and eventual failure of a large earthflow. The primary failure was triggered by a daily rainfall event of 37 mm, following an 8-day period of gradual softening along the basal slip surface. Accordingly, the magnitude of this triggering event was governed by both rainfall intensity and the duration of antecedent weakening processes. Deformation patterns indicate that subsurface water accumulation is primarily controlled by rainfall infiltration along preferential flow paths, rather than by diffuse groundwater movement. These observations support the development of a conceptual model for delineating the extent of potential deep-seated earthflows and inferring their runout magnitude. The proposed model integrates InSAR-derived deformation data, spatial TWI distributions, and mechanical-morphological analysis, providing a practical framework for assessing and mitigating the risks associated with earthflow reactivation.