Advanced InSAR Monitoring of Ground Deformation in the Polog Valley

Summary

Abstract

Understanding ground deformation across valley floors and mountain slopes is essential for assessing terrain stability and enabling the timely detection of landslides. Satellite radar interferometry provides an effective means for high-precision, wide-area deformation mapping, allowing early identification of zones prone to slope instability. This study focuses on the Polog Valley in northwestern North Macedonia and its surrounding slopes, forming a broad intermontane basin bordered by the Shar Planina, Zheden, and Bistra mountains. The objective is to identify and quantify deformation patterns across the valley and adjacent slopes using advanced InSAR techniques. A multi-temporal analysis of Sentinel-1 SAR data (2014–2025) was performed by combining ascending and descending orbits and applying the PSInSAR and QPSInSAR techniques. To obtain physically meaningful motion estimates, line-of-sight (LOS) displacements from both geometries were decomposed into dominant east–west and vertical components. The results indicate that the central plain of the Polog Valley remains stable, with mean LOS deformation rates within ±1 mm/year. In contrast, three active deformation zones were identified near the villages of Gjermo, Selce, and Rakovec. In the northern part of Gjermo, mean cumulative east-oriented displacements are around 35 cm, with about 11 cm mean vertical subsidence, while at the entrance to Selce, mean west-oriented displacements exceed 20 cm with similar mean subsidence. In the southern part of Rakovec, mean east-oriented displacements reach about 12 cm together with roughly 5 cm of mean vertical subsidence. These deformation patterns are consistent with the local slope morphology, with east-inclined slopes at Gjermo and Rakovec and west-inclined terrain at Selce. The findings confirm the effectiveness of advanced InSAR time-series techniques for detecting and characterizing localized slope instabilities. For the identified deformation zones, integrating satellite-based observations with geodetic and geotechnical monitoring is recommended to support long-term stability assessment and timely detection of landslides in the Polog Valley.