Integration of InSAR and Inclinometer Measurements for Evaluating Surface Deformation in an Active Landslide Area

Interferometric Synthetic Aperture Radar (InSAR) enables surface deformation monitoring over large areas with high spatial and temporal resolution, in addition to traditional long-term in-situ monitoring methods such as inclinometers. Conventional Persistent Scatterer InSAR (PS-InSAR) techniques rely on phase-stable point targets and provide reliable results under low deformation rates and high coherence. However, the use of this method is constrained in environments characterized by rapid deformation or vegetation cover. In such environments, the Small Baseline Subset (SBAS) InSAR approach offers a more robust alternative by utilizing interferogram pairs with small spatial and temporal baselines, allowing distributed scatterers to be included in time-series deformation analysis. The main objective of this study is to evaluate surface deformation dynamics in active urban landslide areas through joint InSAR analysis by integrating both SBAS and InSAR results with in-situ inclinometer measurements and to investigate the temporal relationship between deformation behavior and rainfall conditions. The study area is located in the Büyükçekmece region, in the south-western part of Istanbul, Türkiye, where ongoing landslide activity affects a densely urbanized environment and is characterized by predominantly south-westward movement patterns. Historical inclinometer data acquired between 2014 and 2016 were used to characterize subsurface deformation. These measurements were analyzed with SBAS-InSAR LOS (line-of-sight) displacement time series derived from Sentinel-1 descending-orbit data acquired during the same period. Open-source MintPy software within the ASF OpenSARLab was used as a virtual computing environment to process InSAR data and time-series analyses. Low-coherence pixels were excluded, and standard atmospheric phase corrections were applied. Deformation velocities were analyzed by considering their non-linear temporal behavior, evaluating both average velocity patterns and temporal changes in deformation rates. Rainfall data from nearby meteorological stations were incorporated to assess the correlation between precipitation events and deformation acceleration. The novelty of this study lies in the preference for the SBAS-InSAR approach over PS-InSAR; this choice was driven by the rapid deformation characteristics of the landslide area, where the high phase stability required for PS-InSAR is often compromised. Although InSAR does not provide direct subsurface deformation data, the results demonstrate that when integrated with inclinometer measurements, SBAS-InSAR time-series analysis offers a reliable and efficient framework for continuous surface deformation assessment in active urban landslide environments.