A Metro-InSAR Risk Index for corridor-scale displacement risk diagnosis of urban rail infrastructure

Time-series Interferometric Synthetic Aperture Radar (InSAR) provides dense displacement observations along urban rail corridors, yet converting point-based measurements into corridor-scale diagnostic information remains challenging. This study proposes the Metro-InSAR Risk Index (MIRI), a diagnostic index designed to convert multi-dimensional InSAR-derived displacement characteristics into displacement-related diagnostic levels for urban metro corridors. MIRI integrates displacement rate, cumulative displacement, local displacement heterogeneity, height-related variability, and temporal residual variability, thereby extending conventional velocity-based interpretation toward corridor-scale displacement-risk diagnosis. The framework was applied to the Kunming Metro Network using 198 descending Sentinel-1 images acquired from March 2018 to December 2024. The time-series InSAR results show that the network was generally stable, with line-of-sight (LOS) displacement rates mainly ranging from −6 to 3 mm/yr and 97.21% of coherent scatterers falling within the stable interval of −3 to 3 mm/yr. Localized displacement anomalies were mainly observed near line intersections, curved sections, construction-affected areas, and the Dianchi Lake shoreline. Based on MIRI, 78%, 17%, 4%, and 1% of valid points were classified as Levels D, C, B, and A, respectively, indicating a distribution dominated by lower diagnostic levels, with spatially confined Level-B and Level-A anomalies particularly along Lines 4 and 5 and in lakeside soft-ground areas. Multi-source contextual variables and XGBoost-SHAP analysis were further used to interpret the environmental, anthropogenic, operational, and displacement-gradient information associated with the MIRI-based diagnostic patterns. The results indicate that the spatial variation in MIRI is mainly associated with terrain-related conditions, corridor-scale displacement variation, metro operation, and urban development intensity. By deriving diagnostic levels directly from PSI-based displacement indicators, MIRI provides an InSAR-based screening tool for prioritizing displacement-related anomalies and supporting maintenance of urban rail transit systems.