Linking annual land subsidence to built-up density across coastal tourism zones via Sentinel-1 differential interferometric synthetic aperture radar

Uncontrolled urban expansion has intensified land subsidence in many cities, including Bali's coastal tourism zone.To provide the first area-wide assessment for South Bali, we mapped spatiotemporal subsidence from 2014-2022 using Differential InSAR (DInSAR) on Sentinel-1 SLC data, derived built-up density from Sentinel-2, and tested their association.We find persistent subsidence with typical rates of -10 to -60 mm/yr, local extremes near -70 mm/yr, and a multi-year mean near -40 mm/yr.Hotspots cluster in Padang Sambian, Dauh Puri, Sanur, Pemogan, and Dalung; field inspections documented wall cracking, floor settlement, and road depressions consistent with these clusters.Notably, despite drawing global attention over the past decade for overtourism, the Canggu tourism area shows no detectable subsidence in our analysis.At the full-area scale, the subsidence-density relationship is very weak (year-by-year correlations near zero: r -0.11 to +0.008; R 0.0006-0.012).Using decade-average statistics (~2014-2022) and a focused evaluation within zones exhibiting both high built-up density and elevated annual subsidence (n 10,000 pixel samples), the association remains very weak (r = 0.045; R 0.21%), although statistically significant (p < 0.05).Thus, while built-up density exerts a detectable effect, its contribution is minimal, implying that other drivers are more influential at city scale.We recommend using these results as screening evidence for risk-aware planning, prioritizing investigation and monitoring in identified hotspots, and integrating geology, soils, hydrology, groundwater levels, and refined InSAR processing to strengthen attribution and target mitigation.Unrevealed factors remain a key challenge for us and future researchers to determine the primary causes of land subsidence in this tourism zone and urban center.