Assessing Spatial and Temporal Variability of Vertical Land Motion Along Coastal Massachusetts Using Sentinel-1 InSAR and GNSS Time Series Analysis

Anurag Sharma , Shimon Wdowinski

Vertical land motion (VLM) is a critical component of relative sea-level rise (RSLR) assessments and can vary substantially across local to regional scales and over multi-year to decadal timescales due to differences in geology, natural processes, and anthropogenic influences. Coastal Massachusetts (MA) is a low-lying, geologically diverse region encompassing densely urbanized shorelines, glacially conditioned sediments, and environmentally sensitive wetlands, making it particularly vulnerable to the compounding effects of land subsidence and rising sea levels. Despite this vulnerability, spatially resolved, temporally representative characterizations of VLM across coastal MA remain limited, introducing significant uncertainty into regional RSLR projections and flood risk assessments. In this study, we characterize near-decadal (2016-2025) spatial and temporal variability in VLM across coastal MA by integrating continuous Global Navigation Satellite System (GNSS) time series with high-resolution (90-m) Sentinel-1 Interferometric Synthetic Aperture Radar (InSAR) observations. GNSS analysis reveals significant temporal variability in VLM, with distinct regional patterns including accelerating subsidence on the Cape Cod Peninsula, decelerating trends along the South Coast, and relatively stable conditions in Massachusetts Bay. Accounting for this temporal variability, InSAR observations are referenced to temporally representative GNSS-derived VLM rates, revealing a clear north-south gradient in present-day deformation, ranging from near-stable conditions in the North Shore and Massachusetts Bay region to subsidence rates exceeding ~2-3 mm yr⁻¹ toward Cape Cod Peninsula and the Outer Islands. Incorporating locally derived InSAR-based VLM rates into RSLR projections under the SSP2-4.5 scenario leads to spatially variable outcomes compared to projections based on regionally averaged Intergovernmental Panel on Climate Change (IPCC) VLM estimates, with projected RSLR ranging from approximately 0.48 m in the North Shore region to over 0.59 m in the Outer Islands by 2050. These findings highlight the importance of incorporating temporally representative, locally derived VLM rates to improve characterization of present-day coastal deformation and to support location-specific coastal adaptation strategies along coastal MA.