Vertical and horizontal land motion in the Mekong Delta: A long-term record from dual-geometry Sentinel-1 InSAR

The Mekong Delta is highly exposed to land subsidence (i.e. negative Vertical Land Motion, VLM), which increases flood risk, groundwater and soil salinization, land degradation, infrastructure damage, and accelerates relative sea-level rise. InSAR time-series have been widely used to estimate VLM in the delta. However, most studies used short investigation periods of up to 4 years and reported VLM derived from a single viewing geometry by projecting line-of-sight velocities using the incidence angle, which assumes horizontal motion is negligible. Such results are useful, however they can be uncertain where horizontal motion is non-zero and where time-series are noisy or non-linear.

We overcome the limitations of previous InSAR studies in the Mekong Delta by processing a Sentinel-1 time series with SBAS InSAR for an extended observation period of 9 years. The descending dataset covers 27^th February 2015 to 18^th December 2023 (12-day sampling) and provides 942,978 coherent points, while the ascending dataset spans from 13^th March 2017 to 31^st December 2023 and provides 511,972 coherent points. We then apply an ascending-descending (dual-geometry) decomposition to separate VLM and an east-west horizontal component for 190,533 SBAS points, limited to locations coherent in both geometries.

Both tracks show widespread subsidence in the delta with strong spatial variability and local hot spots. In single-geometry results, maximum VLM reaches about -9.5 cm/yr, with mean rates of about -3.3 cm/yr (descending) and -3.6 cm/yr (ascending). The dual-geometry decomposition yields a consistent VLM field with maximum VLM of about -8.4 cm/yr and a mean of -3.2 cm/yr (linear trend), while the east-west component ranges from -3.3 cm/yr (westward) to +2.9 cm/yr (eastward). The lower maximum values in the decomposed solution are expected, because decomposition is only possible for the reduced set of points available in both tracks. VLM is not uniform in time: time series show local accelerations and slowdowns. VLM patterns also differ between land-use types (e.g., urban areas, rice fields, mangroves, aquaculture), suggesting that drivers vary across the delta.

These InSAR-derived land motion estimates should be interpreted with care, because InSAR captures the combined surface response to multiple mechanisms of both natural and human-induced origin. Still, long-term VLM and horizontal motion maps can help compare displacement patterns, support interpretation of potential drivers, and provide a check for subsidence scenarios used in planning, adaptation and mitigation. Robust use of the results requires integration with independent information, including aquifer-compaction modelling outputs and reliable, locally corrected elevation data for relative sea-level rise studies.