Resolving millimeter-scale tectonic deformation in decade-long InSAR time series : from long-term rifting to slow-slip events

Spectacular natural hazards such as large earthquakes or volcanic eruptions are accompanied by smaller-amplitude processes that produce millimeter-scale ground deformation. Although subtle, these signals provide critical insight into the physical state of the system and their associated hazards. Interferometric Synthetic Aperture Radar (InSAR) offers the spatial resolution required to observe such deformation, but its exploitation over long time spans remains challenging due to centimeter-scale noise and systematic biases. Here, we demonstrate how state-of-the-art interferometric processing combined with a Kalman Filter–based Time Series analysis (KFTS) enables the extraction of millimeter-scale deformation from 10 years of Sentinel-1A/B data. 

We present two case studies: the Chaman fault system (Pakistan–Afghanistan) and the Natron rift (northern Tanzania) in the East African Rift. Careful step-by-step corrections of the interferograms include tropospheric and ionospheric corrections, azimuth shift compensation, and rigorous assessment of closure phase biases. Measurement uncertainties derived from coherence are propagated within the KFTS time-series inversion, allowing iterative estimation of phase evolution with associated uncertainties.

In the Chaman fault zone, we detect aseismic deformation characterized by fault creep rates of about 5 mm/yr, as well as a slow-slip event with ~1 cm of cumulative displacement resolved using combined ascending and descending geometries. In northern Tanzania, we resolve long-term rift opening of only a few millimeters per year between 2015 and 2025, consistent with GNSS campaign measurements. We further assess the potential of Independent Component Analysis (ICA) for InSAR signal separation and discuss current limitations imposed by residual noise and vegetation-related biases.