Corrections of Unwrapping Errors, Tropospheric and Ionospheric Effects on SAR Interferometry: Application to the Central Segment of the North Anatolian Fault

The central part of the North Anatolian Fault (NAF) is known to be creeping at least since the last M7.3 earthquake in 1944. Geodetic data collected within the past 20 years and dynamic models suggest that this 80 km-long-section slips aseismically at a velocity of 7-8 mm/yr between the surface and a depth of 4 km. So far, aseismic slip was assumed constant in time within the last 20 years. However, recent slip measurements from a local creepmeter and InSAR time series derived from Cosmo-SkyMed data highlighted that slip occurs within days-to-month long slip episodes, hence calling for a new assessment of the physics of fault slip along this segment.

Detecting millimeter-scale displacements over short time periods with InSAR requires some improvements to data processing. First, the radar signal is affected by changes in refractivity index in the troposphere and in the electronic content of the ionosphere, biasing measurements of deformation. Second, measuring surface displacements from interferograms generated from two SAR acquisitions requires the unwrapping of the interferometric phase which may be biased by phase noise within poorly coherent regions resulting in artificial phase offsets.

Here, we process 714 SAR images acquired along two tracks of Sentinel-1 satellites between 2014-2018 and show how we correct for these sources of noise. We present an original algorithm for correction of unwrapping errors, based on systematic and automatic identification of phase misclosure. We describe the performances of our algorithm and assess its efficiency for time series analysis. We also process 65 PALSAR acquisitions along 4 tracks of the late ALOS satellite to cover the 2006-2011 period. We correct interferograms from stratified tropospheric delays and ionospheric perturbations and highlight the efficiency of such approaches. Finally, we use the Generic InSAR Analysis Toolbox (GIAnT) to perform time series analysis on these 6 tracks. We generate 6 Line-Of-Sight maps of ground velocity and search for transient events within the observation periods. Finally, we use a Bayesian approach to derive the spatial extension of aseismic slip at depth and along-strike. We discuss our results in the light of previous geodetic studies along the creeping section of the NAF.