Land subsidence due to natural gas extraction in the Thrace basin (NW Turkey) and its influence on the North Anatolian fault under the Marmara Sea

Abstract

We map surface deformation in the Thrace region of Turkey using the Sentinel-1 Synthetic Aperture Radar (SAR) data. Interferometric Synthetic Aperture Radar (InSAR) time series analysis of the SAR data acquired between 2014 and 2020 on ascending and descending orbits reveals large-scale subsidence (~110 × 60 km) with rates reaching up to 10 ± 1.5 mm/yr. We relate this deformation to natural gas reservoir operations, such as gas exploitation and extraction activities that have been taking place in the region for decades, an inference being supported by the strong correlation between the InSAR time series and the variation in natural gas production during the same time period in the Thrace region reported by the Energy Market Regulation Authority of Turkey. Assuming that the observed subsidence is caused by compaction of sediments in the natural gas reservoirs, we construct a triangulated surface enveloping roughly at the bottom of the gas extraction wells and use it to invert the amount of negative opening (hence volume loss) on triangular elements that are assumed to be buried in an elastic and homogeneous medium. Coulomb stress changes caused by this volume change on the North Anatolian Fault at the Sea of Marmara are found to be insignificant (less than 10–5 Mega-Pascal) to perturb the state of the stress around the Marmara seismic gap. Yet, the large-scale subsidence revealed in this study needs to be taken into consideration when assessing hazards for the infrastructures, settlements, and other engineering structures, particularly in case of a large earthquake in the Marmara Seismic Gap.