Off-fault damage in the 2023 Kahramanmaraş (Türkiye) earthquakes from SAR images

Abstract

Large strike-slip earthquakes are usually modeled as slip on localized planar fault planes within a homogeneous or layered elastic half-space. However, geodetic fault-slip inversions often show a shallow slip deficit, where the maximum slip is found at a depth of several kilometers with gradually decreasing slip towards the surface. High-resolution satellite images of earthquake surface ruptures also suggest a reduction in on-fault slip, often termed as off-fault damage or distributed deformation. In this study, we refer to this reduction in near-fault deformation as surface absent deformation (SAD). The presence of coseismic SAD, due to off-fault damange and/or compensated for by shallow interseismic fault creep and afterslip, holds significance for earthquake rupture processes, paleoseismology, and earthquake hazard assessments.In our study, we quantify the SAD along the main ruptures of the magnitude 7.8 and 7.6 Kahramanmaraş earthquakes, which occurred on 6 February 2023 near the Türkiye-Syria border, by mapping coseismic three-dimensional (3D) surface displacements using differential interferometry and pixel tracking of satellite synthetic aperture radar (SAR) images. The two earthquakes had a combined rupture length of approximately 500 km, exhibiting multi-meter surface fault offsets and diverse fault geometries, necessitating high-resolution deformation mapping near and away from the fault. We obtained the SAD distribution by analyzing fault-perpendicular profiles of fault-parallel displacement from the SAR-derived 3D displacements. For each profile, an arctan function was used to predict the near-fault (e.g., 0-5 km) elastic displacement based on the far-field (e.g., 5-50 km) observations, yielding an estimate of the lack of deformation close to the fault (i.e., the SAD). The results reveal a clear correlation between SAD and geometrical complexities along the two co-seismic ruptures. Using this strategy to determine the SAD, we find that about 35% of the surface fault slip is “missing” and expressed as SAD within a distance of 5-7 km from the coseismic surface ruptures. By comprehensively analyzing the interseismic, coseismic, and postseismic deformation in this area, we find that shallow interseismic fault creep and afterslip cannot explain the coseismic SAD and that it appears to be dominated by off-fault damage. Notably, existing research on off-fault damage has been concentrated within a range of only 1-2 km. Our results therefore underscore the significance of extending investigations of off-fault damage to 5-10 km from fault ruptures, and suggest that relying solely on on-fault offset measurements may lead to an underestimation of fault slip rate estimations by as much as one third.