The research, conducted by Nihal Tekin Ünlütürk of Erciyes University and Uğur Doğan of Yıldız Technical University, evaluated the effect of tropospheric corrections derived from Global Navigation Satellite System (GNSS) observations against those produced by the Generic Atmospheric Correction Online Service for InSAR, widely known as GACOS. Tropospheric delay is one of the dominant error sources in InSAR processing, and if left uncorrected it can distort the interpretation of earthquake-induced deformation fields—the very patterns scientists rely on to infer fault slip, rupture extent, and seismic hazard. The team’s central question was deceptively simple: which correction approach actually improves the displacement maps, and by how much?

To find out, the researchers processed both ascending and descending Sentinel-1A radar images from the European Space Agency’s Copernicus mission using the Sentinel Application Platform (SNAP) within a standard Differential InSAR (DInSAR) workflow. This involved generating interferometric pairs spanning the earthquake, applying phase filtering to suppress noise, and performing phase unwrapping with the Statistical-Cost, Network-Flow Algorithm for Phase Unwrapping, or SNAPHU—a computational step that recovers continuous displacement values from the wrapped fringes that radar phase measurements naturally produce. Working with both ascending and descending orbits matters because each viewing geometry samples the ground differently along the satellite line-of-sight, providing complementary constraints on the three-dimensional deformation field.

The GNSS-based correction strategy exploited Turkey’s dense TUSAGA-Active continuous GNSS network, whose data were provided by the General Directorate of Mapping. Because GNSS signals also pass through the troposphere, each station continuously estimates the zenith tropospheric delay—the total delay a signal experiences traveling vertically through the atmosphere. By regressing these delays against station elevation, the researchers derived parameters that allowed them to predict the topography-correlated component of the atmospheric delay across the entire interferogram, interpolating between stations and removing the estimated delay from the radar phase. GACOS, by contrast, is an online service that generates correction maps by combining global weather model data with an iterative tropospheric decomposition model, separating the stratified, elevation-dependent part of the delay from turbulent components.

The results are a sobering reminder that atmospheric correction is not a solved problem. Both GNSS- and GACOS-based corrections clearly influenced the spatial distribution of the line-of-sight (LOS) displacement fields, reshaping the apparent deformation patterns in ways that could alter scientific conclusions. Yet their effects were not spatially uniform across the study area. In some regions the corrections agreed closely; in others, the corrected products diverged noticeably, leaving analysts with genuinely different pictures of how the ground moved. The study found that neither correction strategy produced a spatially uniform improvement across all interferometric pairs—a finding that challenges the common assumption that applying any atmospheric correction is automatically better than applying none.

The local discrepancies between the corrected products, the authors suggest, likely stem from a combination of factors: regional atmospheric variability, the rugged topography of the East Anatolian Fault Zone, the acquisition geometry of the satellite passes, and the uneven distribution of GNSS stations. Eastern Anatolia is a region of sharp elevation contrasts, where stratified tropospheric delay can change rapidly over short horizontal distances. Where GNSS stations are sparse, interpolation of zenith delays becomes uncertain, and turbulent water vapor variations—which are poorly correlated with elevation—can dominate. Weather-model-based corrections like GACOS, meanwhile, are limited by the spatial and temporal resolution of the underlying meteorological data, which may not capture localized convective moisture features at the moment of a satellite overpass.

Rather than declaring a single winner, the study advocates a more rigorous, diagnostic approach. The researchers emphasize the need to carefully evaluate atmospheric correction performance using spatial statistics, profile comparisons across the deformation field, and displacement-difference analyses between corrected and uncorrected products. In other words, the choice of correction should be treated as a hypothesis to be tested for each interferogram and each tectonic setting, not a routine checkbox. This methodological caution has practical consequences: deformation signals from the Elazığ–Sivrice rupture feed into models of fault slip and earthquake source parameters, and atmospheric artifacts of comparable magnitude could bias estimates of how much the fault moved and where.

The 2020 Elazığ–Sivrice earthquake itself makes a compelling case study. The event struck the East Anatolian Fault Zone, a major strike-slip boundary accommodating the westward extrusion of the Anatolian plate between the Arabian and Eurasian plates. Previous studies have used InSAR and GNSS data to characterize its rupture behavior, and the region’s tectonic importance has motivated extensive geodetic monitoring. The fault zone’s combination of strong topographic relief, semi-arid climate with pronounced seasonal atmospheric variation, and a growing but still uneven GNSS infrastructure makes it an ideal proving ground for atmospheric correction techniques that must ultimately work worldwide.

The broader significance of the study lies in its framing of GNSS- and GACOS-based corrections as complementary rather than competing tools. Because they draw on fundamentally different information sources—direct physical measurements of atmospheric delay at ground stations versus model-derived estimates from meteorological data—their residuals behave differently. Comparing them side by side provides a way to identify atmospheric contributions embedded in interferometric phase and to assess the uncertainty of InSAR-derived earthquake deformation fields. For hazard assessment, volcano monitoring, and land subsidence studies, where InSAR increasingly underpins operational decision-making, this kind of uncertainty quantification is becoming essential.

The work also fits into a vibrant international research effort. Recent studies have compared GNSS local observations with global weather-based models over tropical volcanoes, applied GNSS spatial interpolation for atmospheric correction in New Zealand, and used neural networks trained on GNSS delays for terrain-challenging regions. The Turkish study adds a critical data point from an active continental strike-slip setting, demonstrating that the performance hierarchy of correction methods is context-dependent. As Sentinel-1 and future radar missions deliver ever more frequent global coverage, and as GNSS networks densify, the message from Elazığ–Sivrice is clear: the atmosphere will always leave its fingerprint on radar interferograms, and taming it demands both better correction products and smarter validation of when, where, and how much those products actually help.

Subject of Research: Comparative assessment of GNSS- and GACOS-based tropospheric corrections for InSAR-derived surface deformation from the 2020 Elazığ–Sivrice earthquake

Article Title: Comparative assessment of GNSS- and GACOS-based tropospheric corrections for InSAR-derived surface deformation: a case study of the 2020 Elazığ–Sivrice earthquake

Article References: Ünlütürk, N. T., & Doğan, U. (2026). Comparative assessment of GNSS- and GACOS-based tropospheric corrections for InSAR-derived surface deformation: a case study of the 2020 Elazığ–Sivrice earthquake. Earth Science Informatics, 19(10), Article 182. https://doi.org/10.1007/s12145-026-02236-1

Image Credits: AI Generated

DOI: 10.1007/s12145-026-02236-1

Keywords: InSAR, GNSS, GACOS, tropospheric correction, surface deformation, Elazığ–Sivrice earthquake, Sentinel-1A, DInSAR, East Anatolian Fault Zone, SNAPHU, atmospheric delay, Earth Science Informatics

The earthquakes, recognized as the Pazarcık and Elbistan earthquakes, measured magnitudes of 7.8 and 7.5, respectively, shaking the region with profound intensity. The Pazarcık earthquake was recorded at 1:17 A.M. GMT, with a focal depth of 10 kilometers. Just nine hours later, the Elbistan earthquake followed, occurring at a shallower focal depth of 7.4 kilometers. Both seismic events resulted in extensive surface ruptures, leading to significant scientific inquiry into their geophysical impacts.

The capacity to rapidly acquire detailed satellite images in the aftermath of these earthquakes has proven invaluable. The research team employed an array of high-resolution satellite data, including resources from Chinese GF-series satellites (GF-2/7), Beijing satellites (BJ-2/3), and various Maxar satellites. These images, with resolutions ranging between 0.3 to 0.8 meters, enabled the researchers to analyze the characteristics of surface ruptures comprehensively. As a result, they discerned that the two earthquakes produced distinct rupture zones, providing essential insights into their individual geological phenomena.

The first earthquake showcased a remarkable coseismic surface rupture extending approximately 280 kilometers along the southwestern segment of the main East Anatolian Fault. This surface rupture was characterized by left-lateral displacements, with measurements revealing shifts reaching up to 6.8 ± 0.68 meters, particularly pronounced 40 kilometers northeast of the event’s epicenter. Conversely, the second earthquake’s rupture exhibited a shorter duration, generating a rupture of about 110 kilometers along an east-west branch of the fault, with maximum displacements recorded at 7.2 ± 0.72 meters. Such findings underscore the varied geological behaviors exhibited by these two seismic events.

Further analysis conducted by Xu’s research group involved juxtaposing the surface rupture distributions and displacement data obtained from high-resolution satellite imagery against results sourced from InSAR and ground-based field measurements. Through this comparative methodology, the researchers concluded that rapid assessments employing high-resolution remote sensing data could significantly enhance scholarly understanding of surface rupture characteristics. Such tools offer scholars and earthquake scientists the ability to comprehend the impacts of seismic events in foreign territories swiftly.

Moreover, the study presents critical observations regarding the nature of cascading ruptures. The research team’s findings reveal that the surface rupture behavior does not adhere strictly to established recurrence intervals, proposing that maximum surface slip is influenced by the accumulative stress energy present within each fault segment. Interestingly, they noted that despite the greater magnitude of the Pazarcık earthquake, its maximum surface offset was surpassed by that of the subsequent Elbistan earthquake. This nuanced understanding of rupture dynamics elucidates the complexity underlying seismic events and challenges previous assumptions held within the earthquake research community.

Additionally, Prof. Xu’s article sheds light on historical seismic activity within the region. He emphasized that the 1822 event, recorded as a Magnitude 7.5 earthquake, likely occurred along the Yesemek segment, rather than along the Amanos segment, which helped prevent the propagation of rupture southward along the Narlı segment during the major Pazarcık earthquake event. By identifying these nuanced historical patterns, his research emphasizes the importance of studying past events to better predict future seismic activities.

The potential implications of these findings extend beyond academic discourse, raising concerns regarding the seismic risks associated with areas such as the Malatya Fault and specific segments of the East Anatolian Fault. The research underscores the necessity of enhancing seismic fortification measures, particularly in valley regions adjacent to these fault segments, which may remain susceptible to future earthquakes. Such fortification efforts can serve as crucial preventive measures aimed at safeguarding the local populations and infrastructures against the disastrous impacts of future seismic events.

The pursuit of knowledge surrounding earthquake rupture mechanisms continues to evolve, propelled by advancements in remote sensing technology. The study led by Xu not only exemplifies the significant role of high-resolution satellite imagery in evaluating seismic occurrences but also highlights the collaborative nature of global scientific inquiry. By sharing their findings, Prof. Xu’s team contributes to a broader understanding of earthquake dynamics that can pave the way for future studies in earthquake preparedness and disaster management strategies.

This pioneering research signifies a noteworthy advancement in the field of earthquake research. It emphasizes the important intersection between technology and geology, highlighting how satellite imagery can facilitate rapid assessments post-disaster while promoting the understanding of complex geophysical processes. As researchers continue to refine these methodologies, the hope is that a greater comprehension of earthquake phenomena will emerge, ultimately fostering improved preparedness against the ever-looming threat of seismic activity.

In conclusion, the study instills a greater understanding of the urgent need for comprehensive examinations of earthquake effects and the geological processes behind them. By recognizing the intricate dynamics of fault ruptures, society can adopt a forward-looking approach, preparing to mitigate the impacts of significant seismic events through scientific innovation and thorough research.

Subject of Research: High-resolution satellite data assessment of surface ruptures in the 2023 Türkiye earthquake doublet.

Article Title: Rapid Assessment of Surface Ruptures in the 2023 Türkiye Earthquakes Using High-Resolution Satellite Data

News Publication Date: October 2023

Web References: N/A

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Image Credits: ©Science China Press

Keywords: earthquake, satellite imagery, surface rupture, seismic risks, 2023 Türkiye earthquake, earthquake assessment, high-resolution data, East Anatolian Fault, cascading rupture patterns, seismic fortification.