The Algeciras Fault System stretches roughly 540 kilometers in a northeast-southwest direction along the eastern flank of Colombia’s Central Cordillera, cutting through the departments of Meta, Huila, Cauca, Nariño, Caquetá, and Putumayo. It is a transpressional structure, meaning it combines sideways strike-slip motion with vertical compression, and it marks part of the boundary zone where the Nazca and South American plates grind against each other. The fault system passes beneath towns including Uribe, Algeciras, Neiva, Gigante, Pitalito, Sibundoy, Mocoa, and Ipiales, across terrain that rises from 600 meters to more than 3,000 meters above sea level. Historical records document destructive earthquakes in the Huila-Caquetá region, and instrumental monitoring confirms persistent moderate-to-strong seismicity along its trace.

What makes the new study unusual is its data sources. The researchers drew on monthly gravity solutions from NASA’s GRACE mission and its successor GRACE Follow-On, which measure tiny changes in Earth’s gravitational field from orbit. Because those satellites also see the weight of water moving through rivers, soils, and aquifers, the team subtracted hydrological contributions using the Global Land Data Assimilation System, or GLDAS, a land-surface model driven by meteorological and satellite observations. Even after this correction, the authors caution that residual non-tectonic signals cannot be fully excluded, so the gravity signals they interpret must be treated carefully.

To check the quality of the satellite gravity data, the researchers compared it against airborne gravimetric surveys flown by Colombia’s National Hydrocarbons Agency in 2006 and 2009 over the Yari-Caguán and Putumayo basins. At coarse regional scales, the agreement was remarkably tight: when the airborne data were smoothed with a 500-kilometer upward-continuation filter, average differences from the satellite solutions dropped to between roughly 0.49 and 1.54 percent. This cross-check supported the team’s decision to use the GFZ-processed RL06 solutions at full 96-by-96 spherical harmonic resolution, which preserve small-amplitude spatial variations that stronger filtering would erase.

The seismic side of the analysis drew on 14,592 earthquakes recorded by the Colombian Geological Survey between May 2002 and July 2020. The team converted moment magnitudes to local magnitudes where necessary and estimated the energy released by each event. When they sorted earthquakes by whether they occurred in areas of gravity increase or gravity decrease, a clear asymmetry emerged: 52.1 percent of events fell in gravity-decrease zones, but those zones accounted for 93.1 percent of the total released seismic energy. Stronger, shallower earthquakes were especially concentrated where gravity was falling.

The most dramatic result came from combining radar interferometry with gravity. Using 85 processed Sentinel-1 satellite images and the Small Baseline Subset technique, the researchers mapped vertical ground motion across the fault corridor and compared it month by month with GRACE-derived gravity variations. They identified two kinds of domains: direct-correlation areas, where gravity and elevation move up or down together, and inverse-correlation areas, where one rises as the other falls. When they tallied the earthquakes falling within each domain type, they found that areas of simultaneous decrease in both gravity and elevation accounted for approximately 99 percent of the total seismic energy released in the analyzed subset. For earthquakes of magnitude 3.0 or larger at depths of 20 kilometers or less, 89 percent of events occurred in these gravity-and-elevation-decrease zones.

This pattern is geophysically meaningful. Simultaneous gravity loss and ground subsidence can indicate mass redistribution at depth, whether through compaction, fluid migration, or the elastic response of the crust to tectonic loading. The authors suggest that such zones may reflect a closer spatial association between mass redistribution and crustal deformation during seismically active periods. Comparable gravity-deformation relationships have been documented in other settings, including Mount Etna in Italy and regions of nonlinear land subsidence, suggesting the pattern is not unique to the Colombian Andes but may be a general feature of actively deforming crust.

The team also cross-correlated gravity variations with ground displacements measured at nine permanent GNSS stations from Colombia’s GeoRED network. Correlation coefficients ranged from nearly zero to 0.61, with time lags stretching up to 45 months. The vertical component showed the strongest correlations of any measured quantity, with the LAB1 station recording a maximum direct correlation of 0.6143 at a four-month lag and a maximum inverse correlation of -0.5593 at six months. Stations close to the main fault trace, like LAB1, tended to show rapid responses consistent with elastic deformation of the upper crust, while stations farther away, like QUIL and OVSC, showed longer-delayed correlations that may reflect deeper viscoelastic processes or rheological differences along the fault corridor.

As a case study, the researchers examined the magnitude 6.0 Mesetas earthquake of December 24, 2019, which struck at a depth of about 13 kilometers. In the months surrounding the event, the GRACE-derived gravity time series displayed a distinctive W-shaped fluctuation, bounded by the two most abrupt gravity variations in the entire 2002-2020 record. At the earthquake location, the team noted a temporal coincidence between a gravity decrease of 0.461 milligal and an elevation change of -0.062 meters. Spectral analysis using the Fast Fourier Transform showed that most of the signal’s power was concentrated at low frequencies, with a dominant peak at roughly 0.2 cycles per month, corresponding to a period of about five months. The authors interpret this sequence as potentially compatible with stress accumulation, rupture, and post-event crustal readjustment, though they emphasize that the coarse spatial resolution of GRACE and possible residual hydrological effects mean the observation is exploratory rather than conclusive.

The study’s authors, Edwin Mauricio Pinzon Monroy and Carlos Alberto Vargas Jiménez, are careful to frame their results as associations rather than causal proof. Satellite gravimetry resolves features hundreds of kilometers wide, far larger than the rupture zone of a magnitude 6 earthquake, and hydrological models cannot perfectly remove water-related signals. GNSS time series vary in length and continuity, and InSAR measurements can be affected by atmospheric conditions and topographic artifacts. Yet the convergence of independent data streams, satellites weighing the Earth from orbit, radar measuring the ground’s rise and fall, and seismometers tallying where energy is released, offers a template that can be applied to other active fault systems worldwide. As gravity missions improve in resolution and longer geodetic records accumulate, this kind of multi-source approach may become a standard tool for identifying which segments of a fault are quietly loading up, and which are already letting go.

Subject of Research: Integration of satellite gravimetry, GNSS, InSAR, and seismicity to analyze the Algeciras Fault System in Colombia

Article Title: Seismic – Gravimetric analysis of the Algeciras fault system (Colombia): spatio-temporal correlations from GRACE and GNSS-InSAR

Article References: Seismic – Gravimetric analysis of the Algeciras fault system (Colombia): spatio-temporal correlations from GRACE and GNSS-InSAR. (n.d.). https://doi.org/10.3389/feart.2026.1938095

Image Credits: AI Generated

DOI: 10.3389/feart.2026.1938095

Keywords: Algeciras Fault System, GRACE, GRACE Follow-On, GNSS, InSAR, satellite gravimetry, seismicity, Colombia, tectonic deformation, Mesetas earthquake, GLDAS, seismic hazard