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On August 10, a magnitude 7.4 earthquake struck near San JosΓ© del Palmar, ChocΓ³, at approximately 103 km depth.
Seven weeks later, Colombia's Geological Survey (SGC) distinguished three seismic patterns. Treating every event as an aftershock misses the geology.
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β’ ChocΓ³ aftershocks: 332 events at 70β105 km, progressively declining in frequency and magnitude.Β β’ ChocΓ³ swarm: over 1,530 events at 30β70 km; 31 at magnitude 4.0 or above, without typical aftershock decay.Β β’ Chaparral, Tolima: at least 979 events of magnitude 2.0 or above since September 20; 22 at magnitude 4.0 or above, all shallower than 30 km.
These are SGC figures as of September 27, not live totals.
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The SGC is investigating whether the ChocΓ³ swarm involves fluids released by dehydration reactions in the subducting plate, potentially aided by stress changes following the mainshock.
Migrating fluids can raise pore pressure and reduce effective normal stress along suitably oriented faults, facilitating slip. Plausible physics; the specific causal mechanism remains preliminary.
For well engineers, the pressureβeffective-stress relationship is familiar from wellbore stability. But an MPD operating window is not a tectonic model: the spatial scale, stress regime, and timescales differ dramatically.
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Colombia is seismically active even without these sequences. The Bucaramanga seismic nest, near 150 km depth, accounts for approximately 45% of the country's recorded earthquakes, according to the SGC.
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Neither these counts nor the fluid-migration hypothesis predict when or where a larger earthquake will occur.
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Classifying the signals correctly is the first step toward understanding the system.
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How often do we mistake different subsurface processes for a single event simply because they occur close together in time?
π SGC official bulletins, September 23 and 27.
The distinction between aftershocks, seismic swarms, and separate crustal seismicity comes from official SGC observations, not from my reading of the news.SGC bulletin of 23 September, which states the fluid migration attribution for the Choco swarm:https://www2.sgc.gov.co/Noticias/Paginas/Boletin-de-actividad-sismica-destacada-del-23-de-septiembre-de-2026.aspx
SGC bulletin of 27 September, with the current counts, depths and the swarm definition:https://www2.sgc.gov.co/Noticias/Paginas/Boletin-de-actividad-sismica-destacada-del-27-de-septiembre-de-2026.aspx
What the Bucaramanga seismic nest is:https://www2.sgc.gov.co/Noticias/Paginas/Que-es-el-Nido-Sismico-de-Bucaramanga.aspx
SGC live earthquake viewer, updated continuously:https://www.sgc.gov.co/sismos
The fluid-migration explanation is still a preliminary hypothesis. Counts are time-dependent, so the official monitoring system is the reference for anything after 27 September, including this post.
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