Silent volcanic gas buildup revealed six months before La Palma eruption

Silent volcanic gas buildup revealed six months before La Palma eruption Lisa Lock Scientific Editor Robert Egan Associate Editor Researchers at Geosciences Barcelona (GEO3BCN-CSIC) have developed a novel way to monitor the silent accumulation of volcanic gases beneath Earth's surface using seismic ambient noise. The results could significantly improve early warning systems for volcanic eruptions. The continuous but invisible release of gases from underground is notoriously difficult to detect, yet it plays a critical role in volcanic hazard assessment. These gases can affect air quality, ecosystems, and public safety, and shifts in their behavior are often among the earliest indicators that magma is moving upward. To tackle this challenge, the study, published in Natural Hazards and Earth System Sciences, introduces a low-cost monitoring technique based on analyzing a property of surface seismic waves known as Rayleigh wave ellipticity. "At the heart of this research are atmospheric tides, a daily physical phenomenon," explains lead author Helena Seivane, who conducted the work with GEO3BCN-CSIC researcher Martin Schimmel. "They provide a natural and periodic reference signal that helps us interpret subtle changes underground." As volcanic systems inject gases from depth, the way atmospheric pressure propagates through the shallow subsurface becomes amplified. By analyzing seismic noise, the team shows that it is possible to detect these subtle changes, effectively tracking the build-up of pressurized magmatic gases long before any visible surface activity appears. Fluctuations in atmospheric pressure naturally compress and expand the ground, invisibly pumping gases through near-surface rocks. The researchers analyzed these variations over eight-hour cycles. "These cycles are purely atmospheric in origin, whereas 12- and 24-hour cycles can also be influenced by other natural forces such as Earth tides," Seivane points out. One of the method's key advantages is its simplicity and resilience. Unlike traditional monitoring approaches that rely on dense and complex sensor networks, this technique can operate with a single seismic station and remains robust even under highly variable noise conditions. Early warning signs at Cumbre Vieja To test the approach, the researchers applied it retrospectively to La Palma in the Canary Islands, an especially challenging case, as the volcano showed little to no surface gas emissions before the 2021 eruption. Data from a seismic station located in a fractured zone near the eventual eruption site revealed clear anomalies in pressure transmission around six months before the eruption began. These "silent signals" pointed to physical changes in the subsurface linked to the injection and accumulation of pre-eruptive gases. The team is now working to validate the technique in other volcanic settings. In collaboration with the Volcanological and Seismological Observatory of Costa Rica, Seivane is applying the method to Poás volcano, which entered an eruptive phase in early 2026. "At Poás we benefit from continuous monitoring and high-resolution CO₂ emission data measured on the scale of minutes," Seivane says. "This will allow much more detailed comparisons than in La Palma, where we only had annual CO₂ flux data." Ultimately, the researchers aim to turn seismic ambient noise into a fully independent monitoring tool, capable of detecting early warning signals without relying on complementary observations. More information Helena Seivane et al, Monitoring diffuse volcanic degassing with seismic ambient noise, Natural Hazards and Earth System Sciences (2026). DOI: 10.5194/nhess-26-2249-2026 Provided by Geosciences Barcelona (GEO3BCN-CSIC)