Monitoring Diffuse Volcanic Degassing with Seismic Ambient Noise

Abstract. Diffuse volcanic degassing is a persistent and often underestimated natural hazard, with potential impacts on air quality, ecosystem health, and volcanic risk management. Detecting and monitoring this process is challenging, especially in volcanoes lacking visible fumarolic activity, where the final stage of gas migration is jointly modulated by the shallow subsurface structure and natural pressure forcings. We present a continuous monitoring approach based on Rayleigh wave ellipticity from ambient seismic noise to track pressure-driven gas transport in the uppermost soil layers. Applied to Cumbre Vieja Volcano (La Palma, Canary Islands), the method detects subdaily seismic velocity variations linked to periodic pressure oscillations, consistent with natural forcings such as atmospheric and solid earth tides. In this work, we focus on the terdiurnal cycle, the only subdaily band at La Palma Island that reflects purely atmospheric pressure forcing. Its temporal evolution reveals long-term fluctuations that broadly coincide with reported CO₂ emission trends, despite differences in spatial and temporal resolution between both estimates. In addition, we identify a clear semi-annual cycle in the long-term ellipticity response at the terdiurnal frequency. This seasonality coincides with the modulation by solid Earth tides and confirms their role in controlling permeability in the upper vadose zone and in the enhancement of barometric pumping efficiency across all stations analyzed. Prior to the 2021 eruption, the terdiurnal cycle reveals contrasting station responses, with TBT exhibiting the earliest and most sustained increase. Together, these observations point to the usefulness of this approach as a complementary tool in the assessment of diffuse degassing and volcanic unrest. This seismic-noise-based approach offers a cost-effective and resilient strategy for detecting hazardous gas migration, supporting early warning and mitigation efforts in volcanic regions.