Soil He and H2 degassing during the recent seismic crisis of São Jorge Island, Azores

São Jorge is one of the nine islands of the Azores Archipelago. On March 19, 2022, a seismic crisis began in the island, with more than 32,300 earthquakes recorded until May 2022, of which nearly 300 were felt by the population. On April 5, 2022, an INVOLCAN team travelled to the Azores and, in collaboration with CIVISA/IVAR of the Universidade dos Açores, carried out a soil gas and CO₂ diffuse efflux survey to monitor the activity in this volcanic island. The absence of visible volcanic gas emissions (fumaroles, hot springs, etc.) at the surface environment of Manadas volcanic system -the most recent volcanic system in São Jorge- makes this type of studies an essential tool for volcanic surveillance purposes.

Soil CO₂ efflux was measured in 400 observation points on the island following the accumulation chamber method using a non-dispersive infrared LICOR-830 CO₂ analyzer. Soil gas samples were taken at 40 cm depth to study the chemical composition of diffuse emanations. The chemical composition of the gas samples (He, Ne, H₂, CO₂, CH₄, N₂ and O₂) was analyzed daily on the island by means of a micro-gas chromatograph (micro-GC).

The results reported herein are related to the soil He and H₂ degassing. The former has exceptional characteristics as a geochemical tracer while the second is one of the most abundant trace species in volcanic systems and it is fundamental in many redox reactions occurring in the reservoir gas. Mapping He and H₂ soil effluxes may be useful to detect hidden tectonic structures. In addition, detection of significant changes in the diffuse emission of both gases in volcanically active areas, as well as changes in their spatial distribution, is usually linked to movements of magma in the subsoil and/or changes in seismic-volcanic activity. These variations may be excellent early warning signals of changes in the activity of the system.

Spatial distribution maps of the diffuse flow of He and H₂ in the 237.59 km² area of the island were constructed following the sequential Gaussian simulation (sGs) procedure to quantify the diffuse He and H₂ emission from the studied area. He fluxes varied from 0 to 3.7 mg·m^-2·d^-1 (mean value, 0.4 mg·m^-2·d^-1), while H₂ fluxes ranged between 0 and 4.7 mg·m^-2·d^-1 (mean value, 0.15 mg·m^-2·d^-1). He and H₂ diffuse emission was estimated in 101 and 28 kg·d^-1, respectively, with diffuse degassing values of 0.369 and 0.102 kg·km^-2·d^-1.

The principal H₂ flux anomaly is in the central part of the island and coincides with the ground deformation obtained by synthetic aperture radar data acquired by the ESA Sentinel-1 satellite at the beginning of the crisis. He highest fluxes are dispersed in different areas and most of the highest values are located in the Manadas volcanic system, where most of the seismicity focused. The spatial distribution maps of He and H₂ fluxes seem to show the existence of areas that could be acting as preferential zones of vertical permeability allowing the migration of deep source gases, which is very important to follow any seismic-volcanic crisis.