The increasing turbulence of the volcanic area of Campi Flegrei, near Naples, in the last two decades may be caused by pressure and heat accumulating in a gas system below it. The process is different from earthquakes caused by the movement of magma, and similar to tremors caused by fluid injection in fracking. The finding comes from an exhaustive analysis of a decade of data collected on site, published in the Journal of Volcanology and Geothermal Research1, by a team based in Italy and Switzerland.
Campi Flegrei is a 40,000-year-old volcanic caldera. Since 2004 it has experienced an increase in small earthquakes, which have caused the ground to lift by 0.75 m. It has also emitted increasing levels of carbon dioxide, up to 5,000 tonnes per day, according to an estimate in the articles. In 2012, the Italian Civil Protection raised the alert level for the area from ‘calm’ to ‘attention’.
Many different data are publicly available on the area, which is heavily monitored. The authors performed a thorough statistical analysis of observations carried out from 2010 to 2020 at Solfatara and Pisciarelli sites. The former is a 1-square-km-wide area with diffuse CO2 degassing, and the latter has the most active fumarolic vents.
First, they derived the temperature and pressure of the water and gas underground from the chemical composition of the gases expelled by fumaroles. They found that the ramping up of these parameters was strongly correlated in time with that of seismicity and CO2 emissions.
In order to understand what is going on, scientists used a picture of the underground structures obtained by a previous study using a technique called audio-magnetotelluric, that measures variations in the Earth’s natural electromagnetic fields. This way, they learned that the majority of the small earthquakes’ hypocentres were concentrated at the top of a plume stretching from 2 to 0.3 km below ground level.
The researchers think that this plume of steam and CO2 is receiving increasing heat and pressure from deeper magmatic fluids or gases. This energy is then transferred to the hosting rocks, causing them to expand and fracture, which results in low magnitude earthquakes. The pressure also explains the increased outflow of CO2.
“A single process is generating all these different behaviours and we believe it can only be the increasing total pressure of the system”, says Giovanni Chiodini, co-author of the paper, at the National Institute of Geophysics and Volcanology in Bologna.
According to Shaul Hurwitz, a volcano researcher at the U.S Geological Survey in Menlo Park who was not involved in this study, similar processes are likely to be happening also in other calderas that share some features with Campi Flegrei, such as Yellowstone and Long Valley in the United States.