Scientists tracked escalating gaseous emissions from magma beneath a volcanic crater near Naples, Italy, and concluded that the related increases in pressure and temperature helped trigger the numerous low-magnitude earthquakes the area has experienced in recent years.

Previously, evidence for links between hydrothermal pressurization, carbon dioxide emissions and volcanic seismicity has been elusive, the team reported in its study of the Phlegraean Fields caldera, published March 30 in the Journal of Volcanology and Geothermal Research.

"Similar processes could affect many other volcanoes of the Earth," said Giovanni Chiodini, research director at the National Institute of Geophysics and Volcanology in Bologna, Italy, and first author of the paper. "Possibly also in other cases we can better understand the signals occurring during a volcanic unrest."

Previously, Chiodini and his colleagues reported a relationship between carbon dioxide emissions and seismic activity in the country's central Apennine Mountains. 

"The structure of Apennine is not a classical hydrothermal system, but we think it is a deep 15- to 20-kilometer zone where gases from the underlying mantle accumulate at high pressure, and high fluid pressure favors seismicity," Chiodini said.

For the new work, he and his team examined the Campi Flegrei, or Phlegraean Fields caldera, a 13 kilometer-wide depression formed by a massive eruption more than 35,000 years ago. The volcano last erupted in 1538, but more recent episodes of seismic activity and ground uplift called bradyseisms have raised concerns that another eruption is approaching. 

In the 1980s, a bradyseism raised the ground about 2 meters and caused thousands of earthquakes, leading to the evacuation of the city of Pozzuoli, Chiodini explained. Another period of inflation began in 2004, which has lifted the ground about 0.75 meters and prompted the Italian Civil Protection Department to raise the caldera's alert status from green, or "calm," to yellow, meaning "attention," in 2012.  

Chiodini's team focused on the caldera's most active zone, which hosts the Solfatara and Pisciarelli hydrothermal sites, where huge amounts of steam and gases released by magma are emitted from the ground. 

"In this work, we summarized in a unique picture all the observations which can be specifically associated with the Solfatara-Pisciarelli hydrothermal site," Chiodini said. 

He and his colleagues examined a range of observations gathered from 2010-20 that included the temperature and chemical composition of gases such as carbon dioxide and hydrogen sulfide sampled at vents called fumaroles, carbon dioxide degassing from the soil and seismic activity. Based on the composition of the gases being vented, the researchers were able to estimate the temperature and pressure conditions of the hydrothermal system feeding the fumaroles.

They found that changes in the seismicity and gas emissions followed strikingly similar patterns over time, and have sharply increased since 2018. At the same time, they calculated, the pressure and temperature of the gases and surrounding rock have increased. The researchers traced these changes to a "gas-dominated zone" around 1 kilometer deep, where many of the low-magnitude earthquakes the region has seen in recent years cluster. 

"We show hydrothermal pressurization is causing energy transfer from the fluids to the host rocks, ultimately triggering low magnitude earthquakes," Chiodini and his colleagues concluded in the paper.

During this process, steam from the magma interacts with hydrothermal aquifers and condenses beneath the surface, where it can find its way into and escape from pre-existing fractures. Meanwhile, the surrounding rock gets hotter and hotter, expands and finally cracks, creating new fractures. The pressure increase itself also causes seismic unrest, Chiodini says.

The findings provide evidence that magmatic gases play an important role in driving volcano seismicity at the caldera. The process that triggers these episodes is similar to how human activities such as hydraulic fracturing can induce earthquakes, Chiodini says. 

It's likely that the phenomena he and his team documented at the Phlegraean Fields also occur at calderas elsewhere in the world, but they may be harder to track in other locations. Detailed data are not available for many volcanic areas, Chiodini says, aside from a few exceptions, such as Yellowstone National Park. 

"In particular, the systematic and [periodic] sampling of the fumaroles is very rare," he said. In the future, he says, more research will be needed on the gases that escape from fumaroles. 

Only weak earthquakes are currently observed around the caldera, which is probably related to the fact that the heating and building pressure are happening at relatively shallow depths, Chiodini says. In other situations, where these processes occur at greater depths, it's likely that more severe earthquakes could be triggered. 

Although the findings don't have direct implications for developing techniques to predict earthquakes, Chiodini says that understanding how pressure and temperature changes drive seismicity will be key to future efforts in this direction. 

The study, "Hydrothermal pressure-temperature control on CO2 emissions and seismicity at Campi Flegrei (Italy)," published March 30 in Journal of Volcanology and Geothermal Research, was authored by G. Chiodini and J. Selva, Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Bologna; C. Cardellini, Università degli Studi di Perugia and Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Bologna; S. Caliro, R. Avino, F. Giudicepietro, W. De Cesare, P. Ricciolino and Z. Petrillo, Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Napoli Osservatorio Vesuviano; G. Bini, ETH Zürich; A. Aiuppa, Università degli Studi di Palermo; and A. Siniscalchi and S. Tripaldi, Università degli Studi di Bari, Aldo Moro.