Campi Flegrei: structure and deep processes at the caldera reconstructed using "ambient noise"

Research team of the INGV Osservatorio Vesuviano and Mainz University analyzes seismic noise at the Campi Flegrei supervolcano over the last decade

17 November 2021


Analyzing the noise recorded at seismic stations deployed on the Earth's surface has helped researchers to come to a better understanding and interpretation of the volcanic processes affecting the Phlegraean Fields, or Campi Flegrei as this area is called in Italian. This result has been achieved using a new imaging technique developed by a team of international researchers of the Vesuvius Observatory, a department of the Istituto Nazionale di Geofisica e Vulcanologia (INGV-OV, Italy), and Johannes Gutenberg University Mainz (JGU) in Germany. The study titled "Fluid migrations and volcanic earthquakes from depolarized ambient noise" has been published recently in Nature Communications.

"Deep fluids can induce earthquakes. Thus, the research team set out to develop a new method to better understand the migration processes of these deep fluids at Campi Flegrei," explained INGV researcher Dr. Simona Petrosino. "This new technique allows 'following' the fluids, which are a combination of liquids and gases, using different times windows – from a few hours to years – of the recorded seismic noise."

The researchers used the disturbances that these deep processes produce on the noise generated at the bottom of the oceans or by atmospheric activity and which are constantly recorded by stations at the volcano surface to scan its interior.

"Sea and wind constantly interact with the caldera and produce waves that scan its depths," the researcher added. "The caldera structures have suffered intense lateral stress over the least 40 years, caused by the extension of the crust, the pressure of magma at depth, and the complex interaction between deep volcanic materials and rain within the volcano."

"Ambient noise waves enter the caldera with their direction changing above faults and magma feeding systems," Dr. Simona Petrosino continued. Our work shows that, while the change of direction is essential to detect structures, the loss of any directionality is a signal of activation. The energy release is followed by migrations of fluids that produce additional noise sources, hindering our ability to reconstruct directionality. Thus, the loss of directionality gives us a tool to track the migration of deep fluids before they reach the surface."

The researchers analyzed noise data recorded over the last decade. They observed a directionality loss in 2018, when deep fluids in fact reached the shallow hydrothermal systems. The researchers infer that these migrations were the likely trigger of the earthquakes that stroke the caldera at the end of 2019.

Migration of fluids towards the eastern caldera

"We created a model of the noise registered and mapped at the volcano," added Professor De Siena of JGU. "TeMaS, one of the High-potential Research Areas at JGU funded by the Rhineland-Palatinate Ministry of Science and Health, helped us create a computerized model of the volcano. We then simulated how the volcano responds to noise generated in the middle of the Tyrrhenian Sea. When combined with the massive amount of knowledge accumulated by the international community about the volcano, these models have allowed us to quantitatively interpret the spatial and temporal losses of directionality."

"The volcano releases its stress through migrations of fluids following paths opened during its intense activity in 1983/1984," De Siena continued. "These deep fluids combine with those from rainfalls, which make the shallow part of the volcano more permeable. This produces strong earthquakes, like those recorded at the volcano in 2019/2020. By observing directionality changes through time, we can now detect the progressive migration of fluids towards the eastern caldera, whose structure suffers the largest stress and which acts as a barrier for further migration toward East."

"The changes in the temporal images depict the increase in stress before the earthquake and its release, coincident with further fluid migrations to the east. These results explain the progressive shift of the volcanic activity towards the east observed in the last decades," the researcher concluded.

The results of this study allow for an improvement in the interpretation of volcanic processes by way of enhanced monitoring of deep fluids, even if at present it has no direct implication for measures that affect the safety of the local population.