By applying a new imaging technique, the scientists reconstructed the deep structures and processes of the Campania volcano.
By analyzing the "noise" acquired by the seismic stations on the surface, we have come to a better interpretation of the volcanic processes affecting the Campi Flegrei.
This result was achieved using a new imaging technique developed by an international team of researchers from the Vesuvius Observatory of the National Institute of Geophysics and Volcanology (INGV-OV) and the Johannes Gutenberg University of Mainz (Germany). I study Fluid migrations and volcanic earthquakes from depolarized ambient noise has just been published in the journal 'Nature: Communications'.
“The deep fluids”, explains Simona Petrosino, INGV researcher, “they can induce earthquakes and to better understand their migration processes, the study team has developed a new method applied to Campi Flegrei. This technique made it possible to "follow" the fluids using different time intervals (from a few hours to years) of seismic noise recordings".
The researchers used the "noise" these processes cause on the noise generated by oceans and atmospheric activity, continuously recorded in volcanic environments.
"Sea and wind”, adds the researcher, “they constantly interact with the caldera, producing waves that plumb its depths. The structures of the caldera are subjected to strong lateral pressures caused by the extension of the crust, by the pressure of the magma at depth and by the complex interaction between the fluids produced by the magma, by the rains and by the superficial fractures of the volcano”.
“The Waves of Noise Penetrating the Caldera”, continues Petrosino, “they change direction as they are recorded above faults and above volcano feed systems. With our research we have shown that while the change of direction is important for reconstructing the volcano structures, the loss of any directionality is a signal of their activation. Indeed, the release of energy is followed by a migration of fluids that produce further sources of noise, corrupting our ability to reconstruct directionality. The lack of directionality itself allows us to trace the migrations before the fluids reach the surface".
The researchers analyzed noise data recorded over the last decade and observed a loss of directionality starting in 2018, when deep fluids reached the shallow hydrothermal system. These migrations, according to the authors, were the probable cause of the earthquakes that have hit the caldera since late 2019.
"We created a model of the noise that was recorded and mapped over time.", adds Professor Luca De Siena of the Johannes Gutenberg University of Mainz. “Thanks to the help of TeMaS, the consortium funded by the Ministry of Science and Health of the Rhine-Palatinate region to find areas with high research potential, a computerized model of the volcano has been developed within which we have propagated "waves of synthetic noise" generated in the middle of the Tyrrhenian Sea. This propagation model, combined with the enormous amount of knowledge accumulated by the international community, has allowed us to quantitatively interpret the loss of spatial directionality over time".
"The volcano", continues De Siena, “relieves stress through fluid migrations following paths that were opened during the intense activity in the years 1983-84. The migration of deep fluids, when combined with rainfall that makes the superficial part of the volcano more permeable, produces strong seismicity, such as that recorded in 2019-20. With our images over time we are able to see the progressive migration of fluids towards the eastern part of the caldera, the structure of which supports a large part of the volcanic stress and which acts as a barrier to further eastward migration of fluids."
"The changes in the time maps”, concludes the professor, “iillustrate stress build-up before earthquakes and its subsequent release, concomitant with further fluid migrations. This picture coincides with the shift of the volcano's activity towards the east, observed in recent decades.
A contribution that could be useful in the future to refine the forecasting and prevention tools of civil protection but which at the moment has no direct implication on measures concerning the safety of the population.
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Campi Flegrei: structure and deep processes at the caldera reconstructed using “ambient noise”
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 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 National Institute of Geophysics and Volcanology (INGV-OV, Italy) and Johannes Gutenberg University Mainz (JGU Mainz, Germany ). The study titled Fluid migrations and volcanic earthquakes from depolarized ambient noise, has just been published in the journal '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 disturbance that these deep processes produce on 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 in the last decade. They observed a directionality loss in 2018, when deep fluids 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.
"We created a model of the noise recorded and mapped at the volcano”, added Professor De Siena of JGU Mainz. “TeMaS, one of the High-potential Research Areas at JGU funded by the 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”, DeSiena 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 greatest 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, coinciding 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.
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Fig 1 - The Transfer Structure (panel a, dark colors and black dotted line) visible in years of low seismic activity (2009 and 2017) opens starting from 2018 (panel b) allowing deep fluids to penetrate the hydrothermal system of the caldera (light colors). These fluids migrated west and east of the transfer facility just before the 2019-2020 seismic sequence, which occurred in the eastern portion of the caldera (panel c). After the sequence, the fluids are visible to the east (panel d), where they intersect the stress-regulating extensional faults at the caldera.
Fig 1 - The transfer structure (panel a, dark color and black dotted line) visible in years of low seismic activity (2009 and 2017) opens up in 2018 (panel b), allowing deep fluids to enter the upper hydrothermal systems (bright colours). These fluids had migrated west and east of the transfer structure just before the 2019-20 seismic sequence, which stroke the eastern portion of the caldera (panel c). After the sequence, fluids are visible in the east (panel d), where they intersect the extensional faults that control stress at the caldera.
Fig 2 - The deep hydrothermal basin located in the central portion of the caldera and reconstructed in the period 2011-13 as a low seismic velocity anomaly (light colors, delimited by the white dashed curve). The study concludes that this basin expanded in the following years, increasing the lateral pressure and becoming a contributing cause of the 2019-20 seismic sequence.
Fig 2 - The hydrothermal reservoir located in the center of Campi Flegrei caldera and reconstructed in 2011-13 as a low seismic velocity anomaly (bright colours, contoured by a white dotted curve). The study concludes that this reservoir has expanded in the following years, increasing lateral stress and contributing to the 2019-2020 seismic sequence.
Fig 3 - From left to right: increase in noise polarization (dark colors) before the earthquake of 6 December 2019 in Campi Flegrei (located in correspondence with number 1 circled in white). The earthquake is followed by fluid migrations to extensional faults to the east. The same migrations are visible in the days following the second earthquake of April 26, 2020 (number 2).
Fig 3 - From left to right: increase in noise polarization (dark colours) before the December 6th, 2019 earthquake at Campi Flegrei (the location of the earthquake is shown by number 1 in the white circle). The earthquake is followed by fluid migrations over the eastern extensional faults. The same migrations are visible a few days after the second earthquake, on April 26th, 2020 (number 2).
