The results of the seismic tomography of Etna obtained with the seismicity that occurred in the last 2 years show areas of accumulation of magma that can fuel the eruptive activity.
Through the application of a seismological technique called "4D Seismic Tomography" a team of researchers from the National Institute of Geophysics and Volcanology (INGV) has defined the structure of Etna, from the summit craters up to 10-12 km deep, discovering that below the central part of the volcano there are three areas that "slow down" the seismic waves, causing an increase in travel times. Scientists have interpreted these anomalies as high-temperature fractured areas containing a percentage of magma equal to 4% of the total volume, an amount that can fuel the eruptive activity for a long time.
These are the results of the study Re-pressurized magma at Mt. Etna, Italy, may feed eruptions for years just published in the journal Communications Earth & Environment and conducted with the seismicity of Etna which occurred between January 2019 and February 2021.
“The processes of formation of magma and those that induce its ascent from the crustal depths towards the surface have always been a topic of great scientific interest, as they are useful for understanding the mechanisms that determine the eruptions and evolution of volcanoesi”, explains Pasquale De Gori, INGV researcher and first author of the study. “The 4D seismic tomography technique”, continues De Gori, “is used very often to study both volcanic complexes and tectonically active areas. When an earthquake occurs, seismic energy - in the form of elastic waves - travels inside the volcanic structure and, crossing volumes of fractured crust containing magma and magmatic fluids, undergoes slowdowns which allow us to define where it is probable that the magma is contained. In this study we calculated the variation of seismic wave speeds in recent years to verify the presence of new magma and possibly the volumes involved, identifying a deep and rather extensive area, located between 4 and 9 km deep, and two other areas more superficial and smaller, close to the summit craters, where seismic waves are particularly slow".
Etna is a very active area from a seismic point of view. The INGV Etna Observatory (OE-INGV) carries out seismic monitoring 24 hours a day through a network of sensors which, between 2005 and 2021, recorded more than 11.000 earthquakes with a magnitude between 1.0 and 4.8 and with depths ranging from a few hundred meters to below the summit craters up to about 30 km depth on the western side of the volcano.
“The ability to use a very large earthquake dataset”, adds the researcher, “allowed us to determine the seismic wave velocity structure "P" of the volcano from 2005 to 2019, providing us with a long-term average picture. The structure obtained with the most recent dataset of the 2019-2021 period, on the other hand, allowed us to define the changes that have occurred in recent years. Considering the activity of Etna over the last two decades, various studies agree on the fact that the dynamics of the volcano has been mainly conditioned by intrusive processes, i.e. magma ascent, accompanied by the activation of faults on the south-eastern flank of the volcano and by instability of the eastern flank, which manifests itself with rapid sliding phenomena towards the Ionian Sea. These two processes are strongly connected as the intrusion triggers the sliding of the eastern flank which, in turn, can favor the eruptions themselves by depressurizing the central volcanic conduits”.
“From the studio”, concludes the researcher, "it emerged that the deep area in which the seismic waves are slow is located at the end of an area characterized, however, by a high speed of the waves, which represents the part of magma that has not erupted and is consolidated and which constitutes the trace of the old activity of Etna during its geological evolution. With this study we hypothesize that the magma coming from the deepest parts of the crust reaches this first accumulation zone and that the new magma creates a pressurization of the system, triggering most of the seismicity observed at Etna between 4 and 12 km of depth. From these depths we are witnessing magmatic ascents in the more superficial accumulation zones, evidenced by the increase in seismicity, which can feed eruptive phases as has happened in recent months".
Link: https://www.nature.com/articles/s43247-021-00282-9
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Seismic tomography defines the internal structure of Mt Etna volcano revealing volumes of magma accumulation at shallow depths
The results of the seismic tomography carried out at Mt Etna by using the seismicity occurred in the last 2 years show volumes of magma accumulation that could feed the eruptive activity.
Through the application of a seismological technique called "4D Seismic Tomography" a team of researchers from Istituto Nazionale di Geofisica e Volcanologia (INGV) studied the crustal structure of Mt. Etna, from the summit craters down to 10-12 km depth. Tomographic results revealed three volumes where seismic velocity is reduced, located in the central part of the volcano. These features are interpreted as volumes of high temperature fractured rocks where magma accumulated in the uppermost volcanic structure. From velocity reduction, scientists have estimated that these rock volumes contain a magma fraction of about 4 % which could feed eruptions for years in the future.
These are the main results of the study Re-pressurized magma at Mt. Etna, Italy, may feed eruptions for years just published in Communications Earth & Environment magazine. The research was conducted by using the Mt. Etna seismicity that occurred between January 2019 and February 2021.
" The study of mechanisms of magma formation and migration from the deep crust up to the surface is of great interest, to better understand the eruptive processes and the evolutions of volcanoes", explains Pasquale De Gori, INGV researcher and first author of the study. “The 4D seismic tomography”, continues De Gori, “is a technique widely used to study both volcanic complexes and tectonically active regions. Seismic waves radiated by an earthquake travel inside the volcanic structure and, crossing a fractured rock mass containing magma and magmatic fluids, are slowed down and recorded later at the seismic stations. Seismic tomography defines the geometrical extexts of such volumes of low velocity where the magma is likely to be contained. In this study we calculated the variation over time of seismic P-waves velocity in order to verify if, in the last few years, new magma has been emplaced in the shallow volcanic structure, and possibly the volumes involved. We identified 3 regions of low P-waves velocity that we interpreted as volumes where magma accumulated. The deepest and the largest region is located between 4 and 9 km depth. In addition, two smaller regions are found at shallower depths, close to the summit craters".
Mt. Etna is characterized by a widespread seismicity. The INGV Osservatorio Etneo (OE-INGV) carries out 24h seismic monitoring through a network consisting of 30 seismic stations. From 2005 to 2021 more than 11,000 earthquakes have been recorded, with magnitudes between 1.0 and 4.8 and with depths ranging from a few hundred meters below the summit craters down to 30 km depth on the western side of the volcano.
“A large seismic dataset,” adds the researcher, “allowed us to determine the P-wave velocity structure of the volcano from 2005 to 2019, providing us with a long-term average image of the volcanic structure. Furthermore, the tomographic model obtained with the most recent dataset (2019-2021) allowed us to define the velocity variations that occurred in recent years. Following the most recent literature, in the last two decades the Mt. Etna dynamics has been mainly controlled by intrusive processes, ie magma rising, by the activation of the faults on the south-eastern flank of the volcano and by the instability of the eastern flank, with rapid sliding episodes. These processes are strictly connected since intrusion triggers the sliding of the eastern flank which, in turn, can favor the eruptions by depressurizing the central volcanic conduits".
"The study shows", concludes the researcher, "that the deepest volume where seismic waves are slowed down is located on the edge of a volume characterized by high P-waves velocities, which represents the part of magma not erupted and now consolidated. It is the trace of the old activity of Etna during its geological evolution.
In this study we hypothesize that the magma coming from the deepest parts of the crust reaches the deepest region of low P-wave velocity. In this volume, the new magma creates a pressurization of the system, triggering most of the seismicity observed at Mt. Etna is between 4 and 12 km depth. Magma rises from the deepest to the shallower low Vp volumes as documented by the increase of shallow seismicity, possibly feeding eruptive episodes, as happened in recent months”.
Fig. a - Velocity distribution of P seismic waves at a depth of 6 km in the period 2005-2019 and in the period 2019-2021. The speed changes between the two periods are also reported. The background model is the 2005-2019 one which represents a "time independent" image of the volcano.
Fig. a - Distribution of P-waves velocity at 6 km depth in the period 2005-2019 and in the period 2019-2021. Velocity changes between the two periods are also reported. The background model is the 2005-2019 one which represents a “time independent” image of the volcano.
Fig. b - Vertical sections SN and WE showing the distribution of speed differences at depth. Seismicity is represented with colored dots based on the time of occurrence of the seismic events (grey=pre 2019, yellow 2019-2020, purple 2020-2021).
Fig b - SN and WE vertical sections showing P-waves velocity changes over depth. Seismicity is represented with dots color-coded for time of occurrence of seismic events (gray = pre 2019, yellow = 2019-2020, purple = 2020-2021).
Photos – The Etna volcano.
Picture - Mt Etna.