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Article

Integrated Monitoring of Soil Radon Gas and Seismic Activity to Detect Volcanic Unrest at Mount Etna (Italy), 2023–2025

by
Salvatore Giammanco
,
Vincenza Maiolino
,
Andrea Ursino
*,
Marco Neri
,
Luca Frasca
,
Salvatore Roberto Maugeri
,
Filippo Murè
and
Paolo Principato
Istituto Nazionale di Geofisica e Vulcanologia, Sezione Osservatorio Etneo, Piazza Roma, 2, 95125 Catania, Italy
*
Author to whom correspondence should be addressed.
Quaternary 2026, 9(1), 16; https://doi.org/10.3390/quat9010016
Submission received: 13 November 2025 / Revised: 12 January 2026 / Accepted: 4 February 2026 / Published: 10 February 2026
(This article belongs to the Special Issue Volcanic, Hydrothermal, and Tectonic Outgassing: Physical, Chemical and Isotopic Signatures)
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Abstract

This work presents the results of an integrated monitoring of soil radon gas and seismic activity at Mt. Etna from August 2023 to May 2025, aimed at enhancing comprehension of magma migration and eruption dynamics. Radon data were collected using a permanent station with an alpha particle probe, aggregated hourly. The INGV-OE network monitored seismic activity at 100 Hz; volcanic tremor was analyzed using Root-Mean-Square (RMS) values from the Serra La Nave station. Earthquakes were located using the Hypoellipse algorithm and a 1D crustal velocity model. A robust correlation was found between radon and RMS anomalies, with the former preceding the latter with increasing probability over time (e.g., 30.1% within 1 day, 46.4% within 3 days). Correlations were also found between radon anomalies and Strombolian activity at the summit craters (e.g., 23.8% within 1 day for the Central Crater), suggesting a potential predictive role for radon. Conversely, correlations with paroxysmal events were weaker in the short term but increased over longer time windows. No clear correlation was found between radon anomalies and seismic strain release, likely due to differing temporal resolutions. These results support the idea that radon plays a role as a short-term precursor in volcanic unrest.

1. Introduction

Monitoring volcanic activity has always required constant improvements in technical ability, from designing new, powerful instruments to obtaining signals with sufficient precision and a high enough sampling frequency to enable effective, reliable correlation with volcanic activity evolution. The correlation between different types of monitoring signals, especially when they come from parameters that carry complementary information, has proven to be very effective for their mutual interpretation and thus for the development of a more accurate and realistic model that explains the behavior of an active volcano.
Mt. Etna volcano (Sicily, Italy) is an excellent natural laboratory for the development of new and/or improved volcano monitoring techniques, due to its high dynamics in terms of the high number of eruptions per year, strong volcano-tectonic activity, and significant release of volcanic gases. Since the early 2000s, the Istituto Nazionale di Geofisica e Vulcanologia-Sezione Osservatorio Etneo (henceforth indicated as INGV-OE), located at the SE foot of Mt. Etna, has deployed an ever-increasing number of monitoring stations to measure a wide range of seismic, geophysical and geochemical parameters. This has led to a remarkable ability to predict volcanic eruptions, especially major summit eruptions defined as paroxysms [1], which release large amounts of volcanic ash into the atmosphere. In more recent years, a great effort has been made to combine seismic parameters such as the rate of earthquake occurrence or the volcanic tremor amplitude (Root-Mean-Square; RMS), whose strong increase is generally related to shallow and fast magma dynamics within the main conduits of the volcano that often precede eruptions [2,3], with geochemical parameters acquired with a high frequency, such as soil gas radon emissions from faults along the flanks of the volcano [4,5,6,7,8]. The temporal changes in soil radon signals at Mt. Etna showed both short-term and mid-term anomalies that have been associated with changes in the level of volcanic activity, either directly or indirectly [4,5,6,7,8,9]. Soil radon emissions at active volcanoes are strongly influenced by processes associated with magma ascent and degassing. Increased magmatic gas flux, stress-induced microfracturing, and transient changes in the permeability of the shallow volcanic edifice can enhance the advective transport of radon-rich carrier gases toward the surface. These mechanisms have been well-documented at Mt. Etna and other volcanoes [5,6,7,8,10,11]. Radon anomalies often reflect shallow rock fracturing, increased pore pressure, or enhanced gas circulation. These phenomena have been linked to magma movement within the volcano, which could result in eruptive activity [5,7,8,12].
In this work, we present findings from the integrated monitoring of soil radon and seismic activity—including both volcanic tremor and strain release—at Mt. Etna. The monitoring period extended from 15 August 2023 to 15 May 2025. These data were analyzed in relation to Mt. Etna’s volcanic activity during the same period, and their interpretation provided a more comprehensive understanding of the mechanisms governing magma migration and release within the volcano (Figure 1).

2. Materials and Methods

2.1. Radon Monitoring

Soil gas radon measurements were performed using a permanent 222Rn monitoring station that is part of the geochemical monitoring network of the INGV-OE and it is located on the upper south side of Mt. Etna at an elevation of about 2000 m above sea level (a.s.l.), near a mountain resort called Rifugio Sapienza (site ERN9 in Figure 1). The station is provided with a radon probe (model Barasol BMC2, AlGaDe, Bessines-sur-Gartempe, France) that consists of a cylindrical chamber (l = 570 mm; ∅ = 60 mm) equipped with an implanted silicon junction specifically designed for alpha particle detection. A built-in microprocessor integrates and stores the radon data into a data logger, with an integration time of 15 min. The station also measures bottom hole air temperature and barometric pressure every 15 min. Differently from the other stations of the radon monitoring network of Mt. Etna, whose setup follows that described in [5,6,14] with the sensor placed at the bottom of a drill hole at a depth of about 1.5 m in a PVC tube closed with PVC stoppers on the top, the radon probe of ERN9 site is placed about 2 m below ground level at the bottom of a relatively open interspace between a large underground water tank and the concrete walls that surround it (Figure 2). The acquisition period was initially configured at 15 min but adjusted to one hour to conserve power and eliminate minor high-frequency radon fluctuations caused solely by background noise. During the first year of acquisition, radon data was downloaded locally to a laptop through a direct serial port connection with the radon probe. Later on, the data were transmitted to the INGV-OE headquarters in Catania every 12 h using a UMTS modem/router equipped with a SIM card. Data transmission from the station to the receiver occurs via an omnidirectional antenna.
Although data from a single radon station may not be representative of soil degassing across the entire volcanic edifice, ERN9 was the only station selected for this study because of its strategic location on the upper southern flank of Mt. Etna. This location makes ERN9 particularly suitable for detecting short-term variations associated with volcanic unrest. ERN9 is closest to the summit craters and main feeder conduits, where rapid changes in magmatic degassing and stress conditions typically occur. Additionally, ERN9 is the only site with a sensor setup that facilitates the detection of radon emission anomalies due to its high signal-to-noise ratio.

2.2. Seismic Monitoring

Mt. Etna seismicity is monitored by the INGV-OE permanent seismic network, which currently consists of about 30 stations deployed around the volcano equipped with broadband three-component Trillium 40s seismometers (Nanometrics™, Kanata, ON, Canada) and recording at a sampling rate of 100 Hz (Figure 1). Seismic signals are transmitted in real-time to the INGV-OE headquarters in Catania, where they are visualized, analyzed and stored in a database.

3. Eruptive Activity

During the study period, Mt. Etna’s volcanic activity was characterized exclusively by summit eruptions from three out of four of its summit craters, namely Voragine (VOR) and Bocca Nuova (BN)—altogether indicated as Central Crater or CC—and Southeast (SEC) and Northeast (NEC) craters (indicated in box a in Figure 1; [15] and references therein). Table 1 shows the list of eruptions during the study period. The intensity of the eruptions ranged from mild Strombolian activity to strong sub-Plinian events (also known as “paroxysms”). Paroxysms are normally short-lived and, in the case of Etna, generally persist for only a few hours [1,16,17]. Lava flows were frequently emitted during eruptive episodes from active craters, either by overflowing the crater rims or via fissures that opened in nearby areas [18]. However, these flows did not impact populated or cultivated zones, owing to the considerable spatial separation between the summit eruptive centers and the inhabited portions of the volcanic edifices.
In detail, the first eruptive activity during the study period was a Strombolian eruption that started on 9 October 2023 at the SEC. It culminated in a paroxysm on 12 November 2023, after which no activity occurred for a week. Strombolian activity resumed at the SEC on 19 November 2023, accompanied by intermittent and weak effusions of lava. As for the previous eruptive episode, this too culminated in a paroxysm on the following 1st December. No significant activity was observed until 10 June 2024, when a Strombolian eruption started at the VOR, after almost three years of total quiescence of this crater. The intensity of this eruption increased progressively with the days, and on 4 July it intensified dramatically until a new paroxysm started, with an impressive lava fountain that lasted about 9 h. After this episode, five more paroxysms occurred at the VOR (6–7, 15, 22–23 July; 4 and 14–15 August). All of those paroxysms at the VOR lasted several hours and were violent. They were all preceded by Strombolian activity that lasted many hours, and they were also characterized by emission of lava flows that, after invading and filling the nearby Bocca Nuova (BN) crater (Figure 1a), overflowed towards the W and the WSW. The accumulation of explosive products during those paroxysms completed the burying of the BN and in part filled the NEC. It also contributed to the formation of a large and tall cone around the VOR crater, whose highest peak became the highest point of Mt. Etna, reaching an altitude of 3403 m asl, a record never reached in at least 2000 years of the volcano’s documented history [19].
Simultaneous to the VOR’s explosive events, the Northeast Crater reactivated on July 14, initiating a series of isolated explosions. This was followed by sporadic Strombolian activity continuing through October, with a major interruption between mid-August and the latter half of September. Furthermore, during August, two isolated explosions occurred at the BN. These events effectively re-established the crater’s outgassing pathways, as they cleared part of the lava cover on this crater produced from the VOR eruptions.
The last relevant activity in 2024 is represented by an isolated paroxysm at the VOR-BN (the eruptive vent was actually located in between the two craters) on the morning of 10 November 2024. Following this eruptive episode, no new activity was reported on Etna until 6 February 2025, when Strombolian activity resumed at the SEC. The intensity of this activity increased over the following two days. On 8 February, a series of fractures opened at the southern foot of the BN cone, resulting in a well-fed lava flow being emitted over the southwestern slopes of the volcano. The combined effusive and Strombolian activity continued, though with some short pauses, until 2 March. After the conclusion of this eruption, a sequence of short-lived Strombolian eruptions started on 15 March at the SEC, often with the emission of small lava flows.
The total volume of lava and tephra erupted during the whole period was about 13.3 × 106 m3 [19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46].

4. Data Analysis

The overview of all the acquired signals over time during the considered period, along with a graphical summary of the concurrent eruptive events that occurred at Mt. Etna, is shown in Figure 3.

4.1. Radon Activity

Soil radon activity data was acquired with consistent continuity during the studied period (Figure 3). Table 2 shows some basic descriptive statistics for the radon data. Radon activity values showed considerable variability, with a range from 0 Bq m−3 (which in reality corresponds to the detection limit of the sensor: 100 Bq m−3) to an impressive 2.4 × 109 Bq m−3, a large standard deviation and an even larger sample variance. Although the highest values may seem unrealistic, they are compatible with the upper limit of the values declared by the manufacturer of the sensor (up to >1 × 109 Bq m−3). In addition, similar very high radon levels were measured and reported on Mt. Etna during previous monitoring of low-temperature fumarole emissions near the volcano’s summit [5]. The very high positive value of skewness, together with the much higher mean value compared to the median and mode, clearly indicates that the radon data follow a strong log-normal probability distribution.
Soil radon signals in general are known to be strongly affected by many environmental parameters, such as air temperature, barometric pressure, rainfall, tidal waves produced from lunar and solar attraction, etc. [47,48,49,50,51]. However, these influences usually impact background levels of radon emissions, particularly in active geodynamic regions. Conversely, high radon values representing anomalous emissions are not significantly affected by environmental factors; rather, they are correlated with volcanic or tectonic parameters [6,51].
In order to determine the anomaly threshold for soil radon activity measured at the ERN9 site, we applied a log probability plot [52] to all 47,434 radon values measured during the monitoring period (Figure 4). The plot shows an evident inflection point at the 97th percentile and two minor ones, at percentiles 17.7 and 99.8, respectively, thus indicating four distinct populations of data. The mean values of radon activity for the four populations are, respectively, 78.5, 911.3, 1.05 × 107 and 7.5 × 108 Bq m−3. The marked inflection point at the 97th percentile, corresponding to a radon value of 2986 Bq m−3, defines an important change in the behavior of radon emissions and can, therefore, be interpreted as the transition between background and anomalous soil 222Rn emissions at the monitoring site. The two populations with the highest radon values, i.e., those exceeding the threshold set at 2986 Bq m−3, are therefore considered to represent anomalous soil radon values.

4.2. Seismic Data

In order to provide a complete picture of the seismic activity, we analyzed the temporal evolution of both volcanic tremor and volcano-tectonic (VT) earthquakes during the period from 15 August 2023 to 15 May 2025.
To follow volcanic tremor amplitude changes, we calculate the Root-Mean-Square (RMS) of the seismic signal recorded by the vertical component of the Serra La Nave seismic station (ESLN), located on the upper southern flank of the volcano at 1778 m a.s.l. (Figure 1). The RMS time series is calculated on 15 min-long time windows of seismic signal filtered in the frequency band 0.5–5.5 Hz [53], which includes most of the volcanic tremor energy (e.g., [3]). This frequency band maximizes the signal-to-noise ratio for detecting changes in tremor amplitude related to shallow magma dynamics, which is the parameter we aim to correlate with radon anomalies and eruptive activity. The temporal evolution of volcanic tremor amplitude is strongly correlated with the intensity of explosive activity [1]. In fact, the volcanic tremor amplitude gradually increases during the activity ramp-up that leads to Strombolian and/or paroxysmal eruptions. In the latter case in particular, volcanic tremor peaks during the climax of the eruption and then decreases rapidly, returning to normal levels at the end of paroxysmal eruptive episodes ([2,3]; Figure 3). As in the case of radon values, we applied a log probability plot [52] to all RMS values measured during the monitoring period (Figure 5). The plot highlights three inflection points that identify four separate statistical populations of data. The most marked inflection point occurs at the 93rd percentile, which corresponds to an RMS log10 value of about 0.398, equivalent to 2.5 mV. Therefore, all RMS values below this threshold can be reasonably assumed to be representative of normal background levels of tremor amplitude.
The daily rate of VT and their associated seismic strain release are shown in Figure 3. Earthquakes were located using the Hypoellipse algorithm [54] and the 1D crustal velocity model proposed for the Etna area by [55], later modified by [56]. The seismic energy (E) in joules (J) is computed using the relationship logE = 9.9 +1.9M − 0.024M2 [57]. The epicentral map of the 992 VT earthquakes with ML ≥ 1.0 ERH = ERZ ≤ 6.0 km (error of the epicentral and vertical coordinates in km, respectively), Nl ≥ 7 (number of P-S readings) is shown in Figure 1 [58].
Seismicity in the Mt. Etna area during this period (Figure 6) was quite modest and mainly concentrated on the eastern and southern flanks of the volcano. The most energetic event (ML = 3.7) occurred along the Pernicana Fault System in February 2025. The pattern of cumulative strain release during the study period is shown in Figure 3, where some steps are evident due to small seismic swarms located in the northern and western sectors of the volcano and characterized by moderate magnitude. In Table 3 we reported the main information about the seismic swarms shown in Figure 6. We also show the focal mechanisms of the earthquakes with ML ≥ 2.7, characterized by left strike-slip focal mechanisms along the Pernicana Fault System, while the western and eastern sectors show dip-slip and strike-slip solutions (Figure 6; [59]).
Figure 3 provides a visual representation of the potential temporal correlations between the parameters examined in this study and volcanic activity. However, the next chapter will address the statistical and mathematical treatment of the data, which was carried out to achieve a more robust cross-correlation between the parameters and a meaningful probabilistic analysis of anomaly occurrences.

5. Discussion

The data presented in this paper provide a comprehensive overview of Mt. Etna’s volcanic activity, radon signals in the soil, and seismic parameters, enabling an integrated analysis to better understand the mechanisms of volcanic unrest. However, given the single-station configuration and the shallow sensitivity of soil radon, it is not possible to statistically resolve the contribution of individual faults or fault depths to the observed radon anomalies. Radon variations at ERN9 should therefore be interpreted as the integrated response, over a large volume of rock, of the shallow ground permeability field to changes in magmatic degassing and stress conditions. A previous study on radon monitoring at Mt. Etna [6] determined that the maximum possible depth for radon sources is in the range of 1200 to 1400 m below the surface, based on typical gas velocities and radon emanation in the local rocks. The correlation analysis between radon anomalies and seismic parameters revealed distinct patterns. Specifically, the anomalous values of soil radon activity, that is, all those higher than 2986 Bq m−3 as defined from the Normal Probability Plot of Figure 4 (totaling 1040), were temporally correlated to the anomalous values of RMS (i.e., >2.5 mV, as defined from the Normal Probability Plot of Figure 5) and to the occurrence of any of the eruptive types listed in Table 1. We calculated the probability that each radon anomaly could be followed by either an anomaly in RMS or an eruptive event of any type in the following 24 h, 48 h and 72 h, using the following:
P(1, 2, 3) (%) = [n. of events (RMS values > 2.5; eruptions) within 1, 2 or 3 days/Total n. of radon values > 2986] × 100
where P(1, 2, 3) is the probability of occurrence of an RMS anomaly or an eruptive event within one, two or three days following any radon anomaly.
The results show that radon anomalies precede RMS anomalies with increasing probability over longer time intervals. Specifically, Table 4 indicates that 30.1% of radon anomalies precede RMS anomalies within 1 day, rising to 38.1% within 2 days and 46.4% within 3 days. This trend suggests a predictive relationship between changes in radon emissions and the intensification of volcanic tremor, a key indicator of shallow magmatic activity. The nature of this correlation, with a time lag, is consistent with the interpretation that increased magmatic degassing and rock microfracturing, which affect radon emissions, may precede the actual ascent of magma responsible for generating tremor. Similarly, radon anomalies show a correlation with eruptive events, although with probabilities that vary depending on the type of activity.
For Strombolian activity, which often precedes paroxysmal episodes, radon anomalies show a significant correlation. For example, radon anomalies precede Strombolian activity at the Central Crater (CC) in 23.8% of cases within 1 day, increasing to 31.4% within 3 days. Similar behavior is observed for Strombolian activity at the Southeast Crater (SEC) and Northeast Crater (NEC), although with slightly lower probabilities. This reinforces the idea that radon may serve as a precursor to the onset or intensification of eruptive activity, reflecting changes in the conditions of the magmatic system prior to visible eruption.
However, the correlation between radon anomalies and paroxysmal events is less pronounced over short time intervals but becomes more evident, though still one order of magnitude lower than for Strombolian activity, as the time window lengthens. For example, radon anomalies preceding CC paroxysms are only 1.5% within 1–2 days but rise to 4.5% within 3 days. For SEC paroxysms, no correlation is observed within 1 day, but it rises to 7.8% within 2 days and 10.8% within 3 days. This difference could indicate that paroxysms, being more rapid and intense events, are preceded by a more complex period of instability that is reflected less directly and immediately in radon emissions than in other parameters. It is possible that the initial stages of preparation for a paroxysm are more subtle and require correlation analysis over longer time scales and/or with the inclusion of other geophysical and geochemical parameters to be fully captured. In addition, paroxysms at Mt. Etna seem to be triggered by a gas-rich magma stored in shallow reservoirs. According to [17], paroxysms occurring at the SEC during a long-standing sequence of episodes from 2020 to 2022 were fed by magma that was stored at depths ranging from 1.8 km above sea level to −1 km below sea level. This range of depth corresponds to the altitude of the ERN9 site. Therefore, given the mechanisms of radon transport and release through Mt. Etna’s volcanic edifice as explained above, radon emissions caused by volcanic factors associated with the preparatory phases of an impending paroxysm are likely to be better observed in areas at higher altitudes than that of our radon station. Contrary to the correlations observed with RMS and volcanic activity, the data do not show a clear correlation between VT seismicity (in terms of cumulative strain release) and radon anomalies. This behavior can be attributed to the difference in the time windows of these parameters: the radon anomaly period is measured in hours, while strain release is calculated on a daily basis. This mismatch in timescales could mask finer correlations that might emerge with a more consistent temporal resolution between the two datasets. However, from previous studies we noticed that radon variations can have rapid changes that would be lost if integrated on a daily scale.
Furthermore, the dataset considered covers a limited time interval and the entire volcanic edifice; this type of correlation might be more effective when applied to specific sectors of the volcano and with a much longer dataset. Another factor to consider is the position of the radon probe relative to the distribution of seismicity. The station’s elevation (approximately 1800 m above sea level) and its distance from the main areas of epicenters and hypocenters of the recorded earthquakes may attenuate, or even entirely suppress, its ability to detect anomalies associated with co-seismic shaking and fracturing. This is particularly the case for seismic events located more than 10–15 km away from the radon station.
The findings of this study align with and expand on previous research on Mt. Etna. Studies such as those of [4,5,6,7,8,51,60,61,62,63] have already highlighted radon anomalies linked to rock microfracturing and fluctuations in soil gas pressure due to stronger magmatic degassing. The present work adds a level of robustness to these observations by quantitatively assessing the probabilities of temporal correlation with other monitoring parameters. Figure 7 summarizes and illustrates the main results of this study.
Looking ahead, the refinement of correlation methodologies, especially for seismicity and radon, could include the application of advanced analytical techniques that account for different time scales and possible nonlinear relationships. The integration of these datasets with other geophysical and geochemical parameters, along with the expansion of the monitoring period, could further enhance the predictive capacity and deepen our understanding of Etna’s underlying volcanic processes.

6. Conclusions

This study presents an integrated analysis of soil radon emissions, seismic parameters, and eruptive activity at Mt. Etna, offering new insights into the temporal dynamics of volcanic unrest. By correlating radon anomalies with RMS values and eruptive events, the research highlights the potential of radon as a short-term precursor for shallow magmatic processes.
Radon anomalies—defined as values exceeding 2986 Bq m−3—show a statistically significant temporal correlation with RMS anomalies above 2.5 mV. The probability of this correlation increases with time: 30.1% within 24 h, 38.1% within 48 h, and 46.4% within 72 h. This trend supports the hypothesis that radon emissions, influenced by magmatic degassing and rock microfracturing, may precede the ascent of magma and the onset of volcanic tremor.
The analysis also reveals meaningful correlations between radon anomalies and Strombolian activity, particularly at the Central Crater (CC), with probabilities rising from 23.8% within 1 day to 31.4% within 3 days. Similar patterns are observed at the Southeast (SEC) and Northeast (NEC) craters, albeit with slightly lower values. These findings reinforce the role of radon as a geochemical precursor to eruptive behavior, especially for activity involving sustained gas release.
In contrast, correlations with paroxysmal events are less immediate but become more evident over longer time windows. For example, radon anomalies precede SEC paroxysms in 10.8% of cases within 3 days. This suggests that paroxysms may be preceded by more complex and subtle preparatory phases, requiring longer observation periods and the integration of additional geophysical and geochemical parameters to be fully understood.
No clear correlation was found between radon anomalies and seismicity, in terms of cumulative strain release, likely due to mismatched temporal resolutions—hourly for radon, daily for strain—and the broad spatial scale of the dataset. Future studies focusing on specific sectors and longer time series may help clarify these relationships.
Overall, the findings align with previous research and add quantitative support for earlier observations. Previous research at Mt. Etna has shown that soil radon anomalies often precede volcanic tremor variations, shallow magma migration, and changes in eruptive behavior. This highlights the importance of integrating geochemical and geophysical parameters in eruption forecasting studies.
Refining correlation methodologies by incorporating nonlinear models and expanding the monitoring network will be essential for improving our forecasting capabilities and for deepening our understanding of Etna’s magmatic system.

Author Contributions

Conceptualization, S.G., M.N., A.U. and V.M.; methodology, S.G., A.U., S.R.M., F.M. and P.P.; validation, S.G., M.N., V.M. and A.U.; investigation, S.G., V.M., A.U., M.N. and L.F.; resources, S.G. and M.N.; data curation, S.G., V.M., A.U., L.F. and P.P.; writing—original draft preparation, S.G., M.N., V.M., A.U. and L.F.; writing—review and editing, S.G., M.N., V.M., A.U. and L.F.; visualization, S.G., M.N., V.M. and A.U.; supervision, S.G.; funding acquisition, M.N., S.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research was partly funded by the European Union-European Social Fund, project European Plate Observing System-Geoscience Research INfracstructure of Italy, grant number PIR01_00013.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Acknowledgments

We thank A. Bruno, owner of the chalet where we installed the radon sensor used for this study, for giving us the permission to work there and for his logistic support. We are also indebted to the technicians of the INGV-OE seismic network staff for enabling the acquisition of seismic data. Lastly, we thank three anonymous reviewers for their useful comments and suggestions that helped improve the manuscript.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

Abbreviations

The following abbreviations are used in this manuscript:
INGV-OEIstituto Nazionale di Geofisica e Vulcanologia-Sezione Osservatorio Etneo
RMSRoot-Mean-Square
VTVolcano-tectonic
VORVoragine Crater
BNBocca Nuova Crater
CCCentral Crater (VOR + BN)
NECNorth East Crater
SECSouth East Crater

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Figure 1. Volcano-tectonic map of Mount Etna, showing the location of radon (blue dots; the radon station used for this article is identified by the code ERN9) and seismic stations (red triangles; ESLN station in pink) belonging to the INGV-OE monitoring networks. The yellow dots indicate the distribution of earthquake epicenters from 15 August 2023 to 15 May 2025; the magnitude scale is shown in the legend. The box (a) highlights the summit area (from [13], modified) and the volcanic products erupted during the same period (maps of individual flows were extracted from weekly multiparametric bulletins published by INGV (https://www.ct.ingv.it). VOR = Voragine; NEC = North-East Crater; BN = Bocca Nuova; SEC = South-East Crater. Regional tectonic context is shown in inset (b); AMC = Appennine-Maghrebian Chain; CGF = Gela-Catania foredeep; HF = Hyblean foreland; ME = Malta Escarpment; CoF = Compressional Front; EF = Extensional Front. Active faults are shown by black lines; the short dashes indicate the downthrown side, and the arrows indicate horizontal movements where present. VdB = Valle del Bove; PFS = Pernicana Fault System; RFS = Ragalna Fault System; TFS = Timpe Fault System; FF = Fiandaca Fault; TF = Trecastagni Fault; ATF = Aci Trezza Fault.
Figure 1. Volcano-tectonic map of Mount Etna, showing the location of radon (blue dots; the radon station used for this article is identified by the code ERN9) and seismic stations (red triangles; ESLN station in pink) belonging to the INGV-OE monitoring networks. The yellow dots indicate the distribution of earthquake epicenters from 15 August 2023 to 15 May 2025; the magnitude scale is shown in the legend. The box (a) highlights the summit area (from [13], modified) and the volcanic products erupted during the same period (maps of individual flows were extracted from weekly multiparametric bulletins published by INGV (https://www.ct.ingv.it). VOR = Voragine; NEC = North-East Crater; BN = Bocca Nuova; SEC = South-East Crater. Regional tectonic context is shown in inset (b); AMC = Appennine-Maghrebian Chain; CGF = Gela-Catania foredeep; HF = Hyblean foreland; ME = Malta Escarpment; CoF = Compressional Front; EF = Extensional Front. Active faults are shown by black lines; the short dashes indicate the downthrown side, and the arrows indicate horizontal movements where present. VdB = Valle del Bove; PFS = Pernicana Fault System; RFS = Ragalna Fault System; TFS = Timpe Fault System; FF = Fiandaca Fault; TF = Trecastagni Fault; ATF = Aci Trezza Fault.
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Figure 2. Field setup of the radon monitoring system used at site ERN9.
Figure 2. Field setup of the radon monitoring system used at site ERN9.
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Figure 3. Time series of soil radon activity (gray line), RMS (red line) and cumulative strain release (blue line) during the monitored period. The colored lines at the top of the graph show the concurrent eruptive events, divided into paroxysms at the SEC (orange dots), paroxysms at the CC (light blue dots), Strombolian activity at the CC (yellow dots and lines), Strombolian activity at the SEC (dark blue dots and lines), Strombolian activity at the NEC (gray dots and lines) and lava effusions (green dots and lines). Arrows with circled numbers mark the steps corresponding to the main seismic swarms described in Figure 6.
Figure 3. Time series of soil radon activity (gray line), RMS (red line) and cumulative strain release (blue line) during the monitored period. The colored lines at the top of the graph show the concurrent eruptive events, divided into paroxysms at the SEC (orange dots), paroxysms at the CC (light blue dots), Strombolian activity at the CC (yellow dots and lines), Strombolian activity at the SEC (dark blue dots and lines), Strombolian activity at the NEC (gray dots and lines) and lava effusions (green dots and lines). Arrows with circled numbers mark the steps corresponding to the main seismic swarms described in Figure 6.
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Figure 4. Normal probability plot of the radon data acquired at site ERN9 (all values expressed as log10 of radon activity in Bq/m3). The original data are shown as black circles; the background values are represented by green and purple circles, whereas the anomalous values are represented by orange and red circles (see text for details). Dashed lines represent the expected theoretical values assuming a normal distribution of the respective populations of data.
Figure 4. Normal probability plot of the radon data acquired at site ERN9 (all values expressed as log10 of radon activity in Bq/m3). The original data are shown as black circles; the background values are represented by green and purple circles, whereas the anomalous values are represented by orange and red circles (see text for details). Dashed lines represent the expected theoretical values assuming a normal distribution of the respective populations of data.
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Figure 5. Normal probability plot of the RMS data acquired at station ESLN (all values expressed as log10 mV). The original data are shown as black circles; the background values are represented by green circles, whereas the anomalous values are represented by yellow, orange and red circles (see text for details). Dashed lines represent the expected theoretical values assuming a normal distribution of the respective populations of data.
Figure 5. Normal probability plot of the RMS data acquired at station ESLN (all values expressed as log10 mV). The original data are shown as black circles; the background values are represented by green circles, whereas the anomalous values are represented by yellow, orange and red circles (see text for details). Dashed lines represent the expected theoretical values assuming a normal distribution of the respective populations of data.
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Figure 6. Epicentral map of earthquakes at Mt. Etna during the monitoring period; the colored circles refer to the areas affected by the seismic swarms indicated in the cumulative strain release curve of Figure 3 (circled numbers). The focal solutions of the earthquakes shown, with ML ≥ 2.7, are reported in [59].
Figure 6. Epicentral map of earthquakes at Mt. Etna during the monitoring period; the colored circles refer to the areas affected by the seismic swarms indicated in the cumulative strain release curve of Figure 3 (circled numbers). The focal solutions of the earthquakes shown, with ML ≥ 2.7, are reported in [59].
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Figure 7. Flow diagram that illustrates how soil radon anomalies are linked with eruptive events and/or with RMS anomalies at Mt. Etna, and how this leads to the interpretation of the observed correlations (when they occur), with definition of the actual limitations to this approach and suggestions for future improvements. Numbers in percentage indicate the range of probability of occurrence of the respective event one, two or three days after a radon anomaly (as from Table 4).
Figure 7. Flow diagram that illustrates how soil radon anomalies are linked with eruptive events and/or with RMS anomalies at Mt. Etna, and how this leads to the interpretation of the observed correlations (when they occur), with definition of the actual limitations to this approach and suggestions for future improvements. Numbers in percentage indicate the range of probability of occurrence of the respective event one, two or three days after a radon anomaly (as from Table 4).
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Table 1. List of eruptive activities recorded at Mt. Etna during the study period, divided by crater, with respective dates of onset and end. VOR = Voragine; NEC = North-East Crater; BN = Bocca Nuova; SEC = South-East Crater.
Table 1. List of eruptive activities recorded at Mt. Etna during the study period, divided by crater, with respective dates of onset and end. VOR = Voragine; NEC = North-East Crater; BN = Bocca Nuova; SEC = South-East Crater.
CraterType of ActivityOnsetEnd
SECStrombolian activity09/10/2312/11/23
SECparoxysm12/11/2312/11/23
SECStrombolian activity and weak lava effusions19/11/2301/12/23
SECparoxysm01/12/2301/12/23
VORStrombolian activity13/06/2405/07/24
VORparoxysm04/07/2405/07/24
VORStrombolian activity06/07/2407/07/24
VORparoxysm07/07/2407/07/24
VORStrombolian activity10/07/2410/07/24
VORStrombolian activity14/07/2416/07/24
VORparoxysm15/07/2416/07/24
VORStrombolian activity20/07/2425/07/24
VORparoxysm22/07/2423/07/24
VORStrombolian activity04/08/2404/08/24
VORparoxysm04/08/2404/08/24
VORStrombolian activity12/08/2415/08/24
VORparoxysm14/08/2415/08/24
BNash explosion18/08/2418/08/24
NECintermittent Strombolian activity14/07/2427/10/24
VOR-BNparoxysm10/11/2410/11/24
SECStrombolian activity06/02/2506/02/25
SEC and high S flankStrombolian activity and lava effusion08/02/2502/03/25
SECStrombolian activity15/03/2516/03/25
SECStrombolian activity19/03/2520/03/25
SECStrombolian activity and lava effusion24/03/2524/03/25
SECStrombolian activity02/04/2503/04/25
SECStrombolian activity and lava effusion07/04/2508/04/25
SECStrombolian activity and lava effusion11/04/2511/04/25
SECStrombolian activity and lava effusion15/04/2515/04/25
SECStrombolian activity and lava effusion18/04/2518/04/25
SECStrombolian activity and lava effusion22/04/2523/04/25
SECStrombolian activity, pulsating lava fountains and lava effusion29/04/2530/04/25
SECStrombolian activity and lava effusion05/05/2505/05/25
SECStrombolian activity and lava effusion12/05/2512/05/25
Table 2. Basic descriptive statistics for the radon activity values measured at the ERN9 monitoring site. All values are expressed as Bq m−3.
Table 2. Basic descriptive statistics for the radon activity values measured at the ERN9 monitoring site. All values are expressed as Bq m−3.
Mean1.1 × 106
Standard error1.4 × 105
Median741
Mode324
Standard deviation3.1 × 107
Sample variance9.4 × 1014
Kurtosis2832
Skewness48
Minimum100
Maximum2.4 × 109
Total counts47,434
Table 3. Main seismic swarms occurred at Mt. Etna during the monitoring period.
Table 3. Main seismic swarms occurred at Mt. Etna during the monitoring period.
Seismic SwarmDateMmaxDepth
1—M. ScavoAugust 20232.920–25 km
2—RagalnaDecember 20232.81.4 km
3—M. MinardoJanuary 20242.414–16 km
4—SolicchiataApril 20242.210–12 km
5—RagalnaJune 20252.71–1.5 km
6—PernicanaFebruary 20253.71–2 km
Table 4. Probabilities (in percentage) that RMS anomalies or eruptive events at Mt. Etna will occur within one, two or three days following any radon anomaly.
Table 4. Probabilities (in percentage) that RMS anomalies or eruptive events at Mt. Etna will occur within one, two or three days following any radon anomaly.
Type of Correlation1 Day2 Days3 Days
% of Rn anomalies preceding RMS anomalies30.138.146.4
% of Rn anomalies preceding paroxysms at CC1.51.54.5
% of Rn anomalies preceding paroxysms at SEC07.810.8
% of Rn anomalies preceding Strombolian activity at NEC4.44.85.9
% of Rn anomalies preceding Strombolian activity at CC23.828.631.4
% of Rn anomalies preceding Strombolian activity at SEC17.821.224.7
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Giammanco, S.; Maiolino, V.; Ursino, A.; Neri, M.; Frasca, L.; Maugeri, S.R.; Murè, F.; Principato, P. Integrated Monitoring of Soil Radon Gas and Seismic Activity to Detect Volcanic Unrest at Mount Etna (Italy), 2023–2025. Quaternary 2026, 9, 16. https://doi.org/10.3390/quat9010016

AMA Style

Giammanco S, Maiolino V, Ursino A, Neri M, Frasca L, Maugeri SR, Murè F, Principato P. Integrated Monitoring of Soil Radon Gas and Seismic Activity to Detect Volcanic Unrest at Mount Etna (Italy), 2023–2025. Quaternary. 2026; 9(1):16. https://doi.org/10.3390/quat9010016

Chicago/Turabian Style

Giammanco, Salvatore, Vincenza Maiolino, Andrea Ursino, Marco Neri, Luca Frasca, Salvatore Roberto Maugeri, Filippo Murè, and Paolo Principato. 2026. "Integrated Monitoring of Soil Radon Gas and Seismic Activity to Detect Volcanic Unrest at Mount Etna (Italy), 2023–2025" Quaternary 9, no. 1: 16. https://doi.org/10.3390/quat9010016

APA Style

Giammanco, S., Maiolino, V., Ursino, A., Neri, M., Frasca, L., Maugeri, S. R., Murè, F., & Principato, P. (2026). Integrated Monitoring of Soil Radon Gas and Seismic Activity to Detect Volcanic Unrest at Mount Etna (Italy), 2023–2025. Quaternary, 9(1), 16. https://doi.org/10.3390/quat9010016

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