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4 June 2026

18 Pages

Tectonic Control on Soil CO2 Emissions in an Active Volcanic System: Insights from Vulcano Island

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Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo, 90146 Palermo, Italy
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Università degli Studi di Milano-Bicocca, Dipartimento di Scienze dell’Ambiente e della Terra, 20126 Milan, Italy
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Università degli Studi della Basilicata, Dipartimento di Scienze di Base e Applicate, 85100 Potenza, Italy
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Author to whom correspondence should be addressed.
Geosciences2026, 16(6), 225;https://doi.org/10.3390/geosciences16060225
This article belongs to the Section Geochemistry
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Abstract

Soil CO2 emissions are widely used to trace fluid circulation in the crust, as faults and fracture networks act as preferential pathways for fluid ascent from depth. Their spatial distribution may reveal tectonic lineaments controlling fluid migration, while temporal variations may reflect stress changes associated with seismogenic processes. In active volcanic systems, however, identifying tectonic influences is challenging because volcanic and hydrothermal activity can mask tectonically controlled signals. Vulcano Island is particularly suitable for investigating these interactions, as it is characterized by both persistent volcanic–hydrothermal activity and a tectonic setting shaped by major regional fault systems. In this study, we analyze continuous soil CO2 flux records and periodic surveys conducted over a fixed measurement grid during the last 20 years. Continuous records show that a clear tectonic signal is recognizable only at the Faraglione site, where the most pronounced increase in soil CO2 flux occurred after the 16 August 2010 M 4.8 earthquake. Spatial analysis reveals two anomalous phases following this event, in September 2010 and January 2011, both showing a NNW-SSE alignment consistent with the regional structural framework. Analysis of data collected during the 2021 unrest confirms that the tectonic framework exerts strong control on fluid release both during quiescence and during phases of enhanced volcanic activity.

1. Introduction

Deep CO2 rising from the crust and upper mantle as part of Earth’s degassing represents a major mechanism of natural carbon transfer from the lithosphere to the atmosphere across a wide range of volcanic and tectonic environments [1,2,3,4,5,6,7,8,9]. These processes occur in several settings, including convergent margins, intraplate volcanic regions, and extensional systems, highlighting the role of the deep carbon cycle in regulating Earth’s carbon budget. In these settings, CO2 emission is closely linked to lithospheric deformation and provides key insights into the dynamics of the deep carbon cycle [5,9,10,11,12,13]. Across volcanic and tectonic environments, soil CO2 emissions can locally exceed background levels by several orders of magnitude, representing a sensitive surface indicator of deep fluid circulation, because the spatial distribution of anomalous CO2 emissions is strongly controlled by tectonic structures. For this reason, soil CO2 flux measurements are widely used to investigate fluid migration, as faults and fracture networks commonly act as preferential pathways for the ascent of deep-seated fluids toward the surface [11,12,13,14].
In tectonically active volcanic and seismic regions worldwide, such as the Azores Archipelago (Portugal) [15], southwestern Sicily (Italy) [16], the Campi Flegrei caldera (Italy) [17], the San Andreas Fault, California [18] the capital area of China [19], the North of China [20], the Anninghe–Zemuhe fault system in the southeastern Tibetan Plateau (China) [21], and the Chihshang fault in eastern Taiwan [22], the spatial distribution of soil CO2 emissions may delineate fault zones and fracture systems, thereby revealing the structural control exerted on crustal permeability and fluid transfer. Temporal variations in soil CO2 flux may also reflect changes in the physical state of the crust, since stress accumulation, seismic rupture, aftershock activity, and post-seismic damage can modify fracture aperture, crack connectivity, and permeability, thereby enhancing or inhibiting fluid transfer toward the surface [23,24,25,26]. From this perspective, soil CO2 emission can be regarded not only as a tracer of fluid pathways, but also as a sensitive indicator of the transient coupling between crustal fluid circulation and tectonic processes. This view is consistent with the broader idea that crustal fluids may actively influence fault behavior and earthquake occurrence by reducing effective normal stress and facilitating slip within fractured volumes [23,24,27]. Accordingly, changes in CO2 output have been documented in association with seismic swarms, stress transfer, and transient permeability reorganization, suggesting that CO2 emissions may respond to evolving mechanical conditions within tectonically active crust [25,26]. In active volcanic systems, however, soil CO2 emissions are governed not only by tectonic structures, but also by variations in volcanic and hydrothermal activity, which can modify the rate of deep fluid supply [28,29,30]. As a result, tectonically controlled signals may be more difficult to investigate, because soil CO2 emissions reflect the combined influence of structural, magmatic, and hydrothermal processes. This makes integrated spatial and temporal analyses especially important in volcano–tectonic settings, where the interpretation of soil CO2 variations requires distinguishing persistent structurally controlled emission patterns from transient perturbations associated with changes in volcanic, hydrothermal, or seismic activity. Vulcano Island, in the Aeolian Archipelago of southern Italy, provides a particularly suitable setting in which to investigate the interaction among these processes (Figure 1). The island is characterized by persistent volcanic–hydrothermal activity, widespread soil CO2 emissions [31,32], and a tectonic framework shaped by major regional fault systems, chiefly the Aeolian–Tindari–Letojanni fault system (ATLFS) and the Sisifo–Alicudi fault system (SAFS) [33,34,35,36] (Figure 1). These structures play a major role in the geodynamics of the southern Tyrrhenian domain and influence both seismicity and volcanism. Previous studies have already suggested that seismic activity can affect fluid circulation at Vulcano [30,37]. Madonia et al. [37], for example, documented significant changes in volcanic fluid related to stress redistribution associated with the 16 August 2010 M 4.8 earthquake. Their observations pointed to a reorganization of the hydrothermal system, marked by reduced discharge in the main La Fossa fumarolic area, increased soil temperature on the outer northeastern flank of the cone, and renewed submarine bubbling in the Baia di Levante area (Figure 2). These results indicate that stress changes induced by seismic processes may modify shallow permeability and redirect fluid circulation toward structurally favorable sectors of the island. In this study, we analyze the results of soil CO2 flux measurements collected over 20 years through monitoring programs conducted in collaboration with the “Dipartimento della Protezione Civile”. The investigated period includes phases of minor magmatic–hydrothermal unrest between 2003 and 2017 [30], as well as periods of major unrest observed in 2018 and culminating in the strongest and most pronounced magmatic unrest starting in 2021 [32].
Figure 1. Geological and tectonic framework of Vulcano Island (Aeolian Archipelago, Italy). (A) Geographic setting of the Aeolian Archipelago in the southern Tyrrhenian Sea. (B) Geological map of Vulcano Island modified after Gioncada et al. [38] and Keller [39]. (C) Map showing the main regional structural systems, including the Aeolian–Tindari–Letojanni Fault System (ATLFS) transfer zone and the Sisifo–Alicudi Fault System (SAFS), together with the distribution of seismicity recorded between 2003 and 2017, including the 16 August 2010 earthquake (M 4.8) (ISIDE database, INGV [40]). Structural lineaments are from Barreca et al. [36] and Cultrera et al. [41]. (The maps were produced using QGIS software version 3.4 with Base map source: Esri, Maxar, Earthstar Geographics).
Figure 2. Location of soil CO2 flux measurement sites. Red and yellow circles represent the survey grid, showing the distribution of the fixed measurement sites used for periodic soil CO2 flux surveys. Yellow circles indicate high-emission sites located near the Faraglione Cone and Grotta dei Palizzi. White triangles represent the monitoring stations: [1] Faraglione; [2] Rimessa; [3] Camping Sicilia; [4] Bordosud; [5] P4max. (The map was produced using QGIS software with Base map source: Esri, Maxar, Earthstar Geographics.).
The analyzed dataset includes both periodic surveys performed on fixed spatial grids, used to generate flux maps and identify anomalous areas under structural control, and continuous records from key sites, used to investigate temporal variations and transient signals associated with seismic and tectonic processes. We consider both quiescent and unrest conditions to evaluate the extent to which the spatial and temporal variability of soil CO2 emissions is controlled by the main structural framework, and whether this control persists across different levels of volcanic and tectonic activity. This analysis is developed within the framework of the CAVEAT project of the central-southern Aeolian islands: Volcanism and tEArIng in the Tyrrhenian subduction system.

2. Geological, Tectonic and Geochemical Overview

Vulcano Island is located within the Aeolian Archipelago in the southern Tyrrhenian Sea. Within the archipelago, Vulcano belongs to the central sector (Figure 1), where volcanic activity and deformation are controlled by the interaction between subduction-related processes, strike–slip tectonics, and localized extensional to transtensional deformation [42,43,44,45,46]. Rather than representing an isolated volcanic edifice, Vulcano is part of the broader Lipari–Vulcano structural domain [46]. Geologically, Vulcano is a stratovolcanic complex composed of overlapping volcanic edifices and caldera structures formed during recurrent eruptive phases throughout the Late Quaternary (Figure 1) [34,38,39]. Its evolution has been strongly controlled by regional tectonics, which influenced magma ascent [34,38,39]. Petrological and geochemical studies indicate that the sub-volcanic plumbing system is vertically articulated and polybaric, with magma storage and differentiation occurring at multiple crustal levels [47]. Rather than being vertically homogeneous, the crust beneath Vulcano is compartmentalized by active fault systems that regulate magma ascent and fluid circulation.
At the regional scale, the tectonic framework is governed by the interaction between two main structural systems (Figure 1): the SAFS a WNW–ESE-trending shear zone affecting the western and central sectors of the Aeolian arc and the ATLFS a NNW–SSE-striking regional tectonic boundary that exerts primary control on the architecture of the central Aeolian sector, including Lipari Vulcano and Salina [33,34,35,41,44,48]. The Lipari–Vulcano area represents a key structural node where these regional lineaments interact. Deformation is partitioned within a complex fault network characterized by active faulting, shallow seismicity, and significant lateral variations in tectonic style. In this context, a NW–SE-to-NNW–SSE-oriented deformation zone between Lipari and Vulcano separates distinct crustal blocks and accommodates differential motion within the central Aeolian sector [41,48]. On Vulcano Island, the influence of the ATLFS is reflected by the persistence of NNW–SSE-oriented structural lineaments, accompanied by subordinate faults with variable orientations [43,45]. The resulting structural configuration is consistent with a dominantly strike–slip regime that locally accommodates extensional to transtensional deformation, promoting the development of secondary fault systems and fracture networks.
Faults and highly fractured zones act as preferential pathways for fluid migration, linking deep magmatic sources with surface structures. Geological and structural data emphasize that the present-day configuration of Vulcano reflects the superposition of regional tectonic trends and local volcano–tectonic processes [41,48]. Therefore, Vulcano represents a structurally controlled volcano–tectonic system in which regional tectonic, active deformation, and magmatic–hydrothermal processes are tightly coupled. In this setting, fault systems play a fundamental role in regulating magma ascent, fluid circulation, and surface fluid emission.
Vulcano hosts a long-lived hydrothermal system sustained by persistent magmatic degassing. The island is also characterized by fumarolic activity, thermal anomalies, diffuse soil CO2 emissions, CO2-rich groundwater and shallow submarine emissions [28,30,31,32,49,50,51,52]. The chemical and isotopic compositions of emitted fluids indicate interaction with deep, hydrothermal and magmatic sources [32,50,52]. The highest soil CO2 emissions are mainly recorded in the area close to Faraglione Cone to the north and in the Grotta dei Palizzi area to the south (Figure 2). At these sites, the carbon isotopic composition of CO2 (δ13C-CO2) indicates a predominant deep origin of the emitted gas [53]. Reported δ13C-CO2 values generally range between about −5‰ and −2‰ vs. V-PDB, consistent with a magmatic-derived component variably modified by hydrothermal interaction and shallow processes [50]. Isotopic surveys further indicate that a contribution of deep-derived CO2 is diffusely present throughout the study area [53,54,55]. This contribution becomes particularly evident during phases of volcanic unrest, when a shift toward less negative δ13C-CO2 values is observed across the island, indicating an increased input of deep CO2 even at sites normally dominated by biogenic emissions [55]. Previous studies documented phases of volcanic unrest associated with enhanced deep CO2 release, notably during the September–November 2018 episode and the unrest that began in September 2021, when a marked increase in soil CO2 emissions and other geochemical anomalies was recorded across Vulcano Island [32,54,55,56]. Similar helium isotopic compositions measured in dissolved gases from the Vulcano Porto aquifer and in La Fossa crater fumaroles indicate a common magmatic origin of these fluids. In particular, the aquifer reaches R/Ra values up to 5.6, indicating a strong mantle-derived helium contribution [52]. These observations support the interpretation that deeply sourced magmatic fluids are diffusely released across the island through tectonic structures and peripheral ascent pathways. Furthermore, anomalous variations in crater fumarolic activity were accompanied by significant changes in groundwater chemistry and soil CO2 emissions in peripheral areas [50,51], highlighting a clear covariation between crater activity and diffuse fluid emission processes throughout the island.

3. Methods

Soil CO2 flux was measured using the dynamic concentration method described by Camarda et al. [57]. This technique is based on measuring the CO2 concentration in a mixture of air and soil gas generated inside a probe inserted 50 cm below the ground surface. The gas mixture is produced by applying a slight negative pressure with a pump operating at constant flow rate, and CO2 concentration is measured in situ by an infrared gas analyzer. After a short equilibration time, generally less than 1 min, the gas mixture reaches a stable value, referred to as the dynamic concentration, which is proportional to the soil CO2 flux. The relationship between dynamic concentration and flux was established experimentally under controlled laboratory conditions by repeated measurements on soils with known and constant CO2 fluxes over a wide range of permeability values representative of natural conditions. Soil CO2 emissions were investigated through both periodic surveys and continuous automatic measurements. Periodic soil CO2 flux surveys (monthly and bimonthly) were carried out at fixed sites distributed across the Vulcano Porto area and along the lower flanks of La Fossa Cone (Figure 2). The measurement grid used in this study is characterized by a relatively homogeneous spatial distribution, with a mean nearest-neighbor spacing of approximately 164 m (median spacing: 157 m) and a standard deviation of ~61 m. Based on these spatial characteristics, the approximate influence radius of each measurement site is estimated at ~82 m, corresponding to an average influence area of ~2.1 × 104 m2 per point. The overall sampling density is approximately 12.5 points/km2. These parameters indicate that the dataset provides a sufficiently regular spatial coverage to investigate the spatial organization of diffuse CO2 emission patterns at the scale of the study area. On the basis of this measurement grid, raster maps were generated using Ordinary Kriging interpolation. A Gaussian Kernel Density Estimation (KDE) was then applied to the extracted anomalous raster cells, producing normalized density surfaces for each survey and principal component analysis (PCA) was used to identify the dominant orientation of the anomaly fields from the spatial distribution of the KDE-derived anomalous cells.
The continuous monitoring datasets used in the present work are reported in Camarda et al. [30]. They consist of data acquired from five automatic monitoring stations at Vulcano Island (white triangle, Figure 2) during the same period covered by the discrete soil CO2 flux surveys discussed in this study. Four stations are located within the same sector investigated by the periodic surveys, whereas one station is installed in the crater. The automatic stations acquire soil CO2 flux data on an hourly basis. Data are stored locally in a data logger and transmitted daily to the INGV-Palermo monitoring center. Each station is powered by a photovoltaic panel coupled with a battery system. The monitoring stations and the grid of discrete survey points are located on terrains belonging to the La Fossa pyroclastic deposits.

4. Results

4.1. Continuous Monitoring

In the first stage of the analysis, seismicity recorded between 2003 and 2017 (Figure 1) was benchmarked against continuous soil CO2 flux data to investigate the relationship between tectonic processes and fluid emissions, as earthquakes reflect seismogenic activity linked to the regional tectonic framework. This time interval was specifically selected because it corresponds to a period characterized only by minor magmatic unrest episodes, thus allowing the assessment of possible seismic effects on soil CO2 emissions under relatively stable volcanic–hydrothermal conditions. Earthquakes occurring in the study area were retrieved from the ISIDE–INGV catalog [40] and are shown in Figure 1. The largest-magnitude event occurred on 16 August 2010 (M 4.8), with an epicenter located about 3 km offshore Vulcano Island. This event was characterized by shallow depth (17 km) and a tectonic origin within a regional extensional stress regime. Focal mechanism solutions indicate normal faulting, consistent with NNW–SSE to NW–SE trending structures that characterize the regional tectonic framework [58]. The earthquake triggered landslides and rockfalls affecting several islands of the archipelago, particularly Lipari and Vulcano, with minor damage reported in the town of Lipari. This earthquake was previously associated with anomalous soil CO2 emissions by Camarda et al. [30]. These authors, by analyzing continuous soil CO2 flux records from five monitoring stations, identified three periods of anomalous CO2 emissions (A-C) (Figure 3). Period A extended from 7 September 2009 to February 2010, period B from August to October 2010, and period C from the end of September 2011 to April 2012. Among these anomalous periods, period B showed distinctive features compared with Periods A and C. In particular, it was shorter in duration and clearly detectable only at site 1, corresponding to the Faraglione site, whereas the anomalies observed during Periods A and C were evident in most of the monitored records. On this basis, Camarda et al. [30] pointed out a unique temporal correspondence between the anomaly recorded at Faraglione and the 16 August 2010 seismic event, suggesting that the earthquake may have induced changes in permeability and fluid circulation. This interpretation is consistent with [37], who reported coeval changes in other geochemical parameters related to stress redistribution associated with the 16 August 2010 M 4.8 earthquake. In Figure 3, we propose an analysis of the weekly mean Z-score time series of soil CO2 flux measured at the five monitoring stations, in order to investigate the temporal evolution of soil CO2 emissions in relation to the August 2010 tectonic event. The use of the Z-score is particularly useful because it standardizes the signal relative to its mean and standard deviation, thereby allowing anomalous variations to be identified more clearly and compared over time in terms of their statistical significance. Among the investigated stations, the most evident increases during the three anomalous periods previously identified by Camarda et al. [30] are observed at Station 1 (Faraglione) and Station 3 (Camping Sicilia). In particular, Station 1 shows the strongest response, with several exceedances above the 2 sigma threshold during periods A, B, and C, and a pronounced peak during Period B reaching values higher than 5 sigma. Following the 16 August 2010 earthquake, soil CO2 flux at Faraglione increased sharply and remained above the 2 sigma threshold during September–October 2010, with additional oscillations above 2 sigma in January 2011. This behavior suggests that the observed anomaly was not limited to an instantaneous perturbation immediately following the earthquake, but rather reflects a prolonged post-seismic enhancement of soil CO2 emissions lasting several months. Station 3 also exhibits anomalous increases during periods A, B, and C, although with lower amplitudes compared to Faraglione. In particular, a moderate exceedance of the 2 sigma threshold is observed during Period B, temporally consistent with the August 2010 tectonic event, suggesting a weaker but still detectable response to seismic activity. By contrast, Station 4 exceeds the 2 sigma threshold mainly during periods A and C and shows a localized anomaly close to the occurrence of the 11 March 2011 Honshu Sendai earthquake (M 9.1). A possible correlation between this earthquake and the observed variation has already been described and recognized by Camarda et al. [30]. The remaining stations do not show significant exceedances above the 2 sigma threshold during the investigated periods, suggesting a more limited response to the tectonic perturbations considered here. Overall, the different behavior observed among the stations may reflect the specific structural, geological, and geochemical characteristics controlling gas migration and emission processes at each site.
Figure 3. Time series of soil CO2 flux at the five monitoring stations expressed as Z-scores. The anomalous periods A, B, and C, yellow box, are that identified by Camarda et al., 2019 [30]. The 16 August 2010 (M 4.8) earthquake is indicated, together with the post-seismic increase in soil CO2 flux recorded on January 2011 (light gray box).
Faraglione is the site characterized by the highest mean soil CO2 flux values within the monitoring network, suggesting that it overlies a sector of persistently efficient CO2 transfer. In such a setting, the system may already operate under conditions of high background permeability and high flux, so that additional increases related to volcanic unrest are less evident than at other sites. Conversely, a tectonic perturbation such as the 16 August 2010 earthquake may directly modify the permeability of the fault/fracture network feeding the area, producing a distinct response in the soil CO2 flux signal. This interpretation supports the idea that the Faraglione station is particularly sensitive to tectonically induced permeability changes, whereas the other stations predominantly record variations related to volcanic–hydrothermal forcing.

4.2. Periodic Survey

To investigate the role of tectonic structures in controlling soil CO2 emission patterns, the spatial analysis was performed following two complementary approaches. In the first phase, sites characterized by high emission rates were excluded, and only data acquired during periods of relative volcanic quiescence, 2003 to 2017, were considered. This strategy reduces the influence of transient hydrothermal and volcanic contributions, thereby allowing structurally controlled anomalies in soil CO2 emissions to be more clearly identified. It also takes into account the greater sensitivity of high-flux sites to atmospheric parameters, which may significantly affect the measured flux and obscure more subtle tectonic-related variations [30]. In the second phase, survey data including high-emission sites and covering periods of the 2021 volcanic unrest were analyzed to assess whether increased emissions are preferentially focused along the main structural features. This approach enables evaluation of the extent to which the tectonic framework controls fluid pathways under both background and high emission conditions.

4.2.1. Spatial Analysis over a Period Characterized by Minor Unrest

As introduced above, to uncover tectonic controls on soil CO2 emissions, the analysis of the periodic survey data was designed to minimize the influence of the volcanic–hydrothermal system and meteorological parameters by excluding high-emission sites (i.e., points with mean values > 50 g m−2 d−1) (yellow circle, Figure 2) and focusing on a period of relatively low volcanic activity. The data were standardized using z-scores, as this approach enhances the identification of anomalous periods and facilitates the spatial visualization of areas characterized by significant deviations from background conditions. In addition, the time series were deseasonalized by removing the mean value of each calendar month, in order to account for seasonal variability that may influence CO2 flux, particularly under low-flux conditions where seasonal effects can be comparable to or exceed the signal of interest.
The resulting time series can be used to identify anomalous periods and to assess whether their spatial distribution reflects the main tectonic structures. Anomalies in soil CO2 flux are commonly evaluated using the temporal evolution of mean values across the monitoring sites. However, in this case, such an approach may not be appropriate, as mean values can be influenced by minor variations in the hydrothermal or volcanic system, potentially masking localized signals. Instead, the analysis focuses on quantifying, for each survey period, the number of monitoring sites exhibiting anomalous values. This is particularly relevant for tectonic processes, which are expected to produce intense spatially localized effects, influencing only part of the network through changes in permeability and fluid pathways. In this context, the key parameter is not the magnitude of the flux increase itself, but the number of sites exhibiting anomalous values. This behavior is consistent with observations from continuous data, where anomalous signals are primarily detected at the Faraglione station.
Accordingly, anomalous periods were identified by quantifying, for each survey, the number of monitoring sites with z-score values exceeding the 2σ threshold (Figure 4). Using this approach, three anomalous periods were identified, defined by the simultaneous occurrence of anomalies at a minimum of four monitoring sites: September 2010, January 2011, and September 2015. To better characterize these three anomalous periods, further statistical and spatial investigations were performed. First, the statistical structure of the anomalies was examined using box plots of the deseasonalized z-score values. A box plot provides a compact summary of the distribution of values for each survey, showing the median, the interquartile range, and the overall spread of the data, while the associated individual points help visualize the presence of extreme values. In the present case, this representation is particularly useful because it allows for disentangling anomalies driven by a few isolated high values from anomalies associated with a broader upward shift in the whole distribution. The box plot comparison (Figure 5) shows that the September 2010 and January 2011 anomalies have similar statistical structures. Although more than four sites exceed the 2σ threshold, the median remains negative in both cases, indicating that the anomalous signal is mainly controlled by a limited number of localized high-value points. By contrast, the September 2015 distribution is shifted toward higher values and is characterized by a positive median value, suggesting that the anomaly affected a larger fraction of the monitored sites and was therefore more spatially widespread.
Figure 4. Number of monitoring sites exceeding the 2σ threshold for each survey between 2003 and 2017. Three main anomalous periods (September 2010, January 2011, and September 2015) are identified.
Figure 5. Box plots of deseasonalized soil CO2 flux Z-scores for selected surveys. September 2010 and January 2011 show negative medians with few high-value outliers (>2σ), indicating localized anomalies, whereas September 2015 displays a positive median and a broader distribution, suggesting a more widespread increase in flux.
After this statistical comparison, the spatial distribution of CO2 flux z-scores was analyzed for each anomalous phase and compared with a reference map representative of background conditions (Figure 6). The distribution of z-score anomalies detected in September 2010 and January 2011 (Figure 6A,B) exhibit a high degree of similarity in terms of spatial geometry, areal extent, and polarity, suggesting that they are driven by a common underlying process. In both cases, the spatial distribution of anomalies shows a marked NNW–SSE alignment (white dashed lines Figure 6A,B), consistent with the main tectonic lineaments of the ATLFS. In this context, the temporal proximity of these anomalies to the August 2010 earthquake, together with their coherent spatial organization, supports the tectonic origin. The box plot analysis is consistent with this interpretation, as both periods are characterized by localized anomalies rather than by an overall increase across the network. In contrast, the anomaly observed in September 2015 (Figure 6C) occurs within a different temporal framework and does not exhibit the same degree of spatial coherence with the regional tectonic structures. Its statistical distribution also differs from the 2010 and 2011 cases, showing a broader upward displacement of values rather than a pattern dominated by a few isolated peaks. This anomaly is therefore more consistent with a perturbation affecting a wider portion of the hydrothermal system, rather than with purely tectonic forcing. The reference map, representative of a quiescent period (Figure 6D), specifically September 2011, highlights the lack of clearly evident anomalies compared to the other periods.
Figure 6. Spatial distribution of soil CO2 flux anomalies during selected survey periods. (A) 09. 2010; (B) 01. 2011; (C) 09. 2015; (D) reference map under quiescent conditions (September 2011). The selected periods correspond to surveys with at least four sites exceeding the 2 sigma threshold. The 2010 and 2011 anomalies show a clear NNW–SSE alignment (white dashed lines) consistent with the ATLFS, whereas the 2015 anomaly appears more spatially diffuse and less structurally organized. (Base map source: Esri, Maxar, Earthstar Geographics).
We performed an additional raster-based KDA on the interpolated CO2 anomaly maps in order to provide a more quantitative assessment of the spatial organization of the emission anomalies.
More specifically, ordinary Kriging raster maps generated from the CO2 dataset connected positive anomaly surfaces characterized by Z-score values > 1σ and were retained, in order to focus the analysis on the most significant emission domains. A KDE was then applied to the extracted anomalous raster cells, producing normalized density surfaces for each survey. The dominant orientation of each anomaly field was subsequently quantified from the covariance matrix of the KDE cell coordinates through the PCA, with the eigenvector associated with the largest eigenvalue defining the principal elongation axis of the anomaly distribution.
The resulting KDE-derived orientations (Figure 7) are ~175.5° for the 09.2010 survey and ~168.8° for the 01.2011 survey, both indicating a dominant NNW–SSE spatial organization of the anomalous CO2 emissions. In contrast, the 09.2015 survey shows a markedly different orientation (~26.7°; NNE–SSW), suggesting a different spatial organization of the emission pattern during that unrest phase. For the 09.2011 survey, no reliable preferential orientation could be identified because only one isolated positive anomaly exceeded the 1σ threshold, preventing a statistically meaningful pattern recognition. The PCA-derived axes do not perfectly overlap the visually inferred anomaly alignments because PCA identifies only the dominant overall orientation of the entire anomaly cluster, whereas the visual interpretation of the anomalies allows recognition of local alignments and spatial relationships among individual anomalies. Nevertheless, the general agreement between the PCA-derived orientations and the visually inferred alignments, both indicating a dominant NNW–SSE trend, supports the overall coherence and reliability of the identified structural controls on fluid migration.
Figure 7. Raster-based Kernel Density Analysis of positive CO2 flux anomalies exceeding the 1σ threshold for the 09.2010, 01.2011, and 09.2015 surveys. The dashed light-blue lines represent the principal spatial orientation derived from PCA of the anomalous raster-cell coordinates and expressed as geographic azimuth. The 09.2010 and 01.2011 surveys show a dominant NNW–SSE organization (~175.7° and ~168.8°, respectively), consistent with the regional tectonic framework, whereas the 09.2015 survey exhibits a markedly different NE–SW orientation (~26.7°), suggesting a distinct spatial organization of the emission anomalies during that unrest phase. KDE values are normalized between 0 and 1 to allow for comparison among surveys.

4.2.2. Spatial Patterns During 2021 Unrest

Periods of elevated soil CO2 emissions can reveal preferential emission pathways, as increases in fluid release tend to highlight zones of enhanced permeability associated with faults and fracture systems. Although these episodes are primarily driven by volcanic unrest, they can improve the detectability of structurally controlled features. Under quiescent conditions, shallow permeability variations related to near-surface geology and superficial structures may partly mask deeper tectonic lineaments in the soil CO2 flux pattern. During unrest, by contrast, the increased ascent of deep fluids can enhance the contribution of deeper permeable pathways, thereby making tectonic structures more clearly expressed in the spatial distribution of soil CO2 emissions. In this analysis, all measurement sites were considered, including points located within the high-emission areas of Faraglione and Grotta dei Palizzi, in order to capture the full spatial extent of the CO2 emission system during perturbed conditions.
The volcanic crisis that began in September 2021 at Vulcano Island, characterized by a marked increase in soil CO2 flux [32], provides a valuable opportunity to better constrain the spatial organization of the structures. Among the available surveys, the November 2021 dataset—corresponding to the peak of CO2 emissions [32]—was selected as a representative of maximum soil CO2 emission conditions (Figure 8A). As shown by the November 2021 map, the spatial distribution of fluxes during this phase of intense CO2 emission clearly outlines anomaly alignments trending NNW–SSE, consistent with the tectonic lineaments of the ATLFS. In this case, the increase in flux associated with volcanic activity enhances the expression of NNW–SSE anomalies, confirming that phases of stronger degassing make tectonically controlled pathways more clearly recognizable in the spatial pattern of soil CO2 emissions. For comparison, a map from a lower-activity period (November 2020) is also presented (Figure 8B), showing that these alignments become evident primarily during phases of elevated CO2 emission.
Figure 8. Maps of spatial distribution of soil CO2 flux. (A) Map of soil CO2 flux during November 2021, corresponding to periods of very intense CO2 emission. The data reveal pronounced NNW–SSE alignments (white dashed lines) consistent with regional tectonic structures. (B) A comparison with a lower-activity period (November 2020) highlights the enhanced expression of structurally controlled pathways during unrest. (C) Raster-based Kernel Density Analysis of positive CO2 flux anomalies exceeding the 600 g m−2 d−1 threshold. The dashed light-blue lines represent the principal spatial orientation derived from PCA of the anomalous raster-cell coordinates and expressed as the geographic azimuth. KDE values are normalized between 0 and 1 to allow comparison among surveys. (Base map source: Esri, Maxar, Earthstar Geographics).
We additionally performed a raster-based KDA combined with PCA on the November 2021 soil CO2 flux map in order to evaluate the spatial organization of the highest emission sectors during this period. In this case, only flux values exceeding 600 g m−2 d−1 were retained to isolate the most significant anomalous domains. The resulting KDE map (Figure 8C) shows a dominant spatial organization characterized by a principal NNW–SSE orientation. Although the PCA-derived axis does not perfectly overlap the visually interpreted anomaly alignments, the overall preferential trend remains consistent with the previously identified NNW–SSE structural organization controlling fluid migration.

5. Discussion

The results of the combined analysis of continuous monitoring and periodic survey data indicate that soil CO2 emission at Vulcano Island is influenced by the tectonic framework and can respond to deformation-related changes in crustal permeability. The continuous monitoring results provide clear evidence of a deformation-related response in connection with the M 4.8 16 August 2010 earthquake, with the strongest signal recorded at the Faraglione site. At this site, the most pronounced increase in soil CO2 flux occurred immediately after the seismic event, rather than during phases of volcanic unrest. Flux values remained elevated through September–October 2010 and increased again in January 2011, indicating that the anomaly was not restricted to a short-lived post-seismic perturbation. Instead, this temporal pattern points to a more persistent enhancement of permeability and fluid transfer following the earthquake, consistent with a tectonic mechanism involving permeability enhancement and stress redistribution, capable of promoting the upward migration of deep-seated fluids.
This interpretation is supported by the periodic surveys, which identified two anomalous phases in September 2010 and January 2011, both shortly after the August 2010 earthquake. These two periods show similar statistical characteristics in the box plot analysis of the deseasonalized z-score values, with anomalies mainly controlled by a limited number of localized high-value sites rather than by a general upward shift in the whole distribution. Their spatial patterns are also highly consistent, both showing a clear NNW–SSE alignment coherent with the regional tectonic framework. Taken together, the temporal, statistical, and spatial evidence suggests that the September 2010 and January 2011 anomalies reflect the same style of perturbation and are plausibly linked to earthquake-induced changes in permeability along structurally controlled pathways.
A tectonic control is also evident during the 2021 unrest, although the increase in CO2 flux during this period was primarily driven by volcanic processes [32,56]. The 2021 unrest was clearly associated with renewed volcanic activity, as shown by the strong intensification of fumarolic degassing, the rapid heating of the hydrothermal system, and the development of widespread geochemical and geophysical anomalies in the La Fossa area and across Vulcano Island [30]. The geophysical evolution of the crisis also included significant ground deformation, repeated phases of increased shallow seismicity, and the occurrence of very long-period seismic signals, interpreted as evidence of fluid-driven dynamics within the shallow system [32,59]. In agreement with this framework, geodetic studies indicate that deformation during the unrest was largely related to shallow pressurization of the volcanic–hydrothermal system and near-radial extension [60,61]. However, the spatial distribution of soil CO2 emission anomalies cannot be interpreted as the simple surface expression of pressurization alone. Rather than being diffuse or radially distributed, the anomaly shows a marked NNW–SSE elongation, consistent with the structural framework of the ATLFS. This geometry suggests that fluid ascent was strongly guided by deep-seated tectonic structures, rather than being controlled solely by shallow pressurization- induced permeability changes. The recurrence of this geometry in the post-seismic anomalies of September 2010 and January 2011, as well as during the high-emission phase of November 2021, suggests that the NNW–SSE tectonic fabric exerts a first-order control on permeability and fluid ascent [41,48]. This interpretation is consistent with the regional geodynamic framework proposed for the Lipari–Vulcano sector, where structural and geodetic studies describe a NW–SE to NNW–SSE right-transtensional fault system, locally associated with both dilatational and shear components [35,41]. Relationships between diffuse CO2 emissions, deep fluid migration, and volcano–tectonic unrest have been observed in several volcanic systems worldwide. At Mammoth Mountain (California), long-term monitoring studies demonstrated that diffuse CO2 emissions are strongly controlled by deep magmatic fluid input and fault permeability. Increased degassing, seismic swarms, and elevated 3He/4He ratios were interpreted as evidence of magma-derived CO2 ascent through fracture systems activated during unrest episodes [62,63]. More recent studies also showed that temporal variations in CO2 flux are linked to fault-valving processes and crustal stress changes, emphasizing the role of tectonic structures in modulating deep gas transport [64]. At the NW rift zone of Tenerife (Canary Islands), Hernández et al. [65] showed that temporal increases in diffuse CO2 emissions were associated with episodes of seismo-volcanic unrest and interpreted them as evidence of enhanced ascent of deep magmatic fluids along tectonic structures. At Campi Flegrei, long-term monitoring of diffuse soil CO2 emissions revealed that increases in degassing and expansion of anomalous areas accompanied phases of caldera unrest, reflecting progressive pressurization and increased input of deep fluids into the hydrothermal system [66]. Similar evidence was also reported at the Los Humeros volcanic system (Mexico), where spatial and temporal variations in diffuse CO2 emissions were linked to permeable fault zones controlling the ascent and circulation of deep fluids within the volcano–tectonic system [67]. In this context, the Vulcano dataset further highlights the importance of integrating continuous monitoring and spatial survey data through a standardized deseasonalized z-score analysis. This approach represents an innovative aspect of the present study because it allows for direct comparison of CO2 emission anomalies across periods characterized by different background conditions, improving the identification of statistically significant temporal changes and spatially coherent structures.

6. Conclusions

The results demonstrate that tectonic structures exert primary control on fluid pathways at Vulcano Island across a wide range of conditions, from background emission to periods of major volcanic unrest. Seismic events can act as triggers that transiently enhance permeability and fluid flow, while phases of elevated emissions improve the detectability of structurally controlled pathways. The observed coherence between temporal signals, spatial patterns, and regional tectonic trends provides robust evidence that soil CO2 emissions are strongly modulated by the tectonic framework of the area. More broadly, the integrated use of continuous monitoring and periodic spatial surveys proved effective in identifying tectonic control even in a complex volcanic setting, where hydrothermal and volcanic processes may obscure structural signals. This approach may therefore be useful in other active volcanic systems where separating tectonic, volcanic, and hydrothermal contributions to fluid emission remains challenging.

Author Contributions

Conceptualization, S.D.G.; methodology, S.D.G.; formal analysis, S.D.G.; investigation, A.P., M.C., G.C., S.D.G., G.M.R., V.P.; data analysis, A.P., M.C. and S.D.G.; writing—original draft preparation, S.D.G.; review and editing, M.C., A.P., G.C., S.D.G., R.M.R.D.M.; visualization A.P., M.C. and S.D.G.; writing S.D.G. All authors have read and agreed to the published version of the manuscript.

Funding

This study has benefited from funding provided by Presidenza del Consiglio Ministri—Dipartimento Protezione Civile (DPC) and the Presidenza della Regione Siciliana—Dipartimento Regionale della Protezione Civile (DPRC). This paper does not necessarily represent DPC and DPRC’s official opinions and policies. This research was partially funded by the INGV Pianeta Dinamico—MT CAVEAT Project, (CUP D53J19000170001).

Data Availability Statement

The data presented in the current study are available from the corresponding author upon reasonable request.

Acknowledgments

The authors would like to thank INGV Palermo’s technical staff for the technical support. During the preparation of this manuscript, the authors used ChatGPT (OpenAI, GPT-5.3) for the purposes of generating preliminary figures and assisting with the revision of the English language. The authors have carefully reviewed and edited all outputs and take full responsibility for the content of this publication.

Conflicts of Interest

The authors declare no conflicts of interest.

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