Understanding Volcanic Eruptions Where Plates Meet

(a) Tectonic map showing the geodynamic framework of the Mediterranean region. (b) Main structural features of Sicily and location of the volcanic areas studied in the V3 Project. Credit: Modified from Billi et al.

A new project elucidates the relationships between tectonics and volcanic systems and how they influence hazards on Italy's Mount Etna and Vulcano and Lipari islands.

Mount Etna and the Aeolian Islands in southern Italy represent ideal natural laboratories to study how magma and tectonics interact in active volcanic zones and their associated hazards. The geodynamic context of both areas is characterized by a tectonic compression running north and south, related to the convergence of the African and Eurasian plates in the central Mediterranean (Figure 1). This compression creates a cluster of volcanoes, some of the most active in Europe.

The region’s proximity to major urban centers is a double-edged sword. Researchers have little trouble visiting volcanoes to study how tectonics influence volcanic plumbing. At the same time, this proximity presents hazards to inland communities and those on nearby shores.

To take advantage of this natural laboratory and to help protect people living nearby, several branches of Italy’s government research community established the V3 Project—so named because it is project number 3 of the Italian Civil Protection Department’s Volcanological Research Program. Data collection for the project ran from 2012 to 2015; now researchers are analyzing the data to build better hazards maps and strengthen understanding of how tectonic and volcanic processes work together to exacerbate risk.

Tectonic Setting
 
A remnant of the Ionian slab (African plate) is subducting toward the northwest beneath the Calabrian Arc, which forms the toe of Italy’s boot [Gvirtzman and Nur, 2001]. The resulting Tindari Fault is a right-lateral strike-slip structure capable of generating moderate earthquakes (the most recent event, in 1978, was M6.2) that extends from the central sector of the Aeolian Archipelago (islands of Vulcano and Lipari) in the southern Tyrrhenian Sea toward the Etna region to the south [Billi et al., 2006]. This tectonic feature, located in eastern Sicily, is considered to represent a stress transfer zone between the two volcanic areas [De Guidi et al., 2013].

A System of Volcanoes
 
Mount Etna (3340 meters above sea level) is the most active volcano in Europe. It features nearly constant summit activity and frequent flank eruptions, with extended lava flows and copious ashfall. Its activity has increased significantly in recent years—not only are eruptions more frequent, but also the volcano emits a greater volume of lava.

At Vulcano and Lipari, in the Aeolian Islands, volcanic unrest is much less frequent but produces more intense explosive activity and emissions of viscous and thick lava flows. The previous outbreak at Lipari, in 1230, produced a renowned eruption of pumice and obsidian. Vulcano erupted more recently, in 1888–1890, but since then it has exhibited only high-temperature fumarolic activity.

The hazards in active volcanic areas are generally related to eruptive activity: lava and flows of hot gas and rock, tephra fallout, fast moving mudflows (lahars), or volcanic gas emission. However, other less devastating kinds of events may also threaten local communities living on the flanks of a volcano, mostly because these events occur frequently or almost continuously.

At Etna, recurrent volcano-tectonic seismicity poses a serious hazard to the 400,000 or so people who live in urbanized areas and to important infrastructure (roads, water/gas/power lines, hospitals, schools, etc.) [Azzaro et al., 2016]. At Vulcano, rockfalls and landslides affecting the active crater put the village and its tourist facilities at risk [Marsella et al., 2015]. These instabilities can happen unexpectedly because of variations in volcanic activity or other such triggering processes as rainfall and seismic activity. Ongoing sea flooding in Lipari’s main town provides overwhelming evidence of land subsidence that, coupled with the expected sea level rise in the next decades [Anzidei et al., 2014], will lead to future permanent inundations.

Addressing the Issues
 
To tackle hazards and risk in this seismically active volcanic zone, the Italian Civil Protection Department (DPC), with the National Institute of Geophysics and Volcanology (INGV, through branches in Catania, Palermo, and Rome), three Italian universities (Cosenza, Catania, and Rome), and the National Institute of Oceanography and Experimental Geophysics (Trieste), launched the V3 Project.

According to the project’s mission statement, V3 aimed to develop “multidisciplinary analysis of the relationships between tectonic structures and volcanic activity.” The project extended previous research programs on the same areas funded by DPC. It dealt with various methodological approaches—tectonic, geophysical, geochemical, petrological, and geotechnical investigations—to interpret ongoing phenomena and assess related hazards.

The V3 Project had four goals:

• to define the tectonic framework controlling the volcanic systems
• to analyze the exceptionally long time series of instrumental data acquired by the multiparametric monitoring and define the relationships among them
• to characterize processes connected with the interaction between tectonic structures and volcanic systems
• to produce hazards maps aimed at mitigating the effects of earthquakes, landslides, and land subsidence
• The results of the project have been presented at conferences and are being published in technical reports [Azzaro and De Rosa, 2016] and scientific papers.

Assessing Hazards at Etna

As a result of the V3 Project, we now have a greater understanding of the volcanic and tectonic mechanisms that drive hazards in the region.

We found significant correlations in the eastern flank of Etna among active fault zones, seismic patterns, variations of crustal geodetic strain, and fluids circulation. This opens new perspectives to understand faulting at Etna. We have obtained an analytical estimation of creep processes, indicating that about 40% of the deformation occurs aseismically. Moreover, we recognized that the pore pressure of fluids circulating in the volcanic rocks (chiefly groundwater) depends on variations in the crustal strains related to volcanic or seismic activity [Mattia et al., 2015].

We performed a full probabilistic seismic hazards assessment at Etna through local seismic sources defined with instrumental and historic earthquake data sets [Azzaro et al., 2015]. The obtained estimations (Figure 2) show that relevant values of ground accelerations are probabilistically likely to occur also in short times (5–30 years) and are intended to complement the 50-year seismic hazards map of Italy [Stucchi et al., 2011]. The assessment can be used to establish priorities for seismic retrofitting of the more exposed municipalities.

V3 efforts improved knowledge of the geometry and structural setting of the sedimentary basement underlying Etna using high-resolution aeromagnetic surveys and offshore seismic profiles. They reveal magnetic anomalies associated with important faults [Nicolosi et al., 2014] and shallow-seated batches of crystalized magma in a framework of active compressive and extensional tectonic structures related to the African plate colliding with Europe [Polonia et al., 2016].

Hazards Elsewhere
 
The regional pattern of north–south crustal shortening in the southern sector of the Aeolian Islands is associated with a diffuse subsidence [Esposito et al., 2015], but at a local scale the dynamics reflect different processes (Figure 3a). At Vulcano a shallow (4-kilometer-deep) deflating magmatic source in the northernmost part of the island is periodically fed by deep fluids coming from the underlying reservoir. At Lipari, long-term land subsidence [Anzidei et al., 2016] is enhanced by coastal dynamics (retreating of submarine canyons into the shelf), with a maximum sea level rise of up to 2.2 meters expected in 2100.

We confirmed the existence of a common plumbing system responsible for the historic eruptions (less than 1000 years ago) of Lipari and Vulcano. This plumbing system, which lies at a depth of 20 kilometers, periodically feeds shallow magma storage zones where processes of magma crystallization occur [Fusillo et al., 2015]. The eruptive activity took place contemporaneously at both islands along a narrow zone characterized by a dominant east–west extensional stress field (Figure 3b). Any future eruption is likely to take place in this narrow zone [Ruch et al., 2016].

The susceptibility of the active crater of Vulcano to landslides, which endanger the main village, is enhanced by hydrothermal alteration of rock mass due to hydrothermal fluid circulation (fumarolic fields, indicated by red in Figure 3a). Fractures and volcano stratigraphic discontinuities control the locations of areas potentially affected by shallow (debris avalanches/flows and rockfalls) and deep-seated instability processes [Cangemi et al., 2016]. This makes large rock volumes prone to slide suddenly, as occurred in 1988.

Science to Mitigate Risk
 
Similar to other coordinated research funded by DPC, the findings of the V3 Project represent the efforts of the scientific community to provide authorities with appropriate instruments to mitigate risks in volcanic areas. To do this, V3 endeavored to improve knowledge on inadequately studied processes.