Detecting ground deformation across the shore at Etna’s unstable southeastern flank

Researchers from PRE-COLLAPSE and accompanying projects are monitoring Etna’s southeastern flank that is slowly moving into the sea. To analyse the behaviour of the sliding block in its connectivity crossing the shoreline, on land two creepmeters have been installed in July 2022 to complement the offshore GeoSEA array that has been running since 2016.

Etna is considered, after Kilauea, the second most productive volcano on earth with several major eruptions per year. Towering above the City of Catania on the Sicilian east coast (Italy), it is the largest and most active volcano in Europe. Etna is located in a complex tectonic setting due to the convergence of the African and European plates. The volcano rests on the active fault between the African plate and the Ionian microplate, which are together being subducted (pulled) beneath the Eurasian plate.  About 10.000 years ago, the structure of the volcano was radically modified by slope failures at the eastern flank that entailed the wide depression of the Valle del Bove. Over the last decades, extensive geodetic surveys focusing on the onshore flanks of Mount Etna have revealed instability of its eastern flank, which continuously moves seawards at rates of up to 50 millimetres per year.

The PRE-COLLAPSE research group at GEOMAR Helmholtz Centre for Ocean Research, Kiel tries to identify and correctly interpret the geometry and dynamics of unstable, slowly sliding volcano flanks in general and in particular to find out if Etna’s unstable flank may be prone to collapse. To observe these slow volcanic flank movements, the deformation of the ground needs to be monitored in detail. The unstable flank is bound by fault systems. One side of the fault moves downhill while the other is more or less stable. The displacement across these bounding faults is a good indicator for possible acceleration of the flank movement. Linking onshore and offshore deformation is necessary in order to understand the failure mechanism of Etna’s flank. The researchers want to understand how strain is distributed over time and space along the fault system that bounds Etna’s moving flank and how the fault activity is related to regional tectonic, seismic, or magmatic activity.

Since decades, the dynamics of these faults, the movement of the flank and the deformation of the volcano have been monitored and analysed by a multi-disciplinary approach by the Italian National Institute of Geophysics and Volcanology, INGV Catania: From space via Satellite-based ground deformation observations - GPS or Interferometric Synthetic Aperture Radar measurements (InSAR) and on land with extensometers that measure the deformation across major faults.

Researchers from PRE-COLLAPSE and related projects want to record the fault movements in more detail at several terrestrial and marine sections of the unstable flank. Since 2016 an acoustic geodetic observatory network consisting of five seafloor geodetic transponders measures the movement of the fault in the offshore part of the moving flank at the seafloor offshore the East Coast of Sicily. The GeoSEA array was deployed on a seafloor structure which is the fault that marks the southern boundary between the downwards sliding southeastern flank of the volcano and its stable surroundings, called “North Alfeo Fault”, which reaches far into the Ionian Sea. The downwards movement of the unstable sector causes distance changes between these transponders over time. Just in front of the coastline, the Alfeo Fault seems to join the offshore continuation of the Acitrezza Fault, that splays into two major segments at the onshore part of the volcanic flank, towards the Etna summit into the Timpe Fault System in the north and Tremestieri-Trecastagni Fault System, identifying the southern boundary of the unstable sector of Etna’s onshore flank.

Complementary to the offshore GeoSEA array, also on land, displacements of this fault system is now continuously measured. In July 2022, two IPOC-type creepmeters have been installed near the coast across the Timpe Fault System and another one is planned to be installed at the Tremestieri-Trecastagni Fault System to best understand the complex behaviour of the distributed faulting onshore. Both arrays will stay for the same observation period of the upcoming three to five years.

A team (Pia Victor, Silvia Crosetto and Thomas Ziegenhagen) from German Research Centre for Geosciences, GFZ Potsdam, Allessandro Bonforte from INGV Catania and others installed the first two creepmeters on the Macchia Fault, which is the northernmost part of the Timpe Fault System that branches of the Alfeo Fault. For the creepmeter installation, nine metre long rods are buried 1-2 m in the soil across the fault. They can detect very small movements of the ground with micrometre precision. A system monitors the relative displacement of the free end of the rod relative to the fixed endpoint. The fault slip is calculated as relative movement of the ground on either side of the fault plane. The creepmeters make two measurements per minute and therefore can detect the slip rate in a high temporal resolution. The Instruments are powered by a solar panel and the data are transmitted via a mobile phone connection that provides direct data access at GFZ Potsdam.

The monitored Macchia fault is a creeping fault but also experienced shallow seismic rupture in the last centuries. Fault creep is the term for the slow, constant slippage that can occur on some active faults without there being a rupture and thus earthquakes. Creeping behaviour happens on all kinds of faults, but it is most obvious and easiest to visualise on strike-slip faults, which are vertical cracks in the earth crust whose opposite sides move sideways with respect to each other, such as those at Etna’s eastern flank. The 2018 Mw 4.9 Fleri earthquake, rupturing the Fiandaca Fault for example occurred at only 1 km depth but along 8 km. Aseismic creeping occurs constantly and also in episodic events that last for several days. These can occur after major earthquakes such as in the case of the Fleri earthquake, but can also happen without any preceding seismicity.  

By simultaneously measuring fault motion offshore and on land over the next few years, the researchers hope to detect the co-occurrence of slip events on Etna's south-eastern flank that occur spontaneously or potentially triggered by volcanic inflation or deeper seated earthquakes. With the two measurement arrays covering the same overarching fault system, they could identify whether the movement starts offshore and propagates onshore, or if it's the other way around. These data could help understand the driving mechanism of the flank movement: Is it gravity that pulls the flank down, or is it movements in the magma chamber that pushes the flank?

Map of Etna’s Southeastern flank showing the major faults and the positions of the Creepmetre and Geodesy (GEOSEA) arrays.

Offset of a gate at San Leonardello Fault (picture: Morelia Urlaub)

The trench for the creepmetre installation in July 2022. (picture: Pia Victor)

The creepmetre with sensor in the front and the rod crossing the Macchia Fault. (picture: Pia Victor)

First signlas of the creep sensors after installation on the Macchia Fault, showing the closing of the trench. (GFZ Potsdam)