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Etna, the most active volcano in Europe, is subject to intensive surveillance by Italian scientists and authorities. Satellite measurements showed that the southeast flank of the volcano was sliding slowly towards the sea, while the other slopes were largely stable. To date, it is unclear whether and how the movements continue under water because it is impossible to perform satellite measurements under the surface of the ocean. With the new geodesic GeoSEA seabed monitoring network, scientists from the GEOMAR Helmholtz Kiel Ocean Research Center, Kiel University, the Kiel Marine Science priority research area and the National Pollution Research Institute and Volcanic Geology (INGV) now has first time the horizontal and vertical motion of a submerged volcanic flank.
The results confirm that the entire south-east flank is moving. The driving force of the flank movement is probably the gravity, not the ascent of the magma, as previously assumed. A catastrophic collapse affecting the entire flank or large parts of it can not be ruled out and would cause a major tsunami with extreme effects in the region. The results of the study were published today in the international journal Progress of science.
"At Etna, we used for the first time a marine-based underwater surveying network, called" marine geodesy, "says Dr. Morelia Urlaub, lead author of the study. directed the investigations in the framework of the project "MAGOMET – Marine Geodesy for the Mount Etna sea monitoring." In April 2016, the GEOMAR team placed a total of five acoustic monitoring transponder stations on the line of a fault that represents the boundary between the sliding flank and the stable slope. "We placed three on the slippery area and two on the presumed stable side of the fault line," said Dr. Urlaub.
During their mission, each transponder would beep every 90 minutes. As the speed of sound in water is known, the transponder signal travel time has provided information on the distances between transponders on the seabed with an accuracy of less than one centimeter. "We found that in May 2017, the distances between the repeaters located on either side of the fault had clearly changed.The sidewall slid four centimeters to the sea and decreased by one centimeter in eight days, "says Dr. Urlaub. This movement can be compared to a very slow earthquake, a "slow slip event". It was the first time that the horizontal movement of such a slow slip event was recorded under water. In total, the system provided data for approximately 15 months.
A comparison with satellite deformation data from the ground showed that the southeast flank above sea level moved at a similar distance during the same period of observation. "So the whole south-east flank has changed position," says Dr. Urlaub.
"Overall, our results indicate that the slope is slipping due to gravity and not because of rising magma," she continues. If the magma dynamics in the center of the volcano caused the deformation of the flank, the displacement of the flank should be greater on land than under water. This is crucial for hazard assessments. "The whole slope is in motion because of gravity, so it is quite possible that it will collapse catastrophically, which could trigger a tsunami throughout the Mediterranean," says Professor Heidrun. Kopp, coordinator of the GeoSEA network and co-author of the study. However, the results of the study do not predict if and when such an event could occur.
"Further fundamental research is needed to understand the geological processes around Etna and surrounding areas, as well as other coastal volcanoes.Our survey shows that the sound-based geodetic monitoring network can be Valuable help in this regard, "summarizes Dr Urlaub.
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