The geometry of the superficial magma chamber of the Stromboli volcano, located between 2 and 4 km deep, has been defined for the first time, thanks to the images acquired with tomographic techniques. The results of the study, conducted by INGV, were published in Geophysical Research Letters
A high-resolution tomography of Stromboli made it possible to precisely define the power system and the geometry of the magma chamber. It was developed by a team of researchers from the National Institute of Geophysics and Volcanology (INGV) - Etna and Vesuvius Observatories, National Earthquake Center (CNT). The study was published in Geophysical Research Letters (http://onlinelibrary.wiley.com/doi/10.1002/2017GL073008/full)
"The project", explains Domenico Patanè, research manager of INGV-OE, was born from the need to better understand the internal structure of the volcano in order to try to define the feeding system and try to identify the magma chamber". For the occasion, 20 temporary seismic stations were installed on the island, in addition to the 13 of the permanent seismic network, integrated by 10 seabed seismometers (Ocean-Bottom Seismometers, OBS), which allowed, for the first time, the exploration of the submarine part of the volcano”.
"The geophysical and geochemical monitoring and surveillance system of Stromboli in recent years has been considerably strengthened by INGV, above all following the eruptive crisis of 2002-2003, with the tsunami of December 30, 2002 and with the paroxysmal event of April 5 2003”, continues Patanè.
The creation of a denser seismic network, the installation of new ground deformation measurement stations (GPS) and geochemical stations were followed by various studies and research for the mitigation of volcanic risk.
Among these, the first seismic tomography experiment which is part of the framework agreement with the Department of Civil Protection (DPC-INGV 2004-2006), conducted at the end of 2006.
"As in other seismic tomography studies, conducted in volcanic areas (e.g. Etna, Vesuvius, Campi Flegrei, Vulcano), seismic images were also obtained at Stromboli", adds the INGV research manager, "which are similar to those acquired by a CT scan medical scan but where, instead of X-rays, seismic waves are used to distinguish rocky bodies with different densities. Seismic waves propagate faster through cold rock, and slower through hot or partially molten rock.
The research integrated the data acquired on board the oceanographic vessel Urania of the National Research Council (CNR) during the 2006 scientific campaign, carried out in collaboration with the Institutes of Marine Sciences and for the Coastal Marine Environment of the CNR, Department of Science of Earth from the University of Florence and Instituto Andaluz de Geofísica, Universidad de Granada (Spain), with recordings of local seismic events from the permanent network.
“It was possible to define, for the first time”, says Patané “the geometry of the superficial magma chamber of Stromboli, located between 2 and 4 km below sea level, which extends from the island to Strombolicchio. The Faraglione Strombolicchio represents the "central chimney" (neck) of the ancient volcano that emerged about 200.000 years ago to the north-east of the current island of Stromboli, today almost totally eroded by exogenous agents. The seismic images show its deepest feeding system that connects the magma chamber with the neck of Strombolicchio".
In accordance with the most recent geochemical and petrological studies, "the tomographic images show two anomalous regions at different depths with different physical characteristics, which contain the magma that currently fuels the persistent activity of Stromboli", continues Patanè.
Furthermore, the inclination of the current power supply system towards the Sciara del Fuoco could explain its propensity to generate landslides of large dimensions (as happened in 1930 and in 2002), during the opening of eruptive fractures, following the increase in magma pressure in this already unstable sector of the volcano.
“Today, thanks to tomographic images of the surface crust, we have a 3D physical model of the volcano's velocity structure and we know the geometry of the magma chamber. The knowledge of the 3D velocity structure of the volcano, in addition to being able to be used to improve the determination of the source parameters of local seismic events, will allow a better modeling of volcanic phenomena in the future, aimed at forecasting studies of eruptive activity". concludes Patanè.
Extended
The shallow magma chamber of Stromboli volcano (Italy)
In this work, we integrate artificial and natural seismic sources data to obtain high-resolution images of the shallow inner structure of Stromboli volcano. Overall, we used a total of 21,953 P readings from an active seismic experiment and an additional 2,731 P and 992 S readings deriving from 269 local events. The well-defined Vp, Vs and Vp/Vs tomograms have highlighted: i) the region where magma cumulates at shallow depths (2-4 km bsl), forming an elongated NE-SW high velocity body (Vp ≥ 6.0 km/s and Vs ≥ 3.5 km/s), with a very fast velocity core (6.5 ≤ Vp < 7.0 km/s) of ~ 2 km3; ii) the presence of some near-vertical pipe-like structures, characterized by relatively high P-velocities values, mainly linked to past activity (eg Strombolicchio) and iii) a near-vertical pipe like volume with high Vp/Vs (1.78÷ 1.85), located beneath the craters (down to ~ 1.0 km bsl), overlying a deeper region (1.0 to 3.0 km bsl) with low Vp/Vs (1.64÷1.69), interpreted as the actual and preferential pathway of magma toward the surface.
Our results demonstrate the importance of combining passive and active seismic data to improve, in a tomographic inversion, the resolution of the volcanic structures and to discover where magma may be stored.
Photo 1 - Stromboli volcano
Figure 2 - 3D schematic model of Stromboli's internal structure
Photo 3 - Mobile seismic station (Etneo Observatory, INGV)
Photo 4 - Release phase of an OBS (Ocean-Bottom Seismometers)