The high seismic productivity of volcanic areas provides the chance to investigate the local stress conditions with great resolution, by analysing the slope of the frequency-magnitude distribution of earthquakes, namely the b-value. Here we investigated the seismicity of Mt. Etna between 2005 and 2019, focusing on one of the largest known episodes of unrest in December 2018, when most of the intruding magma aborted, rather oddly, its ascent inside the volcano. We found a possible stress concentration zone along magma pathways, which may have inhibited the occurrence of a larger eruption. If the origin of such hypothetical loaded region is related to tectonic forces, one must consider the possibility that geodynamic processes can locally result in such rapid crustal strain as to perturb the release of magma. Strong b-value time-variations occurred a few days before the unrest event, suggesting new possibilities for investigating the volcano state and impending eruptions.

Abstract. The Timpe Fault System is the source of very shallow but destructive earthquakes that affect several towns and villages on the eastern flank of Mt. Etna (Italy). In 1984, several seismic events, and specifically on October 25, caused one fatality, 12 injuries and produced serious damage in the Zafferana and Acireale territories. This seismicity was mainly related to the activity of the Fiandaca Fault, one of the structures belonging to the Timpe Fault System. We inverted ground deformation data collected by a geodimeter trilateration network set up in 1977 at a low altitude along the eastern side of the volcano in order to define the Timpe Fault System faulting mechanisms linked to the seismicity in 1984. We found that in the May 1980–October 1984 period, the Fiandaca Fault was affected by a strike slip and normal dip slip of about 27 and 23cm. This result is in fairly good accord with field observations of the co-seismic ground ruptures along the fault but it's notably large compared to displacements estimated by seismicity, then suggest ing that most of the slip over the fault plane was aseismic. The results once again confirm how seismicity and in particular ground ruptures represent a very high hazard to the several towns and villages situated along the Fiandaca Fault.

<p class="p1">The time scale of ground displacement at volcanoes varies between short, sub second seismic events, to days, months or even years. This study is focused on data from seismic and GNSS stations located around Mount Etna. The GNSS and seismic stations operate at different time scales. Data from these different time scales is extracted and combined in order to better understand the subsurface dynamics. The overall aim of this research is to improve volcanic forecasting and monitoring. It does this in a novel way by applying signal processing and machine learning techniques to the rich dataset.</p> <p class="p1">Mount Etna offers an interesting case study as it is a widely monitored volcano with a variety of sensors and with a rich pool of data to analyse. Additionally the volcanic dynamics at Mount Etna are complex. This is a volcano where there is a variety of different sub-surface dynamics due to the movement of both deep and shallow magma. This allows for rich insights to be drawn through the combination of different signal types.</p> <p class="p1">This study looks at combining the information obtained from the seismic array at Mount Etna, with the information obtained from various GNSS stations on the volcano. The seismic array has been able to capture ground velocity data in the frequency range 0.025 Hz to 50 Hz from a range of stations at different locations across the volcano. The GNSS stations measure ground displacement with a sampling frequency of 1 Hz, and they allow for longer term ground dynamic analysis.</p> <p class="p1">We analyse different seismic events, and relate the type and number of the seismic events to the long term ground deformation that we see in the recorded GNSS data. Where links between the two signal types have been identified, research is ongoing to establish a direct connection with known volcanic activity on Mount Etna. This will help establish what the relationship that we are seeing signifies. This integration of data from different types of sensors is a significant step into bridging the gap between seismic and quasi-static ground displacement at active volcanoes and should open the path toward more in depth volcanic monitoring and forecasting.</p>

<p>Since the Late Pliocene - Early Pleistocene, the Calabrian Arc (southern Italy) is affected by extensional and transcurrent tectonic superimposed on the previous collisional context. Various seismogenic sources have been proposed over time to explain such a complex structural framework, but the topic is still matter of debate. </p> <p>In this work we apply a multidisciplinary approach, concerning Geology, Geomorphology, Seismology and Geodesy, to develop a reliable 3D model of the Cittanova and Serre faults. These faults are considered the causative faults for the 1783 seismic sequence (M 6.5-7) as proposed by <strong>Jacques et alii (2001)</strong>. We used CROP data to investigate the crustal architecture of the area and to constrain the geometry at depth of the major structures. through two schematic geological sections orthogonal to these two faults. The shallow geometric patterns of the Cittanova and Serre faults, were verified trough geological, geomorphological and structural field data. Earthquakes hypocentres were analysed and relocated in order to recognize possible cluster alignments useful to constrain the faults geometry at depth. The high-density level of crustal seismicity attests that this domain is seismically active, between 0 km and 23 km and it concentrates along the main faults. To compute the strain and velocity field of the area (time span of the last 20 years) we measured the IGM95  (Instituto Geografico Militare) benchmarks and processed several GNSS permanent stations belonging to the RING Network (http://ring.gm.ingv.it) and TopNETlive Italy Network (https://rtk.topnetlive.com/italy/networks/topnet-live-italy) using GipsyX 1.5 Strain inversion (performed through grid_strain 2D software) allowed us to define a predominant WNW-ESE extensional deformation, in agreement with previous studies). Combining all previous data, we built for the first time a reliable 3D model of the Cittanova and Serre fault planes, that are consistent with:  i) fault magnitude/size empirical relations (Magnitude vs rupture Area, Magnitude vs fault length; ii) geological and geomorphological field observation (fault attitude and kinematic), iii) seismological and geodetic data. Results show that our model is compatible with the seismogenic sources of the 1783 seismic sequence.</p>

<p>The gravity time series from two iGrav superconducting gravimeters and an AQG-B absolute quantum gravimeter, installed at Mt. Etna volcano, reveal a marked gravity decrease during early-June to late-July 2021, a period when more than 20 short-lasting and strongly explosive eruptions (the so-called lava fountain events) took place from one of the summit craters of the volcano. GNSS data indicate that this phase of gravity decrease was associated to deflation of the volcano edifice. <br />We performed a joint inversion of the gravity data and deformation field to define the parameters of the common mass/pressure source. The optimal source is located beneath the summit crater area of Mt. Etna, at a depth between 2 and 3 km below the sea level. Results of the joint modeling also point to a residual mass change that is largely in excess of the corresponding volume change, for any reasonable density of the material extracted from the source. <br />We propose that the observed gravity decrease was mostly driven by a decompression-related density change, i.e., an increase in the proportion of exsolved gas to magma in the source reservoir. This hypothesis is checked through comparing the results of the geophysical data inversion with independent estimates of the change in exsolved gas content due to pressure decrease in the source reservoir.</p>