A feasibility study on the near real-time calculation of the complete seismic moment tensor of the Etna Seismicity: application to the earthquakes occurred during the December 2018 eruption

In a volcanic area, seismicity is characterized by different types of seismic signals, due to the complex interac􏰓on between tectonic stress and volcanic dynamics. Referring to the Etna volcano, seismicity mainly consists in earthquakes associated with the fracturing processes of the rocks and seismic signals mainly caused by fluid dynamics (i.e tremor, LP and VLP events). Complete Seismic Moment Tensor computa􏰓on allows to a complete defini􏰓on of the seismic source through the inversion of the waveforms recorded by the seismic sta􏰓ons. It permits to calculate not only the seismic source Double Couple component (DC), which allows iden􏰓fying the nodal planes of the focal mechanism but also the non ­ DC components. Among the la􏰒er, the Compensated Linear Vector Dipole (CLVD) component can be indica􏰓ve of the genera􏰓on of len􏰓cular cracks and fluids dynamics, while the volumetric component (ISO) is an indicator of the volume varia􏰓ons due to explosions or implosions. INGV rou􏰓nely computes real­􏰓me automa􏰓c seismic moment tensor solu􏰓ons in Italy for Ml greater or equal to 3.5 earthquakes. In this case, the implemented algorithm (TDMT, Dreger et al., 2003), widely employed in many observatories around the world, it is applied mainly to study the DC component of the moment tensor, and the ISO component is constrained to zero. However, for seismicity in a volcanic environment, retrieving the non­DC component is important since it can provide useful insights into the understanding of the source origin and to evaluate the role of magma in the genera􏰓on of earthquakes. Therefore, it would be interes􏰓ng to compute the full moment tensor and to tune the algorithm for this special area. To achieve our goal, we applied the TDMT algorithm to 10 earthquakes with Ml greater or equal to 3.5 occurred in the Etna volcano area during the December 2018 erup􏰓ve episode. To calculate the moment tensor, we used different velocity models and sta􏰓on configura􏰓ons to account for the peculiarity of Etna volcano seismicity and we tested the robustness of the retrieved non­DC components. Finally, the obtained focal mechanisms have been compared with independent es􏰓mates of fault plane solu􏰓ons computed by the first polari􏰓es analysis.