DOI: 10.13127/emts/2008_2020
title: Moment Tensor Solutions for ML ≥ 3.5 Etna earthquakes 2008-2020 (EMTS_2008_2020)
title: EMTS_2008_2020

description: We computed the full seismic moment tensor of the earthquakes (3.5<ML ≤4.8) that occurred in the Etna area in the period May 2008 to December 2020, recorded by the seismic broadband network managed by the INGV - Osservatorio Etneo. 
The full seismic moment tensor computation, through the inversion of the seismic waveforms, allows for a comprehensive definition of an earthquake source. In particular, the moment tensor is directly related to the earthquake fault orientation and kinematics, while the derived moment magnitude, Mw, is the most reliable quantity for measuring the size of an earthquake. In addition, the source tensor can provide information on the decomposition into isotropic (ISO), double-couple (DC) and compensated linear vector dipole (CLVD) components. This distinction is a tool for classifying and physically interpreting seismic sources. Indeed, although most tectonic earthquakes are dominated by shear deformation in narrow area (DC component of the tensor), in volcanic and geothermal areas, other processes, such as the migration of magmatic and hydrothermal fluids or rupture on non-planar faults, can produce earthquakes with significant non-double-couple components. (e.g. Minson et al. 2007; Saraò et al., 2010; 2016).
description: Abbiamo calcolato il tensore completo del momento sismico dei terremoti (3.5<ML ≤4.8) localizzati nell'area etnea nel periodo 2008-2020, registrati dalla rete sismica a larga banda gestita INGV - Osservatorio Etneo. 
Il calcolo del tensore del momento sismico, attraverso l'inversione delle forme d'onda sismiche, consente una definizione completa della sorgente di un terremoto. In particolare, il tensore del momento è direttamente correlato all'orientamento e alla cinematica della faglia che lo ha generato, mentre la magnitudo-momento derivata, Mw, è la grandezza più affidabile per misurare la dimensione di un terremoto. Inoltre, il tensore della sorgente può fornire informazioni sulla decomposizione nelle componenti isotrope (ISO), a doppia coppia (DC) e dipolo vettore lineare compensato (CLVD). Questa distinzione è uno strumento per classificare e interpretare fisicamente le sorgenti sismiche. Infatti, sebbene la maggior parte dei terremoti tettonici sia dominata dalla deformazione di taglio (componente DC del tensore), nelle aree vulcaniche e geotermiche, altri processi, come la migrazione di fluidi magmatici ed idrotermali, o la rottura su faglie non planari, possono produrre terremoti con significativi componenti non a doppia coppia. (es. Minson et al. 2007; Saraò et al., 2010; 2016).
description: The full moment tensor solutions have been calculated using the software gCAP3D (https://www.eas.slu.edu/People/LZhu/home.html) based on the cut-and-paste (CAP) method by Zhu and Helmberger (1996) and improved by Zhu and Ben-Zion (2013). This method is based on the waveform inversion of Pnl and surface wave segments and has proven to be effective for analysing earthquakes over a wide range of magnitudes, even those with magnitudes between 2.5 and 4. 
The CAP method minimizes the misfit between observed and synthetic seismograms using a grid search to obtain the best moment magnitude, source depth and focal mechanism. The inversion technique breaks each waveform into Pnl and surface wave windows. This is because they are sensitive to different parts of crustal structure and have different amplitude decay with distance. The surface waves, although large in amplitudes, are easily influenced by shallow crustal heterogeneities, while the Pnl waves are controlled by the averaged crustal velocity structure and therefore are more stable. The fit is evaluated independently in each phase window and over different frequency bands for P and S waves. In comparison to the whole waveform approach, the separation of P and S waves in both time and frequency domains enhances the contribution of the P-waves.
In order to get reliable source mechanisms, it is necessary to compute synthetic seismograms, which in turn requires a reasonable velocity/attenuation model for generating Green’s functions. We used the frequency–wavenumber (F–K) integration method as described by Zhu and Rivera (2002) and the 1D velocity and attenuation models derived from Alparone et al. (2012) and Martinez-Arevalo et al. (2005). Synthetics and observed ground velocity were filtered in the same frequency bands, from 0.02 to 0.1 Hz for the surface waves and from 0.05 to 0.3 Hz for the Pnl.
The starting dataset of 1 D hypocentral locations, local magnitude and epicentral area is coming from the “Mt. Etna Revised and Concise Seismic Catalog from 1999 ETNA RCSC" (Alparone et al., 2020  https://doi.org/10.13127/ETNASC/ETNARCSC) which collects local earthquakes recorded by the Permanent and Mobile Seismic Network, managed by INGV-OE.
description: The catalog is divided vertically into three sections. In the first one, the origin time, location and local magnitude parameters of the selected earthquakes are reported; they are derived from “Mt. Etna Revised and Concise Seismic Catalog from 1999 ETNA RCSC" (Alparone et al., 2020). The second and third section show for each event the solutions of the moment tensor inversion, considering respectively a pure double-couple mechanism (2nd section) and the full solution, i.e. including DC, ISO and CLVD components (3rd section). Specifically, the reported parameters are the associated moment tensor depth (MT depth), the Moment Magnitude value (Mw), the strike, dip and rake of one nodal plane (in degrees), the Seismic Moment (M0), the six independent components of the tensor (Mxx, Mxy, Mxz, Myy, Myz, Mzz), and the isotropic and CLVD strength.