Description
For a seismotectonic study, it is essential to have a seismic catalogue with good quality earthquake localisations, especially in terms of epicentral coordinates, depth, estimates of localisation errors, magnitude, as well as an adequate knowledge of the regional geology and the rheological principles governing rock deformation mechanisms.
The area of Trentino and its surrounding areas is characterised by the presence of three main tectonic systems, namely the Valsugana fault system (with ENE-WSW orientation), the Giudicarie fault system (with NNE-SSW orientation) and the Schio-Vicenza fault system (with NW-SE orientation). These fault systems have acted at different times in the history of the Alpine chain and are today seismically active.
The current deformation of the faults belonging to these tectonic systems is confirmed by the focal mechanisms, which provide information regarding fault geometry and type for each individual seismic event.
The structure and seismic properties of the Earth's crust in this sector of the central-eastern Southern Alps were determined from a crustal scale seismic tomography study. The investigated crustal volume shows important lateral variations in terms of P (VP) and S (VS) seismic wave velocities, interpreted as lithological and structural variations. Crustal bodies with high velocities (VP> 6.3-6.5 km s-1) represent the magmatic type complexes at depth, while the sedimentary coverings show lower velocities (VP= 5.5-6.3 km s-1), in good agreement with what has been reported in the scientific literature for other areas of the Alps. The VP/VS ratio is very useful for characterising the degree of fracturing and/or presence of fluids within the Earth's crust. High values of this parameter (VP/VS= 1.80-1.90) are observed in correspondence with active geological structures, such as the Southern Giudicarie, suggesting high fracturing of rocks and a probable presence of fluids along these fractures (see figure). Along the southern Giudicarie the observed anomaly is fully compatible with the available geological information and its shape, orientation and thickness suggest important information regarding seismic hazard, in terms of correlation between structures and seismicity for a better definition of seismic potential. In fact, the northern portion of the Giudicarie fault system, where seismicity is much lower in terms of both numerosity and magnitude than in the southern sector, does not present this VP/VS anomaly.
The results of the seismic tomography allowed the construction of a three-dimensional model of seismic velocities at the crustal scale. This model is used to localise recorded seismic events with greater precision and accuracy, so that seismic hypocentres can be interpreted in relation to regional tectonic structures.
Sezione crostale lungo le Giudicarie meridionali (traccia A–B in mappa). Nella sezione in alto è riportata la geologia semplificata, con litologia (colore) e faglie principali (linee nere) (verde: basamento Sudalpino; marrone: vulcaniti, granitoidi e rocce sedimentarie di età Permo-Carbonifera; rosso: plutoni e vulcaniti di età Terziaria; giallo: coperture sedimentarie delle Alpi Meridionali). Nella sezione in basso e nella mappa a lato i colori rappresentano il rapporto tra le velocità delle onde sismiche P ed S (VP/VS). Sono riportati anche i meccanismi focali del terremoto di Salò (S, 24 novembre 2004) ed Avio (A, 29 ottobre 2011), classificati in base alla magnitudo locale (ML). La geologia sovrapposta alla sezione è ricavata da Picotti et al. (1995). La mappa si riferisce ad una profondità di 5 km s.l.m.; le aree oscurate rappresentano quelle dove il risultato dalla tomografia sismica non è interpretabile; le linee in colore bianco sono le faglie principali.