GNGTS 2021 - Atti del 39° Convegno Nazionale

93 GNGTS 2021 S essione 1.1 THE MT. ETNA DECEMBER 26, 2018 EARTHQUAKE: WHAT WE LEARNED ABOUT THE ACTIVE DEFORMATION AT THE EASTERN FLANK OF THE VOLCANO G. Tortorici 1 , S. Catalano 1,2 , F. Pavano 3 , G. Romagnoli 2 1 University of Catania, Department of Biological, Geological and Environmental Sciences, Italy 2 Institute of Environmental Geology and Geoengineering of the Italian National Research Council - Area della Ricerca di Roma 1, Italy 3 Department of Earth and Environmental Sciences, Lehigh University, Bethlem, USA The ground effects of the Mw 4.9 earthquake occurred on December 26, 2018 in the SE flank of Mount Etna provided a clear signature of the seismogenic source in terms of geometry, kine- matics and displacement, precisely fitting the focal mechanism (TDMT solution from INGV web- site, 2018) and the ground deformation evidenced by geodetic data (De Novellis et al ., 2019). The deformation picture of this episodic event was quite different from that normally evidenced by GPS and DInSAR data series (Bonforte and Puglisi 2006; Bonforte et al ., 2011; 2013; Palano 2016; Murray et al ., 2018), constraining a fragmentation of the eastern flank of Mt. Etna into several kinematic blocks, due to the eastward sliding of the shallow crustal horizons, at rate of about 1-2 cm/yr. In the interseismic periods, the permanent scatterers features imaged in the interfero- grams usually correspond to the tectonic boundaries of the sliding blocks. The 2018 seismic event shows several peculiarities that denounce its origin from regional dynamics, different from the active gravitative processes. The kinematics of the event is compat- ible with the contractional regime active at regional scale, due to the Nubia-Eurasia convergence (Romagnoli et al . 2021 and references therein). The earthquake produced a dm-sized ground displacement (Civico et al ., 2019), along a tectonic lineament falling within one of the mobile kinematic blocks, that obscured the bottom signal of the gravitative deformation (Bonforte et al ., 2019; De Novellis et al ., 2019). The ground fracturing developed along two linked fault segments forming an 8 km-long arc- shaped belt. Along the northern NW-SE oriented segment, the co-seismic fractures depict a well- defined dextral shear zone (Tortorici et al ., 2021). The southern NNW-SSE oriented segment dis- played a clear partitioning of the three components of motion, indicating an oblique extensional motion. In particular, the ground fractures accommodated the two horizontal components of motion (strike-slip and heave), while the vertical component produced the rejuvenation of a footwall flexure which is well imprinted on protohistoric lavas (3.9 ky B.P.-122 B.C.; Branca et al ., 2011). The cumulative height of the flexure, evidenced by a sharp 4 m high topographic break, would constrain a short-period vertical slip rate along the structure ranging from 1 to 1.9 mm/yr. The southern branch of the co-seismic belt remobilized part of a main tectonic boundary that extend from the eastern flank of the volcano to the Ionian offshore (Catalano et al ., 2013; Rom- agnoli et al ., 2021 and references therein). This lineament was active during the entire Late Qua- ternary and cumulated large amounts of vertical displacement as revealed by geophysical data (Cassinis et al ., 1970; La Delfa et al ., 2011). The fault displaces the top levels of the pre-Etnean marly clay substratum, hosting submarine tholeiitic lavas dated to about 500 ky B.P. (Branca et al ., 2008; De Beni et al ., 2011), for at least 600 m, with an average minimum throw rate of 1.2 mm/yr. The structure also caused a total vertical separation of of about 120 m that affects the basal unconformity of the alkaline lavas, dating back to about 150 ky B.P. (De Beni et al ., 2011), with a minimum vertical slip rate of about 0.8 mm/yr. Therefore, since the Late Quaternary, the seismo- genic source of the Mw 4.9 December 26, 2018 earthquake cumulated deformation rate 1 order of magnitude less than the active gravitative processes. In conclusion, the 2018 earthquake tells us that at Mt. Etna the effects of the deformation