GNGTS 2015 - Atti del 34° Convegno Nazionale

GNGTS 2015 S essione 1.1 Coulombfailurestresstransferandfault interactionatEtnavolcano: some case-histories in the Timpe fault system R. Azzaro, V. Maiolino, M. Palano Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Etneo - Sezione di Catania, Catania Introduction. In this study we adopt a well-established approach based on the Coulomb failure stress transfer theory to study fault interaction processes at Mt. Etna. Coulomb stress transfer analyses have been applied to this area in order to verify the interaction between magma sources represented by intrusions in the central part of the volcano and tectonic structures located on its flanks. Modelled case-histories showed that volcanic influence have a significant role in promoting or inhibiting fault activity, both before the onset of flank eruptions (Gresta et al. , 2005; Mattia et al. , 2007; Currenti et al. , 2008; Gonzàlez and Palano, 2014) and after the end, when dynamics due to stress readjustment at the scale of the volcanic edifice are usually relevant (Bonanno et al. , 2011; Bonaccorso et al. , 2013). By contrast, “fault contagion” seems to be a possible mechanism of interaction at Etna. Seismic activity migrating from fault to fault during seismic swarms in the eastern sector of the volcano is documented in several cases (e.g., Gresta et al. , 1987, Patanè et al. , 2003; Barberi et al. , 2004), and poses the question if the main source of stress perturbation remains the “volcanic system” or not. On the other hand, evidence for interaction between nearby faults following strong earthquakes (M ≥ 4.0) has been historically observed in the Timpe system, the main seismogenic zone of the Etna region (Azzaro, 2004). In our analysis we consider couples of earthquakes close in space and time – from hours to one month and 3-6 km apart, respectively – whose causative fault can be recognized by the occurrence of coseismic surface faulting (Azzaro, 1999). We performed ����� �� ����������� ��� tests by considering ���the Coulomb failure stress change ���� ����� �� ����� �� ���� �� ��� ���� �� ��� �������� ��� ����� from fault to fault as well as the role of the regional and local stress fields on the “optimal faults”. It has �� �� �������� ��� ��� ����������� �� ���� �������� �� ���� ��� ���� ����� �� ��� to be stressed how the application of this approach at Etna may shed light on the mechanics of faulting but also represent an complementary tool for short-term earthquake rupture forecast, in order to improving seismic hazard assessment in a densely populated area of the volcano (Azzaro et al. , 2013b). Active tectonics and seismicity. Mt. Etna is a Quaternary basaltic stratovolcano located along the eastern coast of Sicily between two first-order tectonic elements: the Apenninic- Maghrebian Chain and the Hyblean Foreland (Branca et al. , 2011) (Fig. 1a). ���� ���� �� This area is characterised by intense geodynamics involving the entire scale of the volcano, with the main process being represented by flank instability which affects the eastern sector of the volcanic edifice. The continuous ESE seaward sliding indeed represents the result of the interaction among regional stress regime, magma intrusion and basement geology (Azzaro et al. , 2013a). In fact, t�� �������� ��� ������� ������� �� ��� ������� ��� ���� ����������� ��� ����������� he northern and western sectors of the volcano lie over metamorphic and sedimentary rocks belonging to the frontal nappes system of the Apenninic-Maghrebian Chain, whereas the southern and eastern ones (i.e. the unstable sector) overlie marine clays of Quaternary age, deposited on the flexured margin of the northward-dipping downgoing Hyblean Foreland (Lentini et al. , 2006). �������� �� ������ ��������� �� ������ ����������� ���� ��� �������� ������� Evidence of active tectonics is mostly distributed over the unstable sector, with a number of volcano-tectonic features controlling dynamics of this area. In particular, the three main faults zones are ������������� ���� ��� ����� �� ��� ����� ����� ��� �� ��� ��������� distinguished from the north to the south � �� ��� ��������� (Fig. 1): i) the Pernicana fault, ii) the Tremestieri-Trecastagni fault system and iii) the Timpe fault system (TFS). Since the role played by these tectonic systems in the geodynamics at a local scale is not relevant in this study, in the following we focus on TFS by describing the main features. TFS crosses the central part of the eastern flank in form of a wide belt of mainly extensional structures, showing well-developed morphological scarps interrupted by hidden sections of the fault (Azzaro et al. , 2012). ��� ���������� ��� �� ����������� ������ ������� ��� ����� The Moscarello and S. Leonardello faults dissect the lower part of the eastern flank ���� � ������� ����� ��� ��� ������������� �� ������������ �������� with a NNW-SSE trend ��� ��� ������������� �� ������������ �������� and are characterized by prevailingly vertical

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