GNGTS 2021 - Atti del 39° Convegno Nazionale

55 GNGTS 2021 S essione 1.1 SEISMOTECTONIC OF THE GUBBIO NORMAL FAULTS SYSTEM AND THE MAY 2021 SEISMIC SWARM M. Menichetti 1 1 Università di Urbino The Gubbio SW dipping extensional faults system is affected by several historical and instrumental moderate earthquake swarms, with thousand low-intensity events, with only a few greater Mw>3 including the sequence of 2013/14 and May 2021 with Mw 3.9 (INGV, 2021). The geology of the area is characterized by a normal fault with NW-SE strike, has an offset of more than 1000 m distributed in several splays, with a total regional extension of 15 % and downthrow the SW limb of a rootless NE verging anticline (Menichetti & Minelli 1991). The quaternary fault activity presents superficial structural and geomorphological expression vis- ible in the field for about 30 km, including the stratigraphic records of Pleistocene intramoun- tain basin. At least three fault segments are kinematically linked with an overall geometry south westward concave. The fault plane is composite, well exposed in the Meso-Cenozoic calcareous rocks, hidden in the mountain slopes where several morphological steps mark the fault-line. The various components of displacement and the geometric relationship related to the bedding attitude show a different mechanism of faults development in different litholo- gies. The fault planes have an inclination variable from 30° to 80°, related to the mechanical properties of the host rocks. Extensional structures originated along the slope involve the pied- mont along the master fault plane trace, producing trenches and historical landslides. Complex and well organized extensional shear zones are localized in calcareous and marls outcrops but also the clay and silts of the Pleistocene fluvio-palu-lacustrine facies (Menichetti, 1992). The evolution of the hydrographic network record the fault activity, where the stream and chan- nels, present catchments with altimetric anomalies related to the uplift of the footwall and the evolution of the hangingwall (Menichetti, 1992). In the last decades, several works have already been done on understanding the predominant seismotectonic frame, integrating active and passive geophysical seismic interpretations. How- ever, in this area, only a few field works and structural studies have focused on the geological ex- pression of the extensional structures (Menichetti & Minelli, 1991; Brozzetti, 1995; Boncio 1998). The geophysical approach leads the model of NE dipping low angle normal faults know in litera- ture as Alto Tiberina faults (Pialli et al ., 1998). This model has mainly based on the interpretation of industrial seismic reflection profiles (Bally et al ., 1986) including CROP 03 (Barchi et al ., 1998). Then the numerous outcrops of the SW dipping planar normal faults in the Umbria Preappenines, are interpreted in-depth with listric geometry to resolve the difference in inclination between the planar surface measurements in the field and seismic nodal plane dips. Accuracy of earthquake locations of few seismic sequences and the aftershock distributions from Colfiorito (Chiaraluce et al ., 2003), Gualdo Tadino (Ciaccio et al ., 2005), Pietralunga (Marzorati, et al , 2014; Latorre et al ., 2016) to Gubbio (Chiaraluce et al ., 2007), clearly show that the aftershock alignments with a planar fault continuity from surface to depth. The focal mechanisms of the seismic sequences of 2010, 2013/14 and May 2021 with Mw 3.9, show nodal planes that dipping towards SW and NE with average dip angles of 60° and 35° respectively. The comparison of hypocentral locations, nodal plane dips and the seismic swarm’s volume, with field structural data, show that they fall within the down-dip projection of the sur- face trace of the faults.