GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 1.1 GNGTS 2023 Seismological Characteristics of the Apennines External Compressive Thrust Front: Seismicity Analysis in The Marche Region (Central-Northern Italy) D. Maceroni 1 , S. Marzorati 1 , V. Castelli 2 and C. Ladina 1 1 Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Nazionale Terremoti, Italy 2 Istituto Nazionale di Geofisica e Vulcanologia, sezione di Bologna, Italy The Marche region belongs to the geologic and seismotectonic setting of the central-northern Apennines, whose seismicity concentrates along two distinct belts: (a) the inner Apennine range, characterised by extensional tectonics that manifests through normal fault activity, and (b) the external region of the Apennine belt, comprising the coastal area and the offshore sector of the Adriatic region, characterised by compressive tectonics, with the activity of reverse faults (thrusts), related to the migration of the Apennines compressional fronts toward the easternmost sectors. While evidence of Quaternary activity along normal faults can be investigated by direct geologic methods (e.g., geological field surveys, aero-photographic analyses, paleoseismological investigations), evidence of the presence and long-term activity of thrust fronts affecting this outer region are mostly derived from indirect investigations, based on geophysical data (such as seismic reflection lines) corroborated by borehole data. These pieces of evidence highlight the structural pattern of the external Apennines thrust front, characterised by a “thrust-related fold” deformation style, associated with thrusts showing evidence of activity in the Quaternary. The recent activity of these structures manifests through historical and instrumental seismicity, characterised by earthquakes of moderate-large magnitude (M w ≥ 5.0-5.8), with epicentres in the Adriatic offshore and along the coastal areas (Fig. 1a). As for historical seismicity, the M w 5.8 Senigallia earthquake in 1930 is one of the largest earthquakes known in the historical seismic catalogues; it affected a vast area in the Marche coastal area, with damage estimated with Intensity of up to 8-9 of the MCS (Mercalli – Cancani – Sieberg) scale. As for instrumental seismicity, consistently to the 2012 Emilia seismic sequence, occurred along the Apennine thrust front to the NW, the seismic sequence that began on November 9, 2022 (M w 5.5) is an example of the described compressive seismotectonic setting, as it has been generated by a reverse structure located offshore, along the outermost thrust front of the Apennine range in this sector (Fig. 1b). The analysis of the historical and instrumental seismicity can contribute to unravel the structural complexity of the Apennines external thrust fronts and to derive possible earthquake scenarios in terms of expected magnitudes associated to single seismogenic segments. The acquisition of data