Seismogenic evidence of ongoing compression in eastern-central Italy and mainland Sicily: a comparison

Abstract

We compare the seismogenic role played by two crustal-scale reverse-type shear zones located at the outer border of the Apennine- Maghrebian fold-and-thrust belt system: the westward-dipping Adriatic Basal Thrust (ABT) in eastern-central Italy and the northward-dipping Sicilian Basal Thrust (SBT) in southern Sicily. The epicentral and hypocentral distributions and, whenever possible, the kinematics of a merged dataset of pre-instrumental (1600 to 1900), early instrumental (1901 to 1980) and instrumental (1981 to 2005) earthquakes with moment magnitude ≥4.5 sited within the surface areas above the ABT and the SBT depth-contour lines (0-30 km) are analysed. They are both first-order crustal-scale seismogenic structures which strongly control reverse, reverse-oblique and strike-slip seismic activity. Relatively deep (10-to-30 km) seismicity occurring in the foothills region and in main- land Sicily may be associated with brittle shearing of the middle and lower crust ABT and SBT thrusts segments, whereas the shallow seis- micity (<10 km) close to the ABT and SBT surface tip line is associated with reverse, oblique and strike-slip shearing of the two upper crust thrust segments and their frontal and lateral splays. Despite that the seismic activity within the two studied areas, and especially in Sicily, is commonly considered minor, several highly damaging earthquakes have struck both areas. In the last four centuries, there is knowledge of 37 events with magnitude >5.0 in the Marche-Coastal Adriatic region, among which are the 1741 Fabrianese (Maw 6.08) and 1799 Camerino (Maw 5.93) events, and of 10 events with magnitude >5.0 in southern and mainland Sicily, including the 1818 Catanese (Maw 6.00) and 1968 Valle del Belice (Maw 6.12) events. Based on the integration of tectonic and seismological con- straints, the boundaries of two seismogenic compressional provinces located above the ABT and the SBT are defined and the associated values of yearly energy release per unit area evaluated. Similar val- ues are obtained for the ABT (~8E+14 erg/y/km2) and for the SBT (~4E+14 erg/y/km2). The recognition of close geometric, kinematic and seismotectonic similarities between the two compressional provinces, further supported by a comparable level of long-term seis- mic activity, has evident implications both for regional tectonic reconstructions and for seismic hazard assessment purposes.