Incipient extension along the active convergent margin of Nubia in Sicily, Italy: Cefalù-Etna seismic zone

Abstract

Recent geodetic data are compatible with NNE–SSW tectonic extension at a rate of ∼5 mm/yr in Sicily, southern Italy, within a broader region of net active compression along the Nubian plate margin (northern Africa). The structures that accommodate such extensional regime and its cause are still unknown. From field structural surveys and seismological analyses, the geometry, kinematics, structural architecture, and seismic potential of an extensional seismic zone linking Cefalù and Mount Etna in central eastern Sicily are defined. The zone includes high‐angle WNW striking normal and right‐lateral strike‐slip faults and subordinate north and NNE striking strike‐slip faults either right or left lateral. The occurrence of small discontinuous faults and the absence of related depressions and sedimentary basins suggest that the extensional regime is still in an incipient stage. The ongoing seismic activity possibly reactivates preexisting faults. Instrumentally and historically recorded earthquakes are lower than about 6 in magnitude, and destructive events are historically unknown since at least 1300 A.D. This apparent upper bound of earthquake magnitudes is consistent with the maximum magnitude values estimated from the length of the longest mapped faults and sources of seismic swarms, which all together suggest a value between 6 and 6.5 as the maximum expected magnitude that can be proposed at the present stage of investigation for earthquakes in the study area. Lateral extension on preexisting faults and upwelling of melt mantle material beneath Mount Etna are considered viable processes to explain, at least in part, the active extensional tectonics along the Cefalù‐Etna seismic zone. Strike‐slip seismic faulting beneath Mount Etna may be part of a previously proposed diffuse transfer zone affecting northeastern Sicily and including the Tindari Fault.

Stress field of Italy — Mean stress orientation at different depths and wave-length of the stress pattern

“…In the central portion of the Gela thrust front, the mean orientation is NNE-SSW perpendicularly to the thrust front itsself. In the southeast of Sicily the mean orientation change from NNW (Hyblean Plateau) to NE (to north of Malta escarpment) probably due to the presence of a transfer zone (Tindari Fault;Billi et al, 2010). The extensional tectonics are still active in the southeastern corner of the Tyrrhenian Sea (i.e., to east of Tindari Fault) and along the Ionian coast of Sicily and southern Calabria (Mattia et al, 2009;Neri et al, 2003Neri et al, , 2005.…”

The Ionian and Alfeo–Etna fault zones: New segments of an evolving plate boundary in the central Mediterranean Sea?

“…Etna to Cefalù along WNW-ENE trends (Neri et al, 2005;Billi et al, 2006Billi et al, , 2010Devoti et al, 2011;Palano et al, 2012). Although Lavecchia et al (2007) interpreted this incipient extension as ensuing from upper crustal stretching above an active thrust belt, Billi et al (2010) favor reactivation of pre-existing faults and upwelling of melt mantle material beneath Mount Etna. A better understanding of active faults in the Ionian Sea is thus crucial to define if upper plate fault systems are controlled by structural development in the offshore region and which are the regional processes driving recent plate-boundary re-organization.…”

“…Extensional processes and associated magmatism could be related to vertical upwelling of the asthenosphere at the SW lateral edge of the Ionian slab (Gvirtzman and Nur, 1999;Doglioni et al, 2001;Billi et al, 2010). Laboratory and numerical experiments show that at the edges of a retreating slab we expect a toroidal component of astenospheric flow.…”

“…Contraction affects mainly the western sector of the Southern Tyrrhenian Sea (Figure 1) where focal mechanism solutions indicate the presence of an E-W oriented compressive belt, which extends from the Aeolian archipelago to the Ustica Island (Neri et al, 2005;Billi et al, 2006Billi et al, , 2007Billi et al, , 2010. This is characterized by 1-1.5 mm/yr of geodetic shortening (Serpelloni et al, 2005;Devoti et al, 2011;Palano et al, 2012) and by frequent, moderate-sized crustal (15-20 km of depth) thrust earthquakes (Pondrelli et al,2006;Billi et al, 2007;Giunta et al, 2009), with P axes constantly trending NW-SE.…”

Historical analysis of rainfall-triggered rockfalls: the case study of the disaster of the ancient hydrothermal Sclafani Spa (Madonie Mts, northern-central Sicily, Italy) in 1851

“…The area of Sclafani, typically mountainous, is subject to freeze-thaw conditions. The earthquake events that produced macroseismic effects in the study area in the first half of the 19th century took place in 1818-1819 and 1823 (Billi et al, 2010). Predisposing factors were many; some were intrinsic (related to the stratigraphic and tectonic setting), while other ones included selective erosion (hard-on-soft landforms).…”