New dating of rapid vertical deformation of Santa Tecla Fault scarps (Mt. Etna volcano, Sicily) by lichenometry method

摘要

The eastern slope of Mt. Etna is characterised by shallow seismicity originating from normal-oblique faulting, Timpe Fault System, related to WNW-ESE regional extension.Recent research has demonstrated that in the fault population of Mt. Etna's eastern flank the minimum earthquake magnitude that will have a ground rupture effect is ca. 2.5. This system is characterised by high frequency seismic activity, due to thinned seismogenic crustal layer. This characteristic, together with the high density of the fault segments, does not always for identification of the segments responsible for the earthquake. The earthquakes, affecting the medium-lower eastern flank, have been historically reconstructed by macroseismic analysis and reported in a macroseismic database, and in recent decades by instrumental seismic registration, which provide the seismological parameters capable of evaluating focal mechanism, hypocentre and relative algorithms related to geometric parameters which control the growth of fault segments.In this paper, we present a methodology to evaluate the age of the rapid exhumation of the free-face fault plane of the NNW-SSE oriented normal fault segment named S. Tecla (Timpe Fault System). It consists of the measurement of the thalli species (Lichenometry method) in order to evaluate the parameters which characterise their growth. The seismic history of the S. Tecla Fault indicates eight certain events from 1865 to 2005 with 3.4 divided by 4.7 Magnitude (De Guidi a al., 2012 and reference therein). We found evidence of two different recent rapid vertical deformation events at the base of the S. Tecla fault escarpment, the oldest 20 m long and 0.25 m in height, and the youngest with a 0.02 m high nude surface exposed. We have observed that there are thalli of Xanthoparmelia conspersa (Ehrh. Ex Ach.) Hale, colonizing part of the nude surface on the escarpment. The results highlight that the oldest thalli was dated at 43.7 years old, showing that rapid vertical deformation generated the surface where the thalli, after 4 years, took root. The displacement of this surface could be related to the seismic events occurring on 3rd August 1973 (3.8 M) in S. M. Ammalati area probably accompanied by intense post seismic deformation. The second and last event could be attributable to a 3.1 M seismic event occurring on 25th September 2014 (ISIDe, 2016).