High Resolution seismic method for detection and understanding of suspected active blind faults: the case of Terme Caronte geothermal area (Lamezia Terme-Calabria, Italy)

A crucial task in earthquake hazard assessment is the problem of investigating buried main faults and secondary features to evaluate their seismic hazard potential, because they cannot be easily detected by surface geological analyses. The Lamezia Terme territory, located in the westernmost edge of the Neogene-Quaternary "Catanzaro Trough", is influenced by the presence of different major trascurrent/oblique faults which belong to the Lamezia Terme-Catanzaro Fault (LCF), a NW-SE trending seismogenetic trascurrent fault zone. The subsurface geometry of LCF is recognizable on a morphological basis and constrained by commercial profiles (VIDEPI Project); however, quality of commercial reflection profiles is often inappropriate to face the extreme structural complexity related to this seismogenetic zone. Although these geophysical surveys have provided insight into the largescale geology, resolvable with high-energy and low-frequency seismic acquisition systems, only high-frequency seismic studies at specific sites can resolve the fine-scale features required to interpret Quaternary processes. In Lamezia Terme area, in fact, the outcropping heterogeneous Quaternary terrigenous successions probably absorbed the rupture's stress related to historical fault activity; this do not permit to delineate active faults with the traditional geological mapping and/or paleoseismic investigation. High Resolution (HR) seismic reflection method, integrated with a pure surface geology approach, can overcame the problem of absence of clear geomorphological/stratigraphic indicators of synsedimentary fault activity, permitting a detailed structural imaging at shallow depth, which is crucial to document recent activity and assess seismogenic potential of blind faults. In Lamezia Terme area, since no detailed information on the faults segment which cross this zone is available, was identified a sector, 600 m-wide (but the extension of the zone is uncertain), subject to environmentally protective restrictions due to the presence of LCF. Therefore, in order to improve the imaging of shallower more recent sequences, we acquired, at "Terme Caronte", near the village of Sambiase, four HR seismic reflection/refraction profiles (total length about 1500 m). Seismic data were collected by a Swept Impact Seismic Source (VibSist-500), appropriate for shallow reflection surveys, and an array of receivers consisting of 96, 4.5 Hz, vertical geophones. Setting the receiver spacing to 5 m and the source interval to 10 m, we recorded dense data within a wide offset range. This study aims to develop an improved seismic stratigraphic framework for Lamezia Terme area in order to recognize seismic units distribution and the possible presence of buried fault segments linked with the principal displacement zone, which are fundamental information for a quantitative approach to seismic hazard assessment and the subsequent land-use planning.