GNGTS 2018 - 37° Convegno Nazionale

GNGTS 2018 S essione 3.2 665 and velocity), obtained by the 1-D Vs profile in the equation of Ibs-Von Shet and Wohlenberg (1999): the frequency values, present in the spectra were converted into depth values, thus allowing to view the distribution of amplitude values of the spectral ratio in the subsoil. The x-axis in these sections shows the distance between the measuring points, with spacing of 10 m, and the y-axis the depth; variations in the amplitude value of the H/V spectral ratio are represented by colours associated with a chromatic scale (Fig. 3). Preliminary results. Generally, the 156 spectra obtained show a H/V ratio below 1 for a wide range of frequencies (Fig. 2a), probably linked to the presence of velocity inversions in the shallow subsoil, in fact in the related single component spectra are observable the decrease of the horizontal components below the vertical one (Fig. 2b). In this condition, the small resonance peaks (amplitude value ≤ 2) present in the spectra, given by local minima in the vertical component, may indicate the presence of impedance contrasts in the subsoil, linked to stratigraphic discontinuities (Castellaro and Mulargia, 2009). In some spectra, resonance peaks with greater amplitude (2 - 4) can be observed at higher frequencies. These values indicate the presence of an impedance contrast at shallow depth. The Vs-depth profile obtained through the MASW survey is characterized by low Vs values (Fig. 2d) that are consistent with the physical-mechanical characteristics of the lithological units at the investigated site and show a shallow velocity inversion, confirming the results of HVSR survey. All the impedance contrast sections (Fig. 3) are characterized by low H/V amplitude (<1) in their shallow parts, probably linked to the presence of muddy material from about 1 to 5 meters depth. Another area at a depth of approximately 40 m, shows the same characteristics, highlighting the possible presence of another zone where this muddy material can be present. The parts of sections characterized by major H/V amplitude values can be related with the transition between the muddy material and the surrounding lithotypes. References Castellaro S. and Mulargia F.; 2009: The effect of velocity inversions on H/V . Pure Appl. Geophys . , 166(4), 567-592. De Guidi G., Barberi G., Barreca G., Bruno V., Cultrera F., Grassi S., Imposa S., Mattia M., Monaco C., Scarfì L. and Scudero S.; 2015: Geological, seismological and geodetic evidence of active thrusting and folding south of Mt. Etna (eastern Sicily): Revaluation of “seismic efficiency” of the Sicilian Basal Thrust . J. of Geodyn., 90, 32-41, DOI:10.1016/j.jog.2015.06.001, 2015. Ibs-von Seht M. and Wohlenberg, J.; 1999: Microtremor measurements used to map thickness of soft sediments . Bull. Seismol. Soc. Am., 89(1), 250–259. Imposa S., Grassi S., De Guidi G., Battaglia F., Lanaia G. and Scudero S.; 2016: 3D subsoil model of the San Biagio ‘Salinelle’mud volcanoes (Belpasso, SICILY) derived from geophysical surveys . Surv. Geophys., 37(6), 1117-1138. INGV RING Working Group; 2016: Rete integrata Nazionale GPS , DOI:10.13127/RING. NakamuraY.; 1989: A method for dynamic characteristics estimation of subsurface using microtremor on the ground surface . Q. Rep. Railw Tech. Res. Inst., 30, 25–33. Lavecchia G., Ferrarini F., De Nardis R., Visini F. and Barbano M. S.; 2007: Active thrusting as a possible seismogenic source in Sicily (Southern Italy): Some insights from integrated structural-kinematic and seismological data . Tectonophysics, 445(3–4), 145–167, DOI:10.1016/j.tecto.2007.07.007, 2007. Lentini F. and Carbone S.; 2014: Geologia della Sicilia. Mem. Descr. della Cart. Geol. d’italia - Vol. 95. Milkov A. V.; 2005: Global distribution of mud volcanoes and their significance in petroleum exploration as a source of methane in the atmosphere and hydrosphere and as a geohazard . In Mud Volcanoes, geodynamics and seismicity pp. 29-34. Springer Netherlands Tortorici L., Monaco C., Di Stefano A., Maniscalco R., De Guidi G., Pappalardo G., Curcuruto E. and Brescia M.; 2014: Note Illustrative della Carta Geologica d’Italia alla scala 1:50000, Foglio 631 Caltanissetta. ISPRA Servizio Geologio D’Italia, ISBN: 978-88-9311-040-2.