GNGTS 2017 - 36° Convegno Nazionale

GNGTS 2017 S essione 1.3 209 results obtained through TF method allowed us to better quantify the horizontal polarization of the ground motion, giving a clear indication which corroborate findings pointing out that the recorded ambient noise is polarized in a narrow frequency band (1.0-4.0 Hz) and follows a trend roughly oriented ENE (see example in Fig. 3a). Moreover the TF polarization analysis revealed the existence of low values of the ellipticity in a wide frequency band (1.0-4.0 Hz) as well as dip values showing an horizontal trend in the same frequency band. It appears therefore clear that the maxima of the horizontal ground motion polarization take place almost perpendicularly to the main buried magma-feeding fractures, which are trending North-North-West. The inversion of the dispersion curve data, obtained by recording ambient vibrations using a “L shape array, allowed us to build a shear wave profile up to 50 m depth. The main observed velocity contrast is between a low velocity layer, having V S in the range 200-400 m/s, and an higher velocity layer with Vs of about 1200 m/s. Although results are preliminary, it is possible to hypothesize the existence in the investigated area of at least 50 m thick pyroclastic deposits overlaying the lava rock cooled within the eruptive fracture NNW oriented. A more detailed geophysical survey is planned in the next future to better constrain the thickness of pyroclastic deposits around the cone and to outline the geometry of the eruptive fracture. References Bard P.Y.; 1998: Microtremor measurement: a tool for site effect estimations? Proc. of the 2nd International Symposium on the Effects of the Surface Geology on Seismic Motion ESG98 Yokohama, Japan, pp 1251-1279 Burjànek J., Moore J.R., Molina F.X.Y., Fäh D.; 2012: Instrumental evidence of normal mode rock slope vibration. Geophys. J. Int. 188, 559-569 Capon J .; 1969: High-resolution frequency-wavenumber spectrum analysis . Proc. IEEE, 1408–1418, doi: 10.1109/ PROC.1969.7278. Corazzato C., Tibaldi A.; 2006: Fracture control on type, morphology and distribution of parasitic volcanic cones: An example from Mt. Etna, Italy . Journal of Volcanology and Geothermal Research 158, 177–194. Crisci G.M., Iovine G.G.R., Di Gregorio S., Lupiano V.; 2008: Lava-flow hazard on the SE flank of Mt. Etna (Southern Italy). Journal of Volcanology and Geothermal Research 177(4):778-796 Di GiulioG., Cara F., RovelliA., LombardoG., Rigano R.; 2009: Evidence for strong directional resonances in intensely deformed zones of the Pernicana fault, Mount Etna, Italy . J. Geophys. Res., 114, doi:10.1029/2009JB006393. Ferrucci F., Gresta S., Patanè D., Rasà R.; 1992: Inferences on the magma feeding system at Mt. Etna volcano from seismological, structural and volcanological data . Atti dell’11° Convegno annuale del Gruppo Nazionale di Geofisica della Terra Solida. Panzera F., Halldorsson B., Vogfjörð K.; 2017: Directional effects of tectonic fractures on ground motion site amplification from earthquake and ambient noise data: A case study in South Iceland . Soil. Dyn. Earthq. Eng., 97, 143–154, doi: 10.1016/j.soildyn.2017.03.024. Panzera F., Lombardo G., Monaco C., Di Stefano A.; 2015: Seismic site effects observed on sediments and basaltic lavas outcropping in a test site of Catania, Italy . Natural Hazard, 79(1), 1-27, doi: 10.1007/s11069-015-1822-7. Parolai S., Bormann P., Milkereit C.; 2001: Assessment of the natural frequency of the sedimentary cover in the Cologne area (Germany) using noise measurements . J. Earthquake Eng. 5, 541-564. Rigano R., Cara F., Lombardo G., Rovelli A.; 2008: Evidence of ground motion polarization on fault zones of Mount Etna volcano . J. Geophys. Res. 113, B10306, doi:10.1029/2007JB005574. SESAME; 2004: Guidelines for the implementation of the H/V spectral ratio technique on ambient vibrations: Measurements, processing and interpretation . SESAME European Research Project WP12, deliverable D23.12, http://sesame-fp5.obs.ujf-grenoble.fr/Deliverables 2004. Vidale J.E.; 1986: Complex polarisation analysis of particle motion . Bull. Seism. Soc. Am. 76, 1393–1405.