GNGTS 2016 - Atti del 35° Convegno Nazionale

552 GNGTS 2016 S essione 3.2 Conclusions. The use of combined active characterization and passive seismic monitoring revealed successful in understanding the behavior of the potentially unstable cliff of Madonna del Sasso, overcoming the limitation of traditional geotechnical and remote-sensing monitoring techniques adopted in past years on the same site. No irreversible changes were detected during the monitored period. A strong thermal control was found to govern the stability of the cliff, with reversible seasonal opening and closing of fractures, resulting from thermal contraction and expansion, thus affecting the resonance frequency and the velocity field of the site. Moreover, microseismic events linked to these cycles of fracture opening/closing were detected, highlighting the dynamic activity of incipient fracturing of rock bridges and frictional sliding along fracture surfaces. References Bergamo P. and Socco L. V.; 2014: Detection of sharp later discontinuities through the analysis of surface-wave propagation . Geophysics, 79 (4), EN77-EN90. Bièvre G., Jongmans D., Winiarski T. and Zumbo V.; 2012: Application of geophysical measurements for assessing the role of fissures in water infiltration within a clay landslide (Trieves area, French Alps) . Hydrol. Process, 26 , 2128-2142. Colombero C., Comina C., Umili G. and Vinciguerra S.; 2015: Multiscale geophysical characterization of an unstable rock mass . Tectonophysics, 675, 275-289. Larose E., Carrière S., Voisin C., Bottelin P., Baillet L., Guéguen P., Walter F., Jongmans D., Guiller B., Garambois S., Gimbert F., and Massey C.; 2015: Environmental seismology: What can we learn on earth surface processes with ambient noise? Journal of Applied Geophysics, 116 , 62-74. Lévy C., Jongmans D. and Baillet L.; 2011: Analysis of seismic signals recorded on a prone-to-fall rock column (Vercors massif, French Alps). Geophys. J. Int., 186 , 296-310. Senfaute G., Duperret A., and Lawrence J. A.; 2009: Micro-seismic precursory cracks prior to rock-fall on coastal chalk cliff: a case study at Mesnil-Val, Normandie, NW France . Nat. Hazards Earth Syst. Sci., 9 , 1625-1641. Fig. 3 – In a: temporal evolution (October 2013 - February 2016) of the first resonance frequency of the site, obtained with the Horizontal-to-Vertical Spectral Ratio method on ambient noise recorded at ST3 (at the top of the unstable sector A). Each spectral ratio is normalized to its maximum to follow the evolution of the peak in time. In b: air temperature monitored at the site in the same period. White time windows correspond to no available monitoring data.