GNGTS 2019 - Atti del 38° Convegno Nazionale

628 GNGTS 2019 S essione 3.2 seismic ambient noise measurements and Multichannel Analysis of Surface Waves (MASW) were carried out in the Poggio Cancelli valley to retrieve a detailed sub-surface model for seismic response analysis. The Laga Fm. was characterized through geomechanical scanlines according to the Marinos and Hoek (2001) classification. In addition, the shallower alluvial deposits were sampled and tested in laboratory to estimate Atterberg limits, weight per unit volume and grain size distribution. Based on the obtained results, different engineering geological units were identified for both flysch and alluvial deposits. Between October 2018 and July 2019, single station seismic ambient noise measurements were performed along 5 NNE-SSW oriented sections across the Poggio Cancelli valley. Two types of instruments were used: i) SL06 24 bit digitizers with built-in SS20 three-component velocimetric sensor (2 Hz eigenfrequency) manufactured by SARA Electronic Instruments; ii) three-component LE-3D/5s Lennartz seismometers (0.2 Hz eigenfrequency) coupled to Reftek 130-01 digitizer. The measurements were carried out with a sampling frequency of 200 Hz and 250 Hz respectively and had a duration of 1 hour. The Horizontal-To-Vertical Spectral Ratio (HVSR) method (Nakamura, 1989) was applied for the analysis as implemented in the Geopsy software (www.geopsy.org ) to evaluate the fundamental resonance frequency (f 0 ) of the investigated site. A Multichannel Analysis of Surface Waves (MASW) test was carried on July 2019 by employing a DoReMi seismic system designed by SARAElectronic Instruments. A linear array of 36 4.5-Hz vertical geophones (spacing 1.5 m) was deployed on the filling deposits in the middle of the valley and 23 shots with different offsets were carried out at both array extremities. The obtained fundamental mode Rayleigh wave dispersion curve (DC) was inverted to retrieve the shear wave velocity profile at the site. As a relatively narrow frequency band (15-30 Hz) was obtained for the DC, a joint inversion of HVSR and DC data was performed by the HV-Inv software (García-Jerez et al. , 2016) to increase the investigation depth. Results. The seismic ambient noise measurements showed clear resonance frequencies (f 0 ) in the range 0.9-2 Hz in the Poggio Cancelli alluvial valley likely due to a significant impedance contrast between the alluvial deposits and the seismic bedrock, which may be represented by the Laga Fm. Some measurements also show a second resonance frequency (f 1 ) at about 4 Hz. Measurements performed over the outcropping Laga Fm. do not show any resonance frequency, as expected. The MASW test allowed estimating the Vs distribution within the alluvial body, which is characterized by low Vs range between 120 and 300 m/s. In addition, the seismic bedrock depth was estimated in the range of 45-60 m. In order to evaluate the seismic response of the valley, including 2D effect, numerical modelling through a finite element approach will be performed across different sections of the valley to derive both AF (amplification factor), available for urban planning, and elastic response spectra to design ordinary or strategic structures according to the present National rule. References Adamoli L., Calamita F. and Pizzi A.; 2007: Note illustrative della carta Geologica d’Italia 1:50.000, foglio 349 “Gran Sasso d’Italia”. Artoni, A.; 2003): Messinian events within the tectono-stratigraphic evolution of the Southern Laga Basin (Central Apennines, Italy) . Boll. Soc. Geol. It., 122(3) , 447-466. Centamore E., Cantalamessa G., Micarelli A., Potetti M., Berti D., Bigi S., Morelli C. and Ridolfi M.;1991: Stratigrafia e analisi di facies dei depositi del Miocene e del Pliocene inferiore dell’avanfossa marchigiano-abruzzese e delle zone limitrofe . Studi Geologici Camerti, 2 , 125-131. Delgado J., López Casado C., Giner J., Estévez A., Cuenca A. and Molina S.; 2000: Microtremors as a geophysical exploration tool: applications and limitations . Pure Appl. Geophys., 157(9) , 1445-1462. Galadini F. and Galli P.; 2003: Paleoseismology of silent faults in the Central Apennines (Italy): the Mt. Vettore and Laga Mts. Faults . Annals of Geophysics, 46 , 5.