GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 2.1 - POSTER GNGTS 2023 These have been characterized from a geological-seismic-technical point of view by down-hole tests, environmental seismic noise measurements and seismic refraction tests (Consorcio Salvador e ITALTEKNA-ITALCONSULT Geotérmica Italiana, 1988; Salazar, 2002). A simplified geological-technical subsurface model extending in the SW-NE direction with a length of 4875 meters (Fig. 2) has been developed. The 2D section consists, starting from the surface, of the following units: Tierra Blanca Joven - TBJ, represented by loose pyroclastic deposits, Tierra Blanca - TB2, represented by medium compact pyroclastic deposits, Ignimbrites - TB3, represented by tuffs, and volcanic slag - MASS. All the above units rest on a bedrock consisting of basaltic/andesitic lavas. In Tab. 1 the physical-mechanical characteristics of the subsurface model are reported. Table 1. The geophysical and geotechnical parameters of the units used for 2D modeling. Material Code Range of thickness (m) ρ (Kg/m 3 ) Vs (m/s) Vp (m/s) Poisson coefficient ν (-) Shear Modulus Reduction curve G/G 0 Damping D curve Loose Pyroclastic deposits TBJ (2-17) 1122 155 290 0.35 EPRI (93) 0-20 ft EPRI (93) 0-20 ft Dense Pyroclastic deposits TB2 (4-14) 1123 255 560 0.40 EPRI (93) 20-50 ft EPRI (93) 20-50 ft Tuff TB3 (4-25) 2243 475 890 0.30 EPRI (93) 50-120 ft EPRI (93) 50-120 ft Volcanic ash MASS (8-25) 2243 530 1090 0.34 EPRI (93) 50-120 ft EPRI (93) 50-120 ft Andesitic / Basaltic Lava - (-) 2447 2100 3240 0.14 Linear=1% Linear=0.1% The thickness of these deposits is not constant along the section but undergoes evident lateral variations. The topography is almost flat and shows isolated shallow depressions. The layers have little variable slopes although they can be considered sub-horizontal. In Fig. 3 some results are shown of the 2D numerical analysis in terms of spectral horizontal acceleration response. At control point 1 (Fig. 3a) there is a significant peak at medium periods (0.5 s - 0.8 s). Only in the case of the first spectrum corresponding to the February seismic event, rich in low frequencies, is it possible to note the presence of an additional peak at about 0.15 s. It can be noted that considering the most superficial layers (TBJ and TB), which have an average shear velocity Vs of 200 m/s and thickness of 16 m, the amplifications are shifted towards longer periods. The tuffs, however, despite having higher seismic velocities, but very low thickness (5.50 m), do not contribute to amplifying the motion on the surface but they are responsible for the peak at the low period of 0.12 s. In Fig. 3b the spectral response of control point 2 is reported: here the thickness of the deeper deposits (TB3 and ASS) increases assuming a total value of 35 m.