GNGTS 2016 - Atti del 35° Convegno Nazionale

416 GNGTS 2016 S essione 2.2 stratigraphic succession of the Sulmona basin is made up of gravelly sand, with silty clay intercalations, terraced alluvial deposits (Late Pleistocene-Holocene) overlaying fine silty-clay and sandy-silt lacustrine sediments with local intercalations of gravel and conglomerate (Early- Middle Pleistocene) (Fig. 1). Accordingly, data from down holes DHs and cross holes CHs generally show a clear decreasing in the velocity values at 20-30m where the boundary between the gravelly alluvial unit and the silty-clay lacustrine one is found (Fig. 2). Furthermore, the carbonate ridges at the boundary of the basin show complex lithostratigraphic and morphological settings due to clastic supply from slope and alluvial fans (Fig. 1c).The superimposition of different geological units that can be subdivided according to their geophysical and geotechnical properties defined several ‘’Stable Zones Susceptible for Local Amplification’’ in the MOPS map (Fig. 1b). ‘’Unstable Zones’’ have been also mapped to highlight the presence of possible instability phenomena that have to be analyzed in the level 2/3 of microzoning (Fig. 1b). Local seismic response. Comparing the seismic intensities suffered from Sulmona city center with its suburbs during several historical earthquakes (e.g., 1349, 1456, 1706, 1915, 1933, 1984 and 2009) it can be pointed out that, in some cases, the city center showed the heaviest damages (DBMI11 database, http://emidius.mi.ingv.it/DBMI11/) . This suggests that, for this city, site effects could be relevant. Four permanent strong motion stations are installed in Sulmona district area by the Italian Civil Protection Office (Fig. 1b). One of them is installed on the outcropping bedrock, the other two in the center of the plain and the last one nearby the Morrone slope. The acceleration tracks recorded at these seismic stations (Fig. 3) highlight that the Sulmona plain may suffer remarkable amplifications. As a matter of fact, the seismic stations placed in the Sulmona plain recorded velocity and displacement peaks about 6 times higher than those at the rock site. The study of the local seismic response performed with STRATA (https://nees.org/ resurces/strata) on two profiles available in the Sulmona area (S1 and S2 in Fig. 1) highlights, in fact, that the calculated spectra through 1D site response analyses by using as input motion accelerograms compatible with 475 years response spectra at outcropping rock conditions are higher than the ones related to the design code NTC08 (2008) for the corresponding subsoil class. The amplifications shown by those spectra fall in the period range Fig. 3 – Acceleration, velocity and displacement values recorded by the seismic stations installed on the bedrock (SUL) and at the Sulmona plain (SULA). These latter recorded velocity and displacement peaks are about 6 times higher than those on the rock site: these data suggest that the Sulmona plain may suffer remarkable amplifications.