A study of the seismic response of the city of Benevento (southern Italy) through a combined analysis of seismological and geological data

Abstract

A previous analysis (Improta et al., 2005) of small magnitude earthquakes recorded at 12 sites within the city of Benevento has stressed the significant role played by near-surface geology in causing variability of the ground motion. In this paper, we extend the study of the seismic response of the city from 12 sites to the entire urban area. We analyze more than 250 boreholes to constrain the near-surface geology in as many as possible sites. In spite of this geological dataset, uncertainties on the subsoil structure remain due to the presence of strong lithological heterogeneities which are responsible for rapid change of the shallow S-wave velocities (from 200 to 1700 m/s). Therefore, based on inferences from the comparison at the 12 sites between earthquake and ambient vibration results, we have collected ambient noise at about 100 sites within the city, intensifying measurements where geological variations occurs. Microtremor H/V spectral ratios are interpreted in terms of near-surface geology and compared to theoretical transfer functions of 1D models along five well-constrained profiles. On the basis of geological, geotechnical, and seismic data, we identify three main typologies of seismic response. Each type of response is associated to zones of the city sharing common soil conditions and similar soil classes according to building codes for seismic design. Moreover, we find that the spatial variation of the seismic response in the old town area is consistent with the damage pattern produced by a very destructive, well-documented historical earthquake that struck the city in 1688, causing intensity of IX-X MCS in Benevento. Finally, we use ground motions recorded during the experiment by Improta et al. (2005) to generate synthetic seismograms of moderate to strong (Ms 6.9, 1980 Irpinia and Mw 5.7, Molise 2002) earthquakes. We calibrate the random summation technique by Ordaz et al. (1995) using recordings of these earthquakes available in Benevento. After a satisfactory fit between observed and synthetic seismograms, we compute response spectra at different sites and speculate on effects of the geology class at large level of shaking, including soil nonlinearity. We find that large discrepancies with design spectra can occur for a wide sector of Benevento, especially for periods < 0.5 sec.