The aim of this work is to verify if (1) the average of shear-wave velocity from the surface to 30 m depth (VS30) is a good proxy of site amplification in a country\ud with complex geology like Italy, (2) the grouping of VS30 in different soil classes and relevant spectra in the Italian seismic codes is adequate, and (3) shallow shear-wave profiles (VS10) could be a more economical tool for site classification. To answer the first two questions, we examined 40 estimates of VS30 derived from microzonation projects we performed in Italy (27 obtained with downhole measurements, plus 13 velocity profiles obtained with surface techniques). In all the sites, we installed seismic\ud instrumentation to record earthquakes and to estimate site response using horizontal to vertical spectral ratios (HVSR). The comparison between HVSR and VS30 showed in about one-third of the sites that VS30 is not a good proxy of observed amplification effects if the site does not have a monotonically increasing velocity profile. The reason\ud VS30 does not provide satisfactory estimates in Italy is linked to peculiar geological settings that are widespread in the country. We then compared the observed amplifications from earthquake HVSR data with the ones provided by the Italian seismic code, noting a substantial underestimation by the code, a somewhat unsettling situation because HVSR is usually considered to be a lower bound for amplification estimated with other techniques. Finally, we studied 45 VS downhole profiles to 30 m depth performed also at sites where earthquake recordings are not available. On this data set, we noticed that VS10 could predict site classification with the same performances of VS30. We consider alternative soil classification schemes that include soil frequency besides the velocity profile. In this two-parameter approach, VS10 could be substituted for VS30

During the 2002 seismic sequence in Molise (Italy), the town of Bonefro suffered moderate damage (I-MCS = VII) except for two reinforced concrete (RC) buildings. These buildings are located on soft sediments, close to each other and very similar in design and construction. The main difference is the height: the most damaged one (European Macroseismic Scale damage 4) has four stories, whereas the less damaged (EMS damage 2) has three stories. The M 5.4 shock on 31 October damaged both of them. The second shock on I November (M 5.3) increased the damage on the four-story building substantially, just while a 5-min. seismic recording was taken. We analyzed the recorded data by four different techniques: short-time fourier transform (STFT), wavelet transform (WT), horizontal-to vertical spectral ratio (HVSR), and horizontal-to-vertical moving window ratio (HVMWR). All the results agree upon the estimate of the main building frequency before the second shock and upon the shift of frequency due to damage. All the fundamental frequencies (pre-, during, and postdamage) are in the range 2.5-1.25 Hz. The fundamental frequency of the less damaged building was estimated at about 4 Hz.\ud \ud To test if the soil-building resonance effect could have increased the damage, we also evaluated the soil fundamental frequency by three different techniques: noise HVSR, strong motion HVSR of seven aftershocks, and ID modeling based on a velocity profile derived from noise analysis of surface waves (NASW) measurements. The results are again in good agreement, showing that resonance frequencies of the soil and of the more damaged building are very close

Recent observations have shown that slope response to seismic shaking can be characterised by directional variations of a factor of 2–3 or larger, with maxima oriented along local topography features (e.g. maximum slope direction). This phenomenon appears influenced by slope material properties and has occasionally been detected on landslide-prone slopes, where a down-slope directed amplification could enhance susceptibility to seismically-induced landsliding. The exact conditions for the occurrence of directional amplification remain still unclear and the implementation of investigation techniques capable to reveal the presence of such phenomena is desirable. To this purpose we tested the applicability of a method commonly used to evaluate site resonance properties (Horizontal to Vertical Noise Ratio – HVNR or Nakamura's method) as reconnaissance technique for the identification of site response directivity. Measurements of the azimuthal variation of H/V spectral ratios (i.e. between horizontal and vertical component) of ambient microtremors were conducted in a landslide-prone study area of central Italy where a local accelerometric network had previously provided evidence of directivity phenomena on some slopes. The test results were compared with average H/V spectral ratios obtained for low-to-moderate earthquakes recorded by the accelerometric stations. In general, noise and seismic recordings provided different amplitudes of spectral ratios at similar frequencies, likely because of differences in signal and instrument characteristics. Nevertheless, both kinds of recordings showed that at sites affected by site response directivity major H/V peaks have orientations consistent (within 20°–30°) with the direction of maximum shaking energy. Therefore, HVNR appears to be a promising technique for identifying seismic response directivity. Furthermore, in a comparative test conducted on a slope mantled in part by a deep-seated landslide we detected spectral peaks with orientations close to the maximum slope direction, whereas no evidence of directivity was found outside the slide boundaries. This indicates the influence of the landslide body on seismic response directivity.