Site Response of Strong Motion Stations in the Umbria, Central Italy, Region

We used near-field and regional records from 16 earthquakes (ML 4.6-5.9) located in the epicentral area of the 1997 Umbria-Marche sequence to estimate the site response of 40 strong motion stations. The distribution of hypocenters covers a distance range from 5 to 100 km. We estimated site amplification factors using two techniques, namely a generalized spectral inversion method and horizontalto-vertical-component spectral ratios (HVSRs) of ground acceleration. For the first approach, we inverted observed spectral amplitudes for site, Q, and source using a reference site. Since the site response estimated using HVSRs assumes that the vertical component of motion is amplification free, we also separated source and site effects by inverting the vertical component of the acceleration spectra. We found that although most of the stations do not show important amplifications on the vertical component, a few of them have a significant vertical amplification at low frequencies (f < 1 Hz). The vertical amplification at those sites ranges from a factor of 2 up to a factor of 25, and we observe that HVSRs underestimate the site response. For the rest of the sites, both techniques give similar site functions (inside the standard error of the estimates). The quality factor Q, representative of the S-wave train, shows a frequency dependence that can be approximated by the relation QS = 31.2f1.2, between 0.3 and 9.5 Hz. However, at high frequencies (f > 10 Hz), this dependence weakens, and QS takes an approximately constant value of 438.