Comparison of empirical and numerical site responses at the tito test site, southern Italy

The town of Potenza (Southern Italy) is one of the test sites for preparing ground-motion scenarios within the framework of the Italian Dipartimento Protezione Civile-Instituto Nazionale di Geofisica e Vulcanologia (DPC-INGV) 2004-2006 projects. An area in the neighboring village of Tito was selected to evaluate different techniques for estimating site effects involving a 40-m-deep instrumented borehole. This two-sensor vertical array records teleseismic, regional, and local seismicity. Close to the borehole, three seismological microarrays (utilizing short-period sensors and digitizers with a high dynamic range) were installed in May 2005 to record seismic noise. Differing acquisition geometries allowed the checking of any dependency in the derived dispersion curves based on the adopted analysis method (extended spatial autocorrelation [ESAC] and frequency wave-number [F-K]). In general, the ESAC method appears to provide more reliable results in the low-frequency range. Furthermore, the soil-velocity profiles obtained from the microarray data were compared with the S-wave velocity profile derived from down-hole measurements. A good agreement was observed in the depth range well constrained by the data. Finally, empirical site responses were compared with those calculated numerically from the S-wave velocity profiles obtained from the microarray data. Although this comparison did not resolve a preference among the derived models, it showed the importance of downgoing waves in modifying the site response at the Tito site.