Statistical analysis of site effect indicators at the Italian seismic network: inside the site characterization database CRISP

Abstract

This study investigates the relationships among various site-effect proxies collected in the CRISP database (http://crisp.ingv.it/), which archives site characterization data of the Italian National Seismic Network. We analyzed the Horizontal-to-Vertical spectral ratio (HVSR), derived from both earthquake and noise measurements at 320 stations, as a primary indicator of site effects. Our research also explored HVSR’s correlation with topography and site classes, lithology, and magnitude residuals. This extensive dataset allowed us to group the HVSR curves into four distinct clusters based on their shapes, facilitating detailed comparisons between earthquake- and noise-derived estimates. The analysis revealed that: (i) approximately half of the permanent stations exhibit significant amplification, with amplitudes exceeding 2; (ii) although HVSR from noise generally mirrored that from earthquakes, it often showed lower or equal amplitudes of the curves but higher amplitude of resonance frequency, likely due to different wavefield compositions. The correlation between HVSR and other proxies displayed a weak but statistically significant dependence on lithology, site classes and magnitude residuals. Specifically, as soil characteristics degrade, the resonance frequency decreases, and its amplitude slightly increases. Furthermore, local magnitude tends to be overestimated at sites exhibiting HVSR amplification at frequencies below 2–3 Hz. No correlation was found with topographic classes. A significant challenge in clearly distinguishing HVSR behavior among soil categories, as defined by current building codes, arises from the considerable standard deviation observed. Nevertheless, our findings suggest that integrating seismological data, including HVSR curves, fundamental frequency, and amplitude, can substantially optimize soil class definitions within the updated Eurocode 8 framework.