Statistical investigation of site effects using earthquake and ambient noise recordings (part 1): correlations between different estimates.

Abstract

The Horizontal-to-Vertical Spectral Ratio from earthquake recordings (HVSR) and from ambient noise recordings (HVN) realistically indicate fundamental frequency but, for the majority of the worldwide examined sites, they do not provide reliable amplification curves as predicted by the earthquake Spectral Ratio to a reference site (SSR). Given the fact that HVSR and especially HVN can be easily obtained, it is challenging to search for a meaningful correlation with SSR amplification functions in the whole frequency band. To this aim we used recordings from 75 sites worldwide and we applied a multivariate statistical approach (the canonical correlation analysis) to investigate and quantify any correlation among spectral ratios and, from a subset of sites in sedimentary basins, with geometrical and geophysical parameters (S-wave velocity and the 2D geometry of the valley). Results show a correlation between SSR and HVN or HVSR that can be statistically quantified, and it is modulated in specific frequency range. Moreover, the larger are the Vs30, the bedrock depth and the valley’s shape ratio, the higher is the low-frequency amplitude in HVSR and HVN, and the lower is the high-frequency value. Despite the limitation of the database, the correlation results can be used to estimate the excepted SSR spectral ratio of specific sites in the investigated areas when only noise measurements are available(see the companion paper De Rubeis et al., 2012).