Statistical investigation of site effects using earthquake and ambient noise recordings (part 2): reconstruction of earthquake spectral ratio from noise

Abstract

Standard spectral ratio from earthquake recordings (SSR) is considered the reference empirical method for assessing site effects as a function of frequency. However, other estimates can be easily obtained from noise measurements (i.e., Horizontal-to-Vertical Spectral Ratio, HVN), even though their reliability in terms of amplitude is controversial. In the framework of the ToK ITSAK-GR (2006-2010) and NERA (2007-2013) EC projects. Recordings from 64 sites from very different geographical areas where analyzed in the companion paper Cultrera et al. (2012); The authors evidenced statistically significant correlations between SSR and HVN through an appropriate linear combination of spectral ratio amplitudes using the canonical correlation technique. In the present paper we go one step further and show how to estimate the SSR spectral ratio of specific sites when only single station noise measurements are available, using the results of the canonical correlation analysis between SSR and HVN recorded at several sites and pertaining to the same area. A sistematic SSR evaluation has been conducted and tested by a cross validation procedure: the expected SSR at each validation site are in turn estimated by a weighted average of the SSR values measured at the other sites; the weights are properly set to account more for the sites with similar behavior in terms of the canonical correlation results between HVN and SSR. To evaluate the goodness of the estimation, we compared all the inferred and the original SSR, and we performed a critical analysis on the spectral characteristics of earthquake site response as recovered from noise measurements.