The Need for Standardized Approach for Estimating the Local Site Effects Based on Ambient Noise Recordings

Abstract

Assessing local site effects reliably is one of the crucial aspects of seismic hazard, which usually cause amplification of ground motions and results in increasing the damage potential of a large earthquake. This phenomenon has been recognized for some time (e.g., Milne, 1898; Takahashi and Hirano, 1941; Gutenberg, 1957). The methods used to determine the site response can be categorized into two major groups, the analytical (theoretical) and the empirical methods. The analytical calculation of the site response, mainly based on inversion techniques, requires a very good knowledge of the geotechnical parameters to constrain the results. Empirical methods are somehow more effective in the sense that they are based on calculating the frequency spectrum directly from the recorded ground motion. Among the empirical methods, the spectral ratio of a sedimentary site with respect to a bedrock reference site, usually referred as the standard spectral ratio technique, is a widely used approach (e.g., Borcherdt, 1970; Borcherdt and Gibbs, 1976; Rogers et al., 1984). Effective use of this technique is demonstrated at different sites and geological conditions following large destructive earthquakes (e.g., Singh et al., 1988; Lermo et al., 1988; Borcherdt et al., 1989; Hough et al., 1990). Other methods include H/V spectral ratios (horizontal vs. vertical components) using single-station recordings (Nakamura, 1989; Lermo and Chávez-Garćia, 1993; Field and Jacob, 1993; Lachet and Bard, 1994), and the cross-spectrum estimate (Safak, 1991; Field et al., 1992). There exists a number of review papers discussing the effectiveness of the different methods used in the site response estimate (e.g., Aki, 1988; Chin and Aki, 1991; Hartzell, 1992; Gutierrez and Singh, 1992; Yu et al., 1992; Aki, 1993; Bard, 1994; Atakan, 1995; Field and Jacob, 1995; Kudo, 1995; Mucciarelli et al., 2003; Atakan et al., 2004a). While most of the methods of site effects estimation use earthquake records, few are based on using microtremors. In the following we will focus on the latter, especially the spectral ratio of horizontal to vertical components of ambient noise recordings. From here on we will refer to this technique simply as the H/V method. Other techniques using array measurements of ambient noise will not be treated in this section.