Abstract
Site-effect assessments performed through earthquake-based approaches, such as the standard spectral ratio (SSR), require good quality records of numerous earthquakes. In contrast, the use of ambient noise appears to be an attractive solution for ease and rapid computation of site responses with sufficient spatial resolution (microzonation), especially in low seismicity areas. Two main approaches are tested here: the horizontal-to-vertical spectral ratio (HVSR) and the noise-based SSR (SSRn). The HVSR uses the relative amplitude of the horizontal and vertical components of the ambient noise. Instead, the SSRn defines the spectral ratio between the seismic noise recorded simultaneously at a site and at a rock reference station, similar to earthquake-based SSR. While the HVSR is currently used in hundreds of site-specific studies, the SSRn approach has been gradually abandoned since the 1990s. In this study, we compare the results obtain from these two approaches with those of earthquake-based SSR. This comparison is carried out for two sedimentary basins, in Provence (southeastern France) and in Argostoli (western Greece). In agreement with the literature, the HVSR does not provide more than the fundamental resonance frequency of the site (f0). The SSRn leads to overestimation of the SSR amplification factors for frequencies higher than the minimal f0 of the basin (f0min). This discrepancy between SSRn and SSR is discussed, and appears to be mainly dependent on the local geological configuration. We thus introduce the hybrid standard spectral ratio (SSRh) approach, which aims to improve upon the SSRn by adding an intermediate station inside the basin for which the SSR is known. This station is used in turn as a local reference inside the basin for the SSRn computation. The SSRh provides site transfer functions very similar to those of the SSR, in a broad frequency range. Based on these results, the SSRn (or SSRh) should be further tested and should receive renewed attention for microzonation inside sedimentary basins.
Highlights
Since the first understanding that seismic signals can be locally modified by the geological conditions of the Earth surface (e.g. Milne 1908), it has been widely demonstrated that site effects can dramatically increase both the amplitude and duration of ground motion
It is currently acknowledged that the horizontal-to-vertical spectral ratio (HVSR) allows for estimation of only f0 of S waves propagating in 1-D sites, mainly due to the surface waves present in ambient noise
Since increasing numbers of studies have focused on site effect analysis based on the HVSR, while the SSRn has been progressively abandoned
Summary
Since the first understanding that seismic signals can be locally modified by the geological conditions of the Earth surface (e.g. Milne 1908), it has been widely demonstrated that site effects can dramatically increase both the amplitude and duration of ground motion. Milne 1908), it has been widely demonstrated that site effects can dramatically increase both the amplitude and duration of ground motion. This is a source of particular concern for seismic hazard assessment, as site effects can greatly increase hazard levels in many cities that are located on sedimentary basins (e.g. Mexico City, Mexico; Los Angeles, USA; Tokyo, Japan; Grenoble, France; among others). After the devastating Guerrero-Michoacan earthquake, the contributions of local site conditions to ground motion became recognized as essential and received particular attention. Assessing seismic site responses from ambient noise is of major interest to locally infer the spatial variations of site effects (i.e. microzonation), especially in low-to-moderate seismicity areas
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