Proposal for a new site classification tool using microtremor data

Abstract

Seismic microzoning at a local scale of a given site or at a global scale of an urban center requires a fairly detailed knowledge of the lateral variations of the site effects, such as the amplification level, the fundamental soil frequency, and the site classification according to a given design seismic code. Over the last decade, the horizontal-to-vertical spectral ratio of background noise has become a widely used tool. However, it still has some shortcomings related to the windows selection as the microtremor contains many unknown and random sources other than the one related to the site effect. In this contribution, one proposes a new approach based on the frequency content to discriminate between the different sources. Site classification scheme recently proposed for seismic accelerations, which define target numerical and analytical horizontal over vertical spectral ratios for the 4 standard soil classes (Rock, soft rock, stiff soil and soft soil) defined in Eurocode-8, is adapted to background noise. In order to test the reliability of the proposed approach, microtremor measurements were performed on 67 sites with available shear-wave velocities down to a depth of at least 30 m. Free-field microtremor measurements are converted to accelerations instead to velocities, and split into 5 s non-overlapping windows. Automatic computation based statistical approach using a classification index which measures the similarity between the measured and target horizontal over vertical spectral ratios is performed for the 67 sites. The obtained results reveal a similar site classification, using the time-averaged shear-wave velocity to 30 m depth (Vs30) and the new approach, which can reach a success rate of about 84%. The automatic calculation provides the 67 sites with two fundamental parameters required for the preparation of a first-order microzonation map, the fundamental soil frequency, and the classification of the site according to Eurocode-8 guidelines. The proposed approach can be very useful for large-scale seismic micro-zoning of urban centers that may involve hundreds of measurement points, or supplement other geophysical measurements for a site specific seismic analysis.