Empirical evaluation of microtremor H/V spectral ratio

The objective of this work is to perform a purely empirical assessment of the actual capabilities of the horizontal-to-vertical (H/V) spectral ratio technique to provide reliable and relevant information concerning site conditions and/or site amplification. This objective has been tackled through the homogeneous (re)processing of a large volume of earthquakes and ambient noise data recorded by different research teams in more than 200 sites located mainly in Europe, but also in the Caribbean and in Tehran. The original recordings were first gathered in a specific database with information on both the sites and recorded events. Then, for all sites close to an instrumented reference, average site-to-reference spectral ratios (“spectral ratio method” (SSR)) were derived in a homogeneous way (window selection, smoothing, signal-to-noise ratio threshold, averaging), as well as H/V ratios (“HVSRE–RF”) on earthquake recordings. H/V ratios were also obtained from noise recordings at each site (either specific measurements, or extracted from pre- or post-event noise windows). The spectral curves resulting from these three techniques were estimated reliable for a subset of 104 sites, and were thus compared in terms of fundamental frequency, amplitude and amplification bandwidth, exhibiting agreements and disagreements, for which interpretations are looked for in relation with characteristics of site conditions. The first important result consists in the very good agreement between fundamental frequencies obtained with either technique, observed for 81% of the analyzed sites. A significant part of the disagreements correspond to thick, low frequency, continental sites where natural noise level is often very low and H/V noise ratios do not exhibit any clear peak. The second important result is the absence of correlation between H/V peak amplitude and the actual site amplification measured on site-to-reference spectral ratios. There are, however, two statistically significant results about the amplitude of the H/V curve: the peak amplitude may be considered as a lower bound estimate of the actual amplification indicated by SSR (it is smaller for 79% of the 104 investigated sites), and, from another point of view, the difference in amplitude exhibits a questioning correlation with the geometrical characteristics of the sediment/basement interface: large SSR/HV differences might thus help to detect the existence of significant 2D or 3D effects.Published75-1084.1. Metodologie sismologiche per l'ingegneria sismicaJCR Journalreserve

MICROZONAZIONE DI NOCERA UMBRA

CNR-GNDT, ROMA, 2000, 228 PP + CD-RO

The Benevento seismic risk project: I - seismotectonic and geotechnical background

Benevento, a town of 62000 inhabitants, has been an European test-site for a detailed hazard and microzonation investigation. The town, approximately 13 sq kilometers, is located in the Southern Appennines (Italy), and in the past was affected by several earthquakes. The ancient part of the city is situated on a hill between the rivers Calore and Sabato ; the subsoil consists mainly of conglomerate formations, fluvio-lacustrine deposits and pliocenic clays, the mechanical properties of which have been obtained through detailed geological investigations and a compilation of the existing borehole and laboratory data, as well as through a literature survey. The regional seismic hazard has been estimated by different teams on the basis of the up-to-date seismicity catalogues and attenuation laws, providing uniform hazard response spectra. This information provides the basis for the site effect evaluation, presented in a companion paper

Analysis of the local seismic hazard for the stability tests of the main bank of the Po river (Northern Italy)

This paper presents the methodology and the results of the local seismic analyses, performed in the bank areas of the Po River (Lombardia and Emilia-Romagna regions, northern Italy) characterized by the highest seismic hazard and finalized for the seismic stability analyses of the banks themselves. The proposed methodology includes the following steps: collection of the pre-existing information and geologic maps; deployment of in-situ investigations by collecting soil samples for geotechnical laboratory tests; definition of the geologic-geophysical and geotechnical model; individuation of the expected seismic inputs; analyses of local seismic response. In this paper, we mainly discuss the results obtained in order to illustrate how the methodology employed enables the definition of the local seismic hazard and allows performing the future stability analyses of the banks. The results are given in term of amplification factors, expected accelerograms and acceleration response spectra modified by the litho-stratigraphic characteristics of the sites