SESAME: Site effects assessment using ambient excitations

• Unité de recherche: PCRD EU
• Numéro de projet: 00.0085-2
• Titre du projet anglais: SESAME: Site effects assessment using ambient excitations

Mots-clé (Anglais)

Ambient vibrations; H/V ratio; numerical modelling; finite difference; earthquake ground motion; site effects; earthquake

Autre Numéro de projet (Anglais)

EU project number: EVG1-2000-00026

Programme de recherche (Anglais)

EU-programme: 5. Frame Research Programme - 1.4a.8 Generic R&D activities

Description succincte (Anglais)

See abstract

Partenaires et organisations internationales (Anglais)

Coordinator: UJF, Grenoble (F)

Résumé des résultats (Abstract) (Anglais)

This is a summary of the contributions of the Swiss Seismological Service to the project SESAME during the first year of the work. Several tasks have started:
1) Develop tools to analyse ambient vibration measurements
2) Define optimum measurement conditions
3) Numerical simulations of ambient vibrations
We have started to develop and test numerical algorithms to be used during the project, in order to analyse ambient vibration wavefields. This is done for single station recordings using H/V spectral ratios, and for array measurements using the propagation characteristics of the fundamental mode Rayleigh wave. Inversion techniques have been developed in order to invert for structural models that explain the observations (Fäh et al., 2002).
A special task is concerned with the further development of a method to determine S-wave velocities from ambient vibration measurements with array techniques. An additional large array experiment was performed in Basle in cooperation with the university of Potsdam. The Basle site is one of the test sites in the SESAME project together with Grenoble, Thesaloniki, Liege and Colfiorito. The cooperation with Potsdam provided for a large number of instruments to be deployed simultaneously. The data will allow for the comparison of several sub-arrays and their results, giving insight in the instrumental set-up for the array technique.
Recordings of ambient vibrations are very sensitive to the measurement conditions due to the low amplitude of ground motion. A series of tests on instrumental set-up has been performed, and also pre-existing data was evaluated for set-up conditions. The Swiss team could in this way contribute to the reference data set collected in the SESAME project, from which now several recommendations for instrumental set-up could be established. These results will help to reduce the error in the ambient vibration recording.
Numerical modelling of synthetic ambient vibrations is used to simulate for realistic structural models. A workshop has been organised in Zurich in order to define the different models. Models are selected with and without lateral heterogeneities in terms of material discontinuities, smooth velocity variations inside layers and large velocity contrasts. Since mainly surface sediments have to be properly modelled, high Poisson's ratios are included. Synthetic ambient vibrations will now be modelled in the second year.

Références bases de données (Anglais)

Swiss Database: Euro-DB of the
State Secretariat for Education and Research
Hallwylstrasse 4
CH-3003 Berne, Switzerland
Tel. [phone]
Swiss Project-Number: 00.0085-2