HR: 0830h
AN: S51E-0085    [PDF]
TI: Seismic Microzonation for the Estimation of Sites Effects in an Urban Area - the Case Study of the City of Mulhouse (NE France)
AU: * Beck, E
EM: elise.beck@lorraine.u-strasbg.fr
AF: Laboratoire Image et Ville, 3 rue de l'Argonne, STRASBOURG, 67000 France
AU: Granet, M
EM: Michel.Granet@eost.u-strasbg.fr
AF: Institut de Physique du Globe de Strasbourg, 5 rue Rene Descartes, STRASBOURG, 67000 France
AU: Weber, C
EM: christiane.weber@lorraine.u-strasbg.fr
AF: Laboratoire Image et Ville, 3 rue de l'Argonne, STRASBOURG, 67000 France
AB: The upper Rhine graben is nowadays characterized by a rather low background seismicity and a rather long seismic cycle. However, on 18 October 1356, an earthquake destroyed the city of Basel (Switzerland), and damaged large parts of this area. It had an intensity of IX to X on the MSK scale and an estimated magnitude of 6.0 to 6.5. The upper Rhine graben is characterized by densely urbanized areas and the presence of several sensitive industries (e.g. nuclear power plants, SEVESO chemical industries). The city of Mulhouse (110 000 inhabitants), located 30 km north to Basel, belongs to this seismically risky region. The aim of the research is to study jointly technological and seismic risks in order to assess the impacts of a strong earthquake in this area. To achieve this goal the different parameters involved are integrated within a GIS (seismic data, for example). The set of data will constitute the basic layers of the GIS. This paper focuses on the estimation of site effects in Mulhouse area. A microzonation experiment has been carried out in order to determine fundamental frequencies and site amplification factors which are closely related to near-surface structures. About one thousand measurements have been done on two embedded grid scales. On the first one, 250 measurements are spatially distributed each 500 meters across the whole studied area, covering a surface of about 50 km$^{2}$. On the second grid, measurements have been positioned each 100 meters around so-called "sensitive" sites (chemical industries, fire station, hospital, etc.). Nakamura's method - horizontal-to-vertical-spectral ratio technique based on ambient noise - was chosen among others as no reference site is needed. It is well adapted where no outcropping nearby bedrock site is present. In fact the whole area here is covered with sediments. This method consists in recording microseismic noise and in calculating the spectral ratio between the horizontal components and the vertical component of the observed ground motion in the same site by assuming that the vertical component is free of near surface influences. Ambient noise was recorded during 30 minutes using two seismometers having respectively a period of 1s and 20s. For each measurement, Nakamura's method was performed on twenty one-minute time windows. Finally, the average of the site effects was calculated and plotted on a map. The first results will be presented showing a local and significant amplification at several sites of the city, using maps of fundamental frequency and normalized amplification.
DE: 7223 Seismic hazard assessment and prediction
SC: Seismology [S]
MN: 2003 Fall Meeting