Determination of Site Factor in and Around Rhine Graben, France in the 1–11 Hz Frequency Band by Non-Reference Network Average Technique

Abstract

The greatest challenge in estimating site response from instrumental recordings of seismic events is removing the source and path effects. Several methods have been suggested in the past in that direction which may be divided into two main categories, depending on whether or not they need a reference site, with respect to which the particular effects at other sites can be estimated. An alternative approach is to determine the site response relative to the network average. In the present paper we propose one such technique for estimating the site factor in the Rhine graben and its surrounding provinces. Judging the higher signal thresh-hold level (≥3 times the background) only sixteen earthquake events with local magnitude varying between 3 and 4.5 are used from the catalogue of 8 years during 1988–1995 recorded by the Fosse Rhenan network using short period single component seismometers belonging to the Ecole et Observatoire des Sciences de la Terre, Strasbourg, Prance and Survey Baden-Wurttenberg, Freiburg, Germany. Sixteen stations in and around Rhine graben are considered for the site factor computation in the 1–11 Hz frequency band. The contour maps prepared from the spatial distribution of non-reference network average site factor estimates at 1, 3, 5, 7, 9 and 11 Hz exhibit high values along the major faults and the minor tectonic features to the western (Vosges) and eastern (Foret-Noire) provinces. Irrespective of the magnitude of the earthquake, the Rhine graben is comparatively stable with low site factors at all the frequencies. As is expected from Coda site amplification studies, the network average contour patterns remained unchanged showing minor or no variation at all with frequencies.