UPPER-RHINE-GRABEN STRONG-MOTIONSEISMIC NETWORK IN GERMANY AND FRANCE

摘要

The Rhine Graben is the most prominent tectonic feature in Central Europe north of the Alps.Earthquake hazard is considered to be moderate. Historical data (Basel 1356 and Düren 1756,for example) and recent paleoseismic results, however, point to infrequent occurrences ofmagnitudes up to about 6.5. Thus a substantial earthquake risk for this densely populatedregion is implied.A 30 station strong motion seismic network has recently been established in and around theUpper Rhine Graben in its segment from Heidelberg via Karlsruhe, Strasbourg, Freiburg,Mulhouse to Basel. This novel network is run in co-operation of the earthquake services ofthe Geological Survey of Baden-Württemberg at Freiburg / Germany (LED) and of the Ecoleet Observatoire des Sciences de la Terre at Strasbourg / France (ReNaSS). It complements theordinary seismic survey networks of both institutions for recording local earthquakes abovemagnitude 4. Clipping of strong motion recording is currently set to 1 g ground acceleration.20-bit data technology of KINEMETRICS and AGECODAGIS data loggers together withthe three component EPISENSOR are installed mostly in freefield locations. Data samplingensures recording of seismic signals up to frequencies of at least 40 Hz. All instruments havebeen thoroughly tested and data correctness has been verified. Data transfer to theheadquarters at Strasbourg and Freiburg is partly achieved by telephone links, data may beobtained from there in GSE format. A station map is provided in Figure 1.Strong motion station sites cover the Upper Rhine Graben area now with a spacing of about20 km and thereby allow to study amplitude distance relations in the graben environment overa length of 250 km. Emphasis has been put to form a station cluster in an area of increasedseismicity in the southern part around the city of Basel to supplement the strong motionnetwork in Switzerland (not shown in Figure 1) and to support microzonation efforts. Astation profile crossing the graben sediments in East-West direction from the Feldberg in thecristalline Black Forest to it's counterpart, the Grand Ballon in the Vosges Mountains willreveal the influence of geological structures and of local site effects on ground motionamplitudes. These influences originate from topography, soft soil layering and from the basinstructure within the graben, which is filled with tertiary sediments extending to 1-2 kmdepth. Typical acceleration response spectra within the graben and on it's shoulders will yieldimprovements to building design spectra with special regard to subsurface structures.