INTEGRATION OF SURFACE GEOPHYSICS AND PALEOSEISMOLOGICAL STUDIES, UPPER RHINEGRABEN, GERMANY

The geophysical investigations presented have been carried out to support paleoseismic studies in the Northern Upper Rhinegraben (NURG). To date, we have focused on the paleoseismic potential of the western border fault (WBF) north of the city of Worms. The fault has been documented from reflection seismic surveys and coring but recent/paleoseismic surface faulting has not been previously identified. The seismicity of this part of the NURG is low. However, geological data, geodetic measurements and geomorphological studies provide good indicators of active tectonics along the WBF. In order to identify near surface faults we applied geophysical techniques such as ground penetrating radar (GPR), electrical profiling and high resolution reflection seismics. For GPR we principally used the 200MHz antenna of a GSSI SIR system, electrical profiling was done using a Wenner array with 2m electrode spacing and reflection seismics used a 50kg drop weight and 1 to 2m geophone spacing. We selected a trench site where all measurements revealed near surface structures and where geological data and geomorphological indications were most promising for active faulting. In the first trench a highly faulted zone in Wuermian alluvial loess occurs. Electrical profiling show a contrast in resistivity and GPR measurements reveal steeply dipping reflectors at the fault zone. In contrast, no significant fault exists in the second trench, only 30m away which has the upper 3m dominated by Holocene paleosols. Measurements at this site reveal subhorizontal dipping of sedimentary units. The applied measurements exhibit good correlation with the observed structures and lithologies in the trenches and enable calibration of measurements. For identification of the location and depth of fault structures the use of shallow geophyics has been very fruitful when integrated with geomorphological analysis, geodetic and geological data of the NURG.