Abstract
The western part of the Bohemian Massif hosts an intersection of two regional fault zones, the SW-NE trending Ohe/Eger Graben and the NNW-SSE trending Marianske Lazne Fault, which has been reactivated several times in the geological history and controlled the formation of the Tertiary Cheb Basin. The broader area of the Cheb Basin is also related to permanent seismic activity of M-L 3+ earthquake swarms.
The Eastern Marginal Fault of the Cheb Basin (northern segment of the Marianske Lazne Fault) separates the basin sediments and underlying granites in the SW from the Krune Hory/Erzgebirge Mts. crystalline unit in the NE. We describe a detailed geophysical survey targeted to locating the Eastern Marginal Fault and determining its geometry in the depth.
The survey was conducted at the Kopanina site near the Novy Kostel focal zone, which shows the strongest seismic activity of the whole Western Bohemia earthquake swarm region. Complex geophysical survey included gravimetry, electrical resistivity tomography, audiomagnetotellurics and seismic refraction.
We found that the rocks within the Eastern Marginal Fault show low resistivity, low seismic velocity and density, which indicates their deep fracturing, weathering and higher water content. The dip of the fault in shallow depths is about 60 degrees towards SW.
At greater depths, the slope turns to subvertical with dip angle of about 80 degrees. Results of geoelectrical methods show blocky fabric of the Cheb Basin and deep weathering of the granite bedrock, which is consistent with geologic models based on borehole surveys.