The gravity field of the Vogtland and NW Bohemia: Presentation of a project

The Vogtland and NW Bohemia are characterized geoscientifically by periodic ally occurrence of swarm earthquakes. The basic geophysical mechanism is no t yet sufficiently clarified, just like detail questions to geology in espe cially the deeper underground. Complex geophysical investigations in the se ismoactive region indicate geodynamic phenomena like mass redistribution or stress accumulation and release (Spicak et al., 1998). According to Grunth al (1989) a weakness zone is suggested in the region of the swarm earthquak es. This zone can be caused by fluid-tectonics (Kampf er al., 1992), a mant le plume (pers. com. J. Svancara, 1999) and/or by the geometry of the geolo gical structures (Neunhofer & Guth. 1988). A three-dimensional gravimetric model can clear up the underground situation. By means of high-resolution g ravimetry a three-dimensional model will be developped for the Vogtland and NW Bohemia region, In the first step a homogeneous Bouguer map of rite Vog tland and NW Bohemia was created (fig. I) containing gravity structures ana lysed by Ibrmajer & Suk (1989) and Blizkovskv et al. (1985). The used gravi metric data were made available by the Saxonian National Office for Environ ment and Geology, by the Czech Geological Survey, Prague and by the GGA Han nover. In the context with the interpretation of the deep-seismic profile MVE 90 a two-dimensional gravimetric modeling was carried out (Behr et al.. 1994), too. Anomaly-producing source bodies apparently do not offer themselves in a two-dimensional model, because after Jung (1961) the length of a gravimet ric source structure must be about four times larger than it's width. The t echnique of the three-dimensional gravimetric modeling by means of any poly hedrons was developed by Gotze (1976. 1984). Gravimetry is a potential meth od and supplies an infinite number of solutions, so the model has to be dev eloped close to other geoscientific results. The aim is to construct a high -resolution three-dimensional underground model. which includes the upper e arth's crust and the deep-seared structures of the,middle and lower crust, too. The determination of the mass distribution in the underground supplies contradicting or supporting facts for geodynamic views in the Vogtland and NW Bohemia for example of Bankwitz et al. (1993). The interpretation of th e Bouguer map of the Vogtland and a three-dimensional gravimetric model oug ht to contribute a substantial, also geodynamic part to understand the orig in and the emergence of the swarm earthquakes LI this region.