© 2010 European Association of Geoscientists & Engineers 311 Near Surface Geophysics, 2010, 8, 311-319 doi:10.3997/1873-0604.2010020 * heidrun.schuetze@hotmail.com nents, which led to the conclusion that the volcanic rock evolved from a primary Si-under saturated, alkali-ultramafic initial melt (Kroner et al. 2006, Fig. 4). This ‘structure’ is situated in a region of uranium deposits and so it was mapped in detail during prospection campaigns of the SDAG Wismut (uranium industry in former East-Germany) in the 1970s. A compilation of geo- logical features of the Ebersbrunn eruptive breccias for the last 40 years was published by Berger (2008). Recently, the structure came back into the focus of geophysical interest because it is situated close to locations of several swarm earthquakes that occurred in September 1986, December 1997/ January 1998 (Hemmann et al. 2003) and August 2006 (Korn et al. 2008). The 1997/98 swarm consisted of more than 200 single events and the 2006 swarm of about 40 events, both with magni- tudes (ML) up to 2.0. Earthquake swarms usually occur at fault intersections (Neunhöfer and Hemmann 2005). Bankwitz et al. (2003) concluded from satellite images, geomorphologic observa- Geophysical modelling of the Ebersbrunn diatreme, western Saxony, Germany Heidrun Matthes 1* , Corinna Kroner 2 , Thomas Jahr 3 and Horst Kämpf 4 1 Alfred Wegener Institute for Polar and Marine Research, Telegrafenberg A45, 14473 Potsdam, Germany 2 Helmholtz Centre Potsdam, GFZ German Centre for Geosciences, Department of Geodesy and Remote Sensing, Telegrafenberg A45, 14473 Potsdam, Germany 3 Friedrich Schiller University Jena, Institute of Geosciences, Department of Applied Geophysics, Burgweg 11, 07749 Jena, Germany 4 Helmholtz Centre Potsdam, GFZ German Centre for Geosciences, Department of Chemistry and Material Cycles, Telegrafenberg A45, 14473 Potsdam, Germany Received June 2009, revision accepted June 2010 ABSTRACT The Ebersbrunn diatreme near Ebersbrunn, western Saxony, Germany, causes an elliptical Bouguer anomaly of about –2.5 m Gal, which covers an area of approximately 2.2 km × 2 km and an ellipti- cal magnetic anomaly with values between –250 nT and 1000 nT about half a kilometre smaller in diameter and situated within the southern part of the Bouguer anomaly. Considering magnetic sus- ceptibility and density values determined from core samples of the diatreme, a symmetrical, cone- shaped body with an oval surface outcrop of 2 km × 1.5 km and a dip angle of the slopes of about 82° is modelled. It comprises a central structure with a diameter of 800–900 m and susceptibility values up to 9000*10 -6 SI higher and densities up to 220 kg/m 3 lower than the surrounding chimney. Additionally, susceptibility within the body increases with depth and decreases with the degree of weathering. The geometry can clearly be associated with the diatreme of a maar-diatreme-volcano. Regarding the petrographic information (the occurrence of pelletal lapilli as juvenile phase, the basaltoid character of the occurring ground mass of magmatic origin and 80% wall rock content) and the final geophysical model, the Ebersbrunn structure can definitely be regarded as the par- tially eroded remnant (diatreme and root zone) of a maar-diatreme-volcano. The association of earthquake swarms in intraplate regions of intersecting fractures and volcanism has led to renewed interest in the eroded maar-diatreme-volcano. INTRODUCTION During a regional survey of the Earth’s magnetic field in the 1960s made by the former VEB Geophysik Leipzig, Germany, an anomaly different from the typical anomalies of the region was detected close to Ebersbrunn, western Saxony, Germany (Jäger 1964). The area of investigation is an integral part of the Variscan Orogen of Central and Western Europe, located at the northern rim of the Vogtland synclinorium (series of Ordovician to Lower Carboniferous, mostly metasediments). The Kirchberg granite, a Variscan late-collisional granite massif of western Saxony (Förster 1999) outcrops about 1 km south of the mag- netic anomaly (Tschudinowski et al. 1983). From subsequent investigation it emerged that the magnetic anomaly is caused by a deeply eroded volcanic structure mainly consisting of breccias of basaltic tuff (Jäger 1964). Petrographic analyses showed pelletal lapilli to be one of the juvenile compo-