SEISMICITY AND STRESS-FIELD IN THE VICINITY OF THE KTB LOCATION

A local, broadband, seismic network of four observatory-quality statio ns (KTB-NET) was operated during the drilling of the KTB hole, within the framework of the interdisciplinary German Continental Deep Drillin g Program (KTB). The aim was to investigate the seismic activity with regard to the tectonic stress field and to compare it with data from i n situ measurements in the 9.1-km deep borehole sections (bottom tempe rature of 260 degrees C and heat flow of 82-85 mW/m(2)). From October 1990 to November 1995, over 80 local microearthquakes with magnitudes from 0.2 to 2.8 ML were recorded: eight small events by the KTB-NET on ly and four earthquake swarms with 73 events by the KTB-NET and statio ns of the Vogtland/Western Bohemia networks. Six of the small events a re located within or close to the KTB-NET. The swarm events occurred a t the southwestern extension of the Ohre rift, in an area 20 km north of the drill site, which is revealed to be part of the Vogtland/Wester n Bohemia seismotectonic unit, characterized by swarm activity. The hy pocenters are limited to the upper 13 km of the crust, with a distinct concentration between 10 and 12 km. All types of fault plane solution s are found, but at depths greater than 8 km, reversed faulting mechan isms predominate. P axes are very uniformly oriented in a NNW-SSE dire ction, corresponding to the well-known regional stress orientation in central Europe and in agreement with the special in situ stress measur ements of the KTB program. The focal mechanism of a ML=1.2 event induc ed by a fluid injection experiment fits into the results obtained from the natural events. Possible indications for the brittle-ductile tran sition are discussed in view of the observed earthquake depth and foca l mechanism distributions.