One peculiarity of the Saxothuringian Seismotectonic Province is the appearance of earthquake swarms in the Vogtland/NW-Bohemia region. In the years 1997/1998, an earthquake swarm occurred outside this area, in West Saxony, near Werdau. This is the northernmost earthquake swarm detected in this region so far. The geological, the geotectonic and the hydrochemical structures in the areas Vogtland/NW-Bohemia and Werdau are different, e.g., recent volcanic activity is missing in the northern part of the Saxothuringian Seismotectonic Province, but there are hints for sub-recent volcanic activities. The important tectonic fault-zones are the N–S trending Rostock-Leipzig-Regensburg photo lineament and the NW–SE trending Finne– and Gera–Jachimov fault-zones. Over 200 events were recorded from 26 December 1997 to 21 January 1998 (magnitudes range between Ml=−1.5 and Ml=2.0). The spatial extension of the swarm area is in the order of some kilometers. Five different clusters at depths between 10 and 15 km could be distinguished. Waveform cross correlation and relative hypocenter relocalisation were carried out for 50 events recorded at least at 9 stations with suitable azimuthal distribution. By means of cross-correlation the similarity coefficients were calculated between all P- and S-phases of all stations. The relative relocalisation was performed without a master-event, and is based on the inversion of phase-onset time differences of all stations with regard to their spatial and time differences. The evaluation of the CELEBRATION 2000 profile in southern Thuringia provided the 1-D velocity–depth model necessary for the inversion. Results of waveform data are shown by similarity matrices. Three multiplets could be distinguished with average correlation coefficients between 0.75 and 0.95. After the application of the relative relocalisation the relative distances of the events in multiplet 1 decreased strongly. In multiplets 2 and 3, the related distances of the hypocenters are less than 50 m, and the entire multiplet had an extension of about 200 m. After the relative relocalisation and the absolute localisation of the center of the multiplets, tectonic structures become visible, that can be linked to geological and tectonic structures. It seems that the hypocenter configuration is related to tectonic faults, parallel to the Rostock–Leipzig–Regensburg lineament (about N–S). Only multiplets 2 and 3 fulfill the λ/4 requirement. From the distribution of the hypocenters and the extension of the multiplets we can conclude: The character of the seismic sequence changed with time from pre-dominant single events at the beginning to a real earthquake swarm at the end.

The East Thuringian Seismic Network (OTSN) was installed in 1997. It started its operation with five and now consists of six seismic stations, the GRSN (German Regional Seismic Network) station MOX and a control and analysis centre. All stations are equipped with 3-component GÜRALP and short-period seismometers, RefTek 24-bit data acquisition systems (dynamic range 23.5 bit), hard disks, GPS-receivers, modems and communication computers for dial-up purposes. The seismic signals are sampled at 100 Hz and stored on the hard disk. Simultaneously, the signals are processed by a STA/LTA detector which generates an extended event list. The central station calls these event lists once per day, analyses them, produces a list of ‘real’ seismic events and calls the waveform data for these events only from the single stations. All stations operate completely autonomously and the whole system works automatically, but all operations can also be carried out interactively. The event analysis is performed manually using common seismic analysis programs.