Refined locations of the swarm earthquakes in the Novy Kostel focal zone and spatial distribution of the January 1997 swarm in western Bohemia, Czech Republic

In order to improve the accuracy of the spatial distribution of earthquake foci in the principal Novy Kostel focal zone, refined focal locations of about 1500 micro-earthquakes of the 1991 - 1997 period were determined using the relative Master-Event location method. To estimate the reduction in the scatter of located hypocentres, the results were compared with those obtained by routinely used FASTHYPO method and cluster analysis (the nearest-neighbour method) was applied to the located foci to evaluate the spatial distribution of the foci. Based an the results of refined location and of the cluster analysis, a concept of seismic energy release in space and time in the main focal zones was developed. Especially the January 1997 earthquake swarm was studied in detail: 946 events were located with the Master-Event location method and the dimensions and geometry of focal clusters were determined. Type analysis was applied to waveforms to divide approximately 800 located events the eight multiplet groups to each of which a characteristic source mechanism Ir as assigned The spatial distribution of the foci as well as of the eight types of source mechanisms was revealed in this way and also the planes fitting the clusters of foci with two predominant source mechanisms were determined fairly well.