Static Source Parameters of the West Bohemia/Vogtland Earthquake Swarms

The static source parameters like seismic moment, stress drop and size of the fault plane are important quantities in assessing the earthquake effects. The static stress drop is commonly accepted to be almost constant but recent studies show that this is not valid in many cases. Thus the confirmation of the validity of scaling laws in the case of small earthquakes can significantly contribute to the assessment of the seismic potential of a particular fault zone. One of the areas with such small intraplate earthquakes is the West Bohemia/Vogtland region in the Central Europe where earthquake swarms occur. The area is continuously monitored since 1990 by WEBNET seismic network which nowadays consists of 13 permanent stations and similar amount of temporary stations. The main instrumentally recorded swarms occurred in 1997 (ML<=3.0), 2000 (ML<=3.2) and 2008 (ML<=3.8). In our study we focused on the swarms 2000 and 2008 for which the similarity analysis using the cross-correlation method was done and earthquakes were classified into multiplets. We applied the spectral ratios method to event pairs inside multiplets to evaluate corner frequencies of inividual earthquakes. Using the absolute moment tensors of specific earthquakes which are known from previous studies and assumption of reasonable rupture propagation velocity (3 km/s) we can figure out the stress drop distribution over the range 1e13 to 2e15 Nm of seismic moment (0.0<=ML<=3.5). To get reliable source parameters the propagation and attenuation effects should be eliminated what can not be sufficiently done using the single event spectral analysis. Comparison of results using both techniques - spectral ratios and single event spectral analysis - shows the advantages and drawbacks.