Monitoring the West Bohemian earthquake swarm in 2008/2009 by a temporary small-aperture seismic array

The most recent intense earthquake swarm in West Bohemia lasted from 6 October 2008 to January 2009. Starting 12 days after the onset, the University of Potsdam monitored the swarm by a temporary small-aperture seismic array at 10 km epicentral distance. The purpose of the installation was a complete monitoring of the swarm including micro-earthquakes (ML <0).We identify earthquakes using a conventional shortterm average/long-term average trigger combined with sliding-window frequency-wavenumber and polarisation analyses.

The resulting earthquake catalogue consists of 14,530 earthquakes between 19 October 2008 and 18 March 2009 with magnitudes in the range of −1.2 ≤ ML ≤ 2.7. The small-aperture seismic array substantially lowers the detection threshold to about Mc = −0.4, when compared to the regional networks operating in West Bohemia (Mc > 0.0).

In the course of this work, the main temporal features (frequency– magnitude distribution, propagation of back azimuth and horizontal slowness, occurrence rate of aftershock sequences and interevent-time distribution) of the recent 2008/2009 earthquake swarm are presented and discussed. Temporal changes of the coefficient of variation (based on interevent times) suggest that the swarm earthquake activity of the 2008/2009 swarm terminates by 12 January 2009.

During the main phase in our studied swarm period after 19 October, the b value of the Gutenberg–Richter relation decreases from 1.2 to 0.8. This trend is also reflected in the powerlaw behavior of the seismic moment release. The corresponding total seismic moment release of 1.02 × 1017 Nm is equivalent to ML,max = 5.4.