International audienceA swarm of earthquakes of magnitudes up to  = 3.8 stroke the region of West Bohemia/Vogtland (border area between Czechia and Germany) in October 2008. It occurred in the Nový Kostel focal zone, where also all recent earthquake swarms (1985/1986, 1997, and 2000) took place, and was striking by a fast sequence of macroseismically observed earthquakes. We present the basic characteristics of this swarm based on the observations of a local network WEBNET (West Bohemia seismic network), which has been operated in the epicentral area, on the Czech territory. The swarm was recorded by 13 to 23 permanent and mobile WEBNET stations surrounding the swarm epicenters. In addition, a part of the swarm was also recorded by strong-motion accelerometers, which represent the first true accelerograms of the swarm earthquakes in the region. The peak ground acceleration reached 0.65 m/s. A comparison with previous earthquake swarms indicates that the total seismic moments released during the 1985/1986 and 2008 swarms are similar, of about 4E16 Nm, and that they represent the two largest swarms that occurred in the West Bohemia/ Vogtland region since the  = 5.0 swarm of 1908. Characteristic features of the 2008 swarm are its short duration (4 weeks) and rapidity and, consequently, the fastest seismic moment release compared to previous swarms. Up to 25,000 events in the magnitude range of 0.5 <  < 3.8 were detected using an automatic picker. A total of nine swarm phases can be distinguished in the swarm, five of them exceeding the magnitude level of 2.5. The magnitude-frequency distribution of the complete 2008 swarm activity shows a value close to 1. The swarm hypocenters fall precisely on the same fault portion of the Nový Kostel focal zone that was activated by the 2000 swarm ( ≤ 3.2) in a depth interval from 6 to 11 km and also by the 1985/1986 swarm ( ≤ 4.6). The steeply dipping fault planes of the 2000 and 2008 swarms seem to be identical considering the location error of about 100 m. Furthermore, focal mechanisms of the 2008 swarm are identical with those of the 2000 swarm, both matching an average strike of 170° and dip of 80° of the activated fault segment. An overall upward migration of activity is observed with first events at the bottom and last events at the top of the of the activated fault patch. Similarities in the activated fault area and in the seismic moments released during the three largest recent swarms enable to estimate the seismic potential of the focal zone. If the whole segment of the fault plane was activated simultaneously, it would represent an earthquake of ~5. This is in good agreement with the estimates of the maximum magnitudes of earthquakes that occurred in the West Bohemia/Vogtland region in the past

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Two stations monitoring concentrations of carbon dioxide and radon in soil gas (Oldřišská and Nový Kostel) and one station monitoring flow of carbon dioxide at a mofette (Soos) have been operated in the area of the West Bohemian earthquake swarms. We present preliminary results obtained on the base of four-year observations. We found that data are not influenced considerably by barometric pressure. Although the CO 2 concentration varies greatly, the long-term trends at stations Oldřišská and Nový Kostel are similar, which indicates that the CO 2 flow is controlled by common geogenic processes. Also temporal trends of CO 2 and Rn concentrations in soil gas at individual stations are analogous. We found diurnal variations of both CO 2 concentration in soil gas and the CO 2 flow at mofettes due to the earth tides. A response to tides of semi-diurnal period is insignificant in CO 2 concentration and only weak in the CO 2 flow. We also examined possible pre-seismic, co-seismic and post-seismic effects of the intensive 2008 earthquake swarm on the CO 2 concentration at Oldřišská and Nový Kostel, and on the CO 2 flow at Soos. However, all potential indications were insignificant and there has not been proven any influence of the swarm on the CO 2 concentration as well as on the CO 2 flow. Nevertheless, a gradual decrease of amplitudes of diurnal variations before the swarm and the lowest amplitudes during the swarm is a noteworthy phenomenon, which might indicate the strain changes of the rock associated with earthquake swarm.

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