Source mechanisms of the 2000 earthquake swarm in the West Bohemia/Vogtland region (Central Europe)

Abstract

An earthquake swarm of magnitudes up to ML = 3.2 occurred in the region of West Bohemia/Vogtland (border area between Czech Republic and Germany) in autumn 2000. This swarm consisted of nine episodic phases and lasted four months. We retrieved source mechanisms of 102 earthquakes with magnitudes between ML = 1.6 and 3.2 applying inversion of the peak amplitudes of direct P and SH waves, which were determined from ground motion seismograms. The investigated events cover the whole swarm activity in both time and space. We use data from permanent stations of seismic network WEBNET and from temporal stations, which were deployed in the epicentral area during the swarm; the number of stations varied from 7 to 18. The unconstrained moment tensor (MT) expression of the mechanism, which describes a general system of dipoles, i.e. both double-couple (DC) and non-DC sources, was applied. MTs of each earthquake were estimated by inversion of three different sets of data: P-wave amplitudes only, Pand SH-wave amplitudes, and P-wave amplitudes along with the SH-wave amplitudes from a priori selected four “base” WEBNET stations, the respective MT solutions are nearly identical for each event investigated. The resultant mechanisms of all events are dominantly DCs with only insignificant non-DC components mostly not exceeding 10 per cent. We checked reliability of the MTs in jack-knife trials eliminating some data; we simulated the mislocation of hypocentre or contaminated the Pand SH-wave amplitudes by accidental errors. These tests proved stable and well constrained MT solutions. The massive dominance of the DC in all investigated events implies that the 2000-swarm consisted of a large number of pure shears along a fault plane. The focal mechanisms indicate both oblique-normal and oblique-thrust faulting, however, the obliquenormal faulting prevails. The predominant strikes and dips of the oblique-normal events fit well the geometry of the main fault plane Nový Kostel (NK) and also match the strike, dip, and rake of the largest ML=4.6 earthquake of a strong swarm in 1985/86. On the contrary, the 2000-source mechanisms differ substantially from those of the 1997-swarm (which took place in two fault segments at the edge of the main NK fault plane) in both the faulting and the content of non-double-couple components. Further we found that the scalar seismic moment M0 is related to the local magnitude ML used by WEBNET as M0  10 which differs from the scaling law using moment magnitude Mw, i.e. M0  10 (Kanamori, 1977).