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A major swarm of microearthquakes (main shock: M L = 4.6) occurred in December 1985 and January 1986 in the western Bohemia region (50.2°N, 12.4°E). Here, we revisit the 1985/86 series by modelling some pronounced features of the observed P waveforms frequently seen in the records of the CLL, BRG and the GRF seismograph stations. Travel times and waveforms can be explained by a simple two-layer crustal model consisting of an upper crust with V P = 5.9 km/s and thickness t = 18 km, and a lower crust with V P = 6.8 km/s and t = 11 km. The crust-mantle boundary is at 29 km depth. Waveforms recorded at distances between about 100 and 120 km consist of a low-amplitude first P wave which has travelled through the lower crust. This phase is followed about 0.5 to 1 s later by the large-amplitude main event. About 3 s after the main event another phase stands out with similar amplitude. It probably represents the depth phase sP m P which leaves the focus upwards as an S wave, is converted at the surface to P, and then follows the path of P m P, the reflection from the crust-mantle boundary, to the receiver. We used this depth phase to constrain the focal depth of the main shock to 9 km. Depths of the stronger events ( M L > 3) of the swarm are within the depth range from 7 to 10 km. Inversion of body wave amplitudes and P wave polarities for the focal mechanism of the main event reveals a strike-slip mechanism which agrees well with previously published results. |
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