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in the Bohemian Massif and Central Europe and coincides with the results of the main Variscan tectonic events in that area. This indicates that the anisotropy is caused predominantly by alignment of textural elements and minerals in the rocks, which developed in early geological stages rather than by a preferred orientation of cracks or microcracks due to present-day stress. References Vavryčuk V., Hrubcová P., Brož M., Málek J., and the ALP 2002 Working Group, 2004. Azimuthal variation of Pg velocity in the Moldanubian, Czech Republic: observations based on a multi-azimuthal common-shot experiment. Tectonophysics 387, 189-203. Search for triggering mechanisms and driving forces of earthquake swarms in the western part of the Bohemian Massif by the WEBNET group Forces and mechanisms accountable for origin and development of the earthquake swarms in the West Bohemia/Vogtland region are the issue for understanding the geodynamics of the Western Part of the Bohemian Massif. Seismic observations of the recent intensive 2000-swarm, which occurred in the main focal zone of the West Bohemia/Vogtland earthquake swarms – the area of Nový Kostel in the period from August to December 2000, has been a proper base for investigation of triggering mechanisms and driving forces of the swarm. More than 20 000 earthquakes of magnitudes ML ≤ 3.4 were recorded by 8 permanent and 3 temporary local stations. The swarm took place in nine swarm phases isolated by quiescence periods and showed a pronounced bottom => top => bottom and north => south migration along the fault plane (Fischer, 2003). The recent investigation of the 2000- swarm was focused to (a) precising the earthquake locations and determining the focal mechanisms; (b) search for possible driving forces by analysis of interactions between subsequent events; (c) analysis of multiple-events showing complex rupture process. a) Over 5400 well-recorded swarm events were relocated by the master event method and for the 133 largest, M ≥ 1.7 swarm earthquakes focal mechanisms were determined. The majority of focal mechanisms were, respectively, oblique strike-slips with average strike, dip and slip values of 164°, 70° and -30°, which correspond very well to the geometry of the Nový Kostel focal zone, and showed sinistral movements (Fischer and Horálek, 2005). b) The presence of causal relationship of the swarm earthquakes was examined based on the spacetime relations between consecutive events – the prior event and the immediate aftershock (IA), (Fischer and Horálek, 2005). For this purpose we determined relative position vectors and interevent times for each consecutive event pair. The spatial distribution of the relative positions evinced two pronounced attributes: (i) a high density of IAs near the origin decaying with distance (Fig. 20) and (ii) preferential occurrence of IAs in the slip-parallel direction. The temporal distribution showed further relevant features: (iii) a notably large span of the inter-event times ranging from less than 1 second to more than 104 seconds, and (iv) speedy interaction of some 37 along dip, m 2000 1000 0 -1000 number of IA / 100m2 -2000 -2000 -1000 0 1000 2000 along strike, m 0.01 Fig. 20. The areal density of the immediate aftershocks (IAs) projected onto the fault plane striking 169°. 20 6 2 0.6
consecutive event pairs occurring systematically at greater distances (called fast IAs as opposed to remaining ones denoted slow IAs). These characteristics point out an evident causal link between consecutive swarm earthquakes and a triggering effect of the coseismic stress perturbation, which is induced by a fault slip during the prior earthquake. To examine conditions of triggering the aftershocks upon the main fault plane on account of the prior events we calculated the time-space distribution of both dynamic and static components of the Coulomb stress change on the fault plane surrounding a synthetic rupture (see Fig. 21). We found a close relation between the spatial and temporal distribution of the relative position vectors and of the Coulomb stress changes. The static Coulomb stress field decays rapidly with distance likewise the average areal density of the IAs; oval-like character of the isobars and their elongation in the slip direction matches well the spatial distribution of the slow IAs. The dynamic stress changes generated by transient deformation due to passage of the near-field seismic waves decay much slower with distance and show a two-lobe shape resembling the pattern of the fast IAs. This implies that both static and dynamic coseismic stress changes significantly affect the stress field in the surroundings of the rupture and so they have a substantial impact on self-organization of the swarm activity. Majority of subsequent events at smaller distances are presumably triggered by the static stress field, while the fast IAs occurring at greater distances appear to be generated by the dynamic stress transition. The relatively small magnitude of Coulomb stress changes upon the fault plane (a few hundredths of MPa at distances of a few radii of the prior rupture) might still be capable of governing the aftershock distribution as reported e.g. by Harris (1998). Moreover, highpressured crustal fluids, generally presumed in the region concerned, could locally elevate the pore pressure and thus substantially reduce the shear strength of the fault. Fig. 21. a) Space distribution of the complete stress field on the fault plane surrounding the rupture induced by an instantaneous stress drop of 10 MPa due to strike slip on a circular area with radius of 100 m in a homogeneous half-space. The shear stress component at the time of 0.35 s after the rupture is depicted (normal stress is zero). The stress field breaks with time into the permanent (static) part as a result of the nearfield deformation and the transient (dynamic) part carried by the passage of seismic waves. b) Angular dependence of the rate of IAs. The left panel shows all aftershocks, the right panel shows the fast aftershocks linked by speed higher than 100 m/s. c) The interactions between subsequent earthquakes of the 2000-swarm at the shortest interevent times were studied by Fischer (2005) by the analysis of the multiple-events whose complicated waveforms indicated multiple rupturing episodes. The source-time functions of such earthquakes consist of several pulses whose relative positions provide information on the mutual position of the sub-events. In order to determine the P- and S-wave onsets of the sub-events the waveform modelling method was used (Fig. 22). The P- and S-multiple waveforms were modelled as the sum of waveforms of single sub-events with different hypocentre coordinates and scalar moments, the 38
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Page 5 and 6: Estimates of P-wave anisotropy from
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Page 9 and 10: Organization Units of the Geophysic
Page 11 and 12: Staff of the Geophysical Institute,
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Page 15 and 16: The air-ground temperature monitori
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Page 19 and 20: fourth logging. A passage of the fr
Page 21 and 22: Fig. 3. The borehole climate change
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Page 25 and 26: Fig. 6. Residual gravity anomalies
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Page 29 and 30: Fig. 10. A simplified sketch showin
Page 31 and 32: Deep structure, seismotectonics and
Page 33 and 34: References Špičák A., Hanuš V.
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Page 37 and 38: References Babuška V., Plomerová
Page 39: References Brož M., Hrubcová P.,
Page 43 and 44: Amplitude ratios for complete momen
Page 45 and 46: References Vavryčuk V., 2005. Foca
Page 47 and 48: Fig. 28. Projections of the 3D ray
Page 49 and 50: Fig. 30 shows comparison of QI (red
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Page 53 and 54: with the maximum/minimum resistivit
Page 55 and 56: spectral methods (Hejda and Reshetn
Page 57 and 58: vertical profiles of magnetic susce
Page 59 and 60: ) The rapidly varying geomagnetic f
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Page 63 and 64: Drilling the Eger Rift in Central E
Page 65 and 66: provided by IAGA, was used to cover
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Page 69 and 70: Structure of the budget of the Geop
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Page 73 and 74: Selected publications Alcalá M.D.,
Page 75 and 76: Vavryčuk V., Hrubcová P., Brož M
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