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The intraplate West Bohemia region is characterized by relatively frequent earthquake swarms mostly concentrated in the vicinity of the Nový Kostel village. Records of selected microearthquakes of the 2008 seismic swarm from a small circular area around Nový Kostel with a radius of 11 km are processed with focus on availability of precise onset readings from all the selected events at all the selected stations. These data were used in the joint hypocenter-velocity inversion, employing the so-called isometric method which is especially efficient for this task. The results are two new one-dimensional velocity models, the constrained (smoother) and the unconstrained one, of the upper crust down to the depth of 11 km, and absolute locations of the selected earthquakes in the new constrained model. The foci delineate two segments of a steeply dipping fault zone. Time residuals at each of the stations display a remarkable azimuthal dependence, for both P and S waves, caused by anisotropy. No systematic dependence on epicentral distance was found even for stations outside the target area, which means that the new 1D models seem to be reasonable approximations of the real structure in the whole, much larger, West Bohemia region. No time dependence indicating possible changes of v P , v S and their ratio during the swarm was observed. A set of P- and S-wave station corrections that should be used to correct onset times in order to reach precise absolute locations was derived as an integral part of the new models. [ABSTRACT FROM AUTHOR]

The ratio of P- to S-wave velocities, V/V, is an important parameter characterizing rock composition and fluid saturation. We have studied properties of the ratio in the West-Bohemian seismically active region, using data from the earthquake swarm which occurred here in 2008. The earthquake swarm was well recorded by 23 seismic stations from epicentral distances less than 25 km. We selected a subset of 158 events with local magnitudes between 1.5 and 3.8. Applying the Wadati method to the measured arrival times of P and S waves, we arrived at an average value of V/V =1.68 ± 0.01. This differs a little from the value of V/V = 1.70, which is routinely used for earthquake locations in the region at present. Moreover, it was recognized that the points in the Wadati graphs for some stations were systematically deviated from the mean straight lines. In particular, the stations with the largest positive deviations (above the mean straight lines) are situated close to the Mariánské Lázně Fault and to some intensive mofettes. Further analyses revealed reduced P- and S-wave velocities along the seismic rays toward these anomalous stations. In our opinion, the seismic waves arriving at the anomalous stations probably propagated along a fault or another zone of weakness. In this way, our results support the hypothesis that the Mariánské Lázně Fault is a deep-seated fault continuing down to the seismically active zone of local earthquakes. From a general point of view, this study demonstrates that even some narrow structural anomalies in the crust, such as fault zones, can be recognized by the simple Wadati method if data from a dense seismic network are available. [ABSTRACT FROM AUTHOR]