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The western part of the Bohemian Massif (West Bohemia/Vogtland region) is characteristic in the relatively frequent recurrence of intraplate earthquake swarms and in other manifestations of past-to-recent geodynamic activity. In this study we derived 1Danisotropic qP-wave model of the upper crust in the seismogenic West Bohemia/Vogtland region by means of joint inversion of two independent data sets -travel times from controlled shots and arrival times from local earthquakes extracted from the WEBNET seismograms. We derived also simple 1-D P-wave and S-wave isotropic models. Reasons for deriving these models were: (a) only simplified crustal velocity models, homogeneous half-space or 1D isotropic layered models of this region, have been derived up to now and (b) a significant effective anisotropy of the upper crust in the region which was indicated recently by S-wave splitting. Both our anisotropic qP-wave and isotropic P-and S-wave velocity models are constrained by four layers with the constant velocity gradient. Weak anisotropy for P-waves is assumed. The isotropic model is represented by 9 parameters and the anisotropic one is represented by 24 parameters. A new robust and effective optimization algorithm -isometric algorithm -was used for the joint inversion. A two-step inversion algorithm was used. During the first step the isotropic P-and S-wave velocity model was derived. In the second step, it was used as a background model and the parameters of anisotropy were sought.Our 1D models are adequate for the upper crust in the West Bohemia/Vogtland swarm region up to a depth of 15 km. The qP-wave velocity model shows 5% anisotropy, the minimum velocity in the horizontal direction corresponds to an azimuth of 170°. The isotropic model indicates the V P /V S ratio variation with depth. The difference between the hypocentre locations based on the derived isotropic and anisotropic models was found to be several hundreds of meters. K e y w o r d s : West Bohemia/Vogtland region, earthquake swarm, anisotropy, 1D velocity model, inverse problem, earthquake location Stud. Geophys. Geod., 49 (2005), 501−524 501 © 2005 StudiaGeo s.r.o., Prague J. Málek et al.

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A B S T R A C TUsing a set of synthetic P-and S-wave onsets, computed in a 1D medium model from sources that mimic a distribution of microseismic events induced by hydrofrac treatment to a monitoring geophone array(s), we test the possibility to invert back jointly the model and events location. We use the Neighbourhood algorithm for data inversion to account for non-linear effects of velocity model and grid search for event location. The velocity model used is composed of homogeneous layers, derived from sonic logging. Results for the case of one and two monitoring wells are compared. These results show that the velocity model can be obtained in the case of two monitoring wells, if they have optimal relative position. The use of one monitoring well fails due to the trade-off between the velocity model and event locations. *

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Ð The ASPO method (Amplitude Spectra and POlarities) for the focal-mechanism retrieval from relatively weak events is based on a widely available instrumental setup: A few broadband stations within a denser short-period network. Collectively all stations provide the epicenter location. Complete records are taken from three-component broadband stations, without selecting a particular wave type, or picking amplitudes. It makes the method suitable for automated data processing, and enables studies of the interference crustal phases. Only the amplitude spectra are inverted. This is a robust feature which makes the method insensitive to any timing problems (such as those due to uncertain origin time, or due to technical failures). The ®rst-motion polarities serve as an additional constraint of the amplitude-spectra inversion; only few (clear) polarities are taken from the nearest stations, where they mostly belong to direct P waves. The method seeks ®ve parameters: The focal depth, scalar moment, strike, dip, and rake. Green's function, automatically including possible near-®eld eects and interference (e.g., surface) waves, is calculated by the discrete wavenumber method. ASPO works below the corner frequency, and the time function is not being retrieved. This feature not only minimizes the number of the inverted parameters, but also speeds up the calculation, because the lower the frequency, the faster the discrete wavenumber run. Instead of an exceedingly slow 5-parameter grid search, the inversion is organized in two steps: (i) the depth and moment determination with a coarse grid search of the strike, dip and rake, and (ii) a ®ne grid search of the three source angles. Uncertainty of the best-®tting solution is assessed from the minimum error value and from the scatter of the nodal lines (and/or P and T axes) between min and min + 10%. The method was tested on the clustered M » 3.5 earthquakes recorded by a temporary network of three CMG3-T broadband stations in western Corinth Gulf. A fundamental problem is that the broadband stations suer systematically from event-induced instabilities at horizontal components if earthquakes of the studied magnitudes occur at short distances, 10±30 km. Therefore, the ASPO method could not be applied below 0.1 Hz. As such, the results are sensitive with respect to unknown crustal structure details, and the focal mechanisms remain rather uncertain (minimum error higher than 0.34). Compared to synthetic tests with perturbed data, in which the error is lower than 0.2, it is concluded that the crustal model needs further improvement.

As part of the international refraction measurements in Central Europe in the year