Layered Velocity Models of the Western Bohemia Region

A new robust and effective optimization algorithm – isometric algorithm – was used for the inversion of layered velocity models, with constant gradient in each layer, to find suitable 1-D models for the location of microearthquakes in the individual four subregions of the West Bohemian earthquake swarm region. Models which are considered as optimal yield the minimum sum of the absolute values of the travel-time residua in locating the whole group of earthquakes in the given subregion. The results obtained from the inversion of P and S waves and from P waves only are shown. For comparison, optimum homogeneous models derived by the grid search method, again using both P and S waves and P waves only, are given. The computations indicate that the models for the individual subregions differ from each other. For layered models the differences are more pronounced, as expected, in the upper parts, down to depths of about 5 km. In comparison with the subregions Nový Kostel and Plesná, the P and S wave velocities for subregion Lazy are relatively higher and the P and S velocities for subregion Klingenthal relatively lower. In the lower parts the differences are smaller and the velocities have practically identical gradients. The highest velocities were obtained for subregion Lazy and the lowest velocities for subregion Klingenthal, as well for the homogeneous models. The model that represents the whole swarm region was determined in a similar way. This model is compared with the previously published velocity-depth distribution, obtained from DSS profile VI/70 in the vicinity of the area under study.