Amplitudes of core waves recorded in Macedonia

Summary The amplitude-distance function of the short period PKP waves, recorded at Macedonian seismological stations from sources in the SW Pacific Ocean, is investigated. The dependences found for sources with depths under 100 km and for sources with depths over 500 km are compared with the results of some other authors.     

Theoretical ray seismograms of short periodPKP waves

Summary This paper shows some examples of theoretical seismograms of short period PKP waves calculated by the zero approximation of the ray theory. The influence of the epicentral distance, source time function and the Earth's crust on the form of the seismograms is shown.     

Vertically Inhomogeneous Models of the Upper Crustal Structure in the West-Bohemian Seismoactive Region Inferred from the Celebration 2000 Refraction Data

As part of the international refraction measurements in Central Europe in the year 2000, three profiles traversed the region of earthquake swarms in West-Bohemia/Vogtland. The shots were also recorded at the permanent stations of the local seismic networks. The travel times of P-waves, observed in the West-Bohemian region, are discussed and interpreted in the present paper. In general, significantly lower P-wave velocities were found in the Saxothuringian (northern) part of the studied area than in the adjacent southern parts. The observed travel times are interpreted separately for the individual geological units, in particular for the plutons, crystallinicum, and the Mariánské Lázn (Marienbad) Complex. After smoothing the selected data using rational approximations, the Wiechert-Herglotz method was used to compute vertically inhomogeneous velocity models. The characteristic features of the derived models are relatively low P-wave velocities at the surface and prominent velocity increases within the uppermost crust down to a depth of about one kilometre.     

Focal Mechanisms of Weak Earthquakes from Amplitude Spectra and Polarities

—?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 first-motion polarities serve as an additional constraint of the amplitude-spectra inversion; only few (clear) polarities are taken from the nearest stations, wher e they mostly belong to direct P waves. The method seeks five parameters: The focal depth, scalar moment, strike, dip, and rake. Green's function, automatically including possible near-field effects 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 fine grid search of the three source angles. Uncertainty of the best-fitting 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 suffer 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.