Revision of the earthquake catalogue and seismicity of Croatia, 1908–1992

The paper presents the results of Croatian earthquake catalogue revision for the period 1908–1992. The revised catalogue lists a total of 4853 events (4486 after 1908), of which 3700 are mainshocks. All primary data sources available to us (seismograms, phase onset time readings, macroseismic questionnaires…) were consulted in order to improve the quality and reliability of magnitude, intensity and location. Along with the most important earthquake parameters the catalogue contains entries that enable assessment of the reliability of location and bibliographic references. Using the temporal analysis of the maximum-likelihood estimate of the b -value in the frequency-magnitude relation, the catalogue is estimated to be complete for events with M ≥ 4.0 throughout the investigated period. The magnitude completeness threshold decreases to 3.8 after the middle of the century, and to about 3.6 in the last three decades. It is noted that the b-value variation with time is closely tied to the seismic activity, as well as that at present there seems to be a lack of seismic energy release corresponding to the M = 6.0–6.5 earthquake.     

Scattering of elastic waves by a general anisotropic basin. Part 2: a 3D model

Scattering of plane harmonic waves by a three‐dimensional basin of arbitrary shape embedded within elastic half‐space is investigated by using an indirect boundary integral equation approach. The materials of the basin and the half‐space are assumed to be the most general anisotropic, homogeneous, linearly elastic solids without any material symmetry (i.e. triclinic). The unknown scattered waves are expressed in terms of three‐dimensional triclinic time harmonic full‐space Green's functions. The results have been tested by comparing the surface response of semi spherical isotropic and transversely isotropic basins for which the numerical solutions are available. Surface displacements are presented for a semicircular basin subjected to a vertical incident plane harmonic pseudo‐P‐, SV‐, or SH‐wave. These results are compared with the motion obtained for the corresponding equivalent isotropic models. The results show that presence of the basin may cause significant amplification of ground motion when compared to the free‐field displacements. The peak amplitude of the predominant component of surface motion is smaller for the anisotropic basin than for the corresponding isotropic one. Anisotropic response may be asymmetric even for symmetric geometry and incidence. Anisotropic surface displacement generally includes all three components of motion which may not be the case for the isotropic results. Furthermore, anisotropic response strongly depends upon the nature of the incident wave, degree of material anisotropy and the azimuthal orientation of the observation station. These results clearly demonstrate the importance of anisotropy in amplification of surface ground motion. Copyright © 2003 John Wiley & Sons, Ltd.     

Scattering of plane harmonic SH,SV, P and rayleigh waves by non-axisymmetric three-dimensional canyons: A wave function expansion approach

Scattering of elastic waves by three-dimensional canyons embedded within an elastic half-space is investigated by using a wave function expansion technique. The geometry of the canyon is assumed to be non-axisymmetric. The canyon is subjected to incident plane Rayleigh waves and oblique incident SH, SV and P waves. The unknown scattered wavefield is expressed in terms of spherical wave functions which satisfy the equations of motion and radiation conditions at infinity, but they do not satisfy stress-free boundary conditions at the half-space surface. The boundary conditions are imposed locally in the least-squares sense at several points on the surface of the canyon and the half-space. Through a comparative study the validity and limitations of two-dimensional approximations (antiplane strain and plane strain models) have been examined. It is shown that scattering of waves by three-dimensional canyons may cause substantial change in the surface displacement patterns in comparison to the two-dimensional models. These results emphasize the need for three-dimensional modelling of realistic problems of interest in strong ground motion seismology and earthquake engineering.     

Amplification of elastic waves by a three dimensional valley. Part 1: Steady state response

Steady state scattering of incident P, SV, SH and Rayleigh waves by general non-axisymmetric three dimensional dipping layers is investigated by using an indirect boundary integral equation method. Material of the half-space and the layer is assumed to be linear, weakly anelastic, homogeneous and isotropic. Systematic comparisons between three dimensional and two dimensional models demonstrate that the validity of a two dimensional approximation for a given basin shape may be affected strongly by changes in azimuthal angle of incidence, type of incident wave and frequency. The discrepancies of two dimensional modelling appear to be much more pronounced for the case of an incident SH wave. Another important feature of the results is the existence of strong coupling between P/SV and SH modes, which has no correspondence in two dimensional models. Such off-azimuthal mode conversions are particularly strong for an incident SH wave.     

Numerical Modeling of the Ston-Slano (Croatia) Aftershock Sequence

The rate of aftershock occurrence after the M6 Ston-Slano (Croatia) earthquake is modeled as the Epidemic Type Aftershock Sequence (ETAS). Increase of the modeled cumulative number of aftershocks with time was fitted to observations by the least-squares criterion using the combined grid-search and Monte-Carlo approach. This enabled not only the estimation of the most probable ETAS parameters, but also the determination of their confidence limits, as well as the estimation of the bias between them. It has been found that the bias is significant for some of the parameter pairs, regardless of the threshold magnitude assumed. Residual analyses revealed that all strong aftershocks (M _L 4.5) occurred during the periods of normal to high aftershock activity. There were two periods of quiescence in the sequence, both of which were followed by a strong aftershock.     

Seismic Zoning of Croatia

The territory of Croatia and neighboring regions is divided into 17 seismic source zones, considering available seismological and geological data. On this basis, seismic hazard elements (seismicity rate, maximum magnitude, b-value, probabilities of exceedance and return periods for a predefined set of magnitudes) are computed using the maximum likelihood method appropriate for treating data-sets with variable completeness thresholds. The values of long term expected peak horizontal acceleration obtained by using a combination of the deterministic and the probabilistic procedure are the highest in the Dubrovnik zone, while the Zagreb zone has the highest earthquake hazard in the continental part of the country. This revised version was published online in June 2006 with corrections to the Cover Date.     

Peak surface motion due to scattering of plane harmonic P,SV, or Rayleigh waves by a rough cavity embedded in an elastic half-space

Scattering of plane harmonic P, SV, or Rayleigh waves by a two-dimensional rough cavity completely embedded in an isotropic elastic half-space is investigated by using a direct boundary integral equation method. The cavity’s roughness is assumed to be in the form of periodic or random perturbations of arbitrary amplitude superimposed to a smooth elliptical shape. For the randomly corrugated cavities the normal or the uniform probability distribution functions are assumed. Based on multiple random cavity results, the corresponding average surface response is computed. These are compared with the corresponding periodically corrugated and smooth cavity responses. The surface response is evaluated for different cavity shapes and incident waves and for a range of frequencies. The surface motion results are used to determine the peak surface motion frequencies. They depend strongly upon the basic inclusion shape (the principal axes) and the nature of the incident wave. Strong similarity in the peak surface motion frequencies can be observed for the rough and smooth cavity models for both circular and elliptical shapes. In order to quantify the importance of the cavity corrugation upon the surface motion, a roughness influence factor is defined in terms of the rough and smooth cavity surface responses. This factor strongly depends upon the type of the incident wave, the nature of the cavity corrugation, the basic cavity shape, and the frequency. The factor clearly shows the effect of the cavity roughness upon the surface motion.     

Azimuthal anisotropy of the Pg-wave velocity in hypocentral volumes of NW Croatia

Based on a large set of arrival times of the Pg phase reported by local and regional stations, we estimate azimuthal anisotropy of the Pg-wave velocity in focal volumes of the upper crust in NW Croatia. The method is based on analyses of the azimuthal dependence of ratios of cumulative differences of arrival times and travel paths between foci of earthquake pairs, computed for rays propagating within narrow azimuthal windows. The results clearly indicate the presence of anisotropy of 3.3% with the direction of fast velocity (approximately NNE-SSW) coinciding with the direction of the maximum tectonic pressure as revealed by 23 available focal mechanisms and previous geological investigations. Although a large part of observed anisotropy can be explained assuming that focal volumes are pervaded by a system of vertical extensive-dilatancy anisotropy (EDA) cracks aligned under the influence of local tectonic stress field, there is indication that — to a smaller extent — some role was also played by alignment of structural features in the region.     

Consistency of Magnitudes of the Largest Earthquakes of the First Half of the 20TH Century Determined on the Basis of Göttingen and Zagreb Wiechert Seismograms

The magnitudes (M _S , m _bP , m _bS ) of the largest historical earthquakes which occurred in the first half of the 20 ^th century, calculated on the basis of records of Wiechert horizontal seismographs in Göttingen (Germany) and Zagreb (Croatia), are compared with one another, as well as with the magnitudes reported in worldwide catalogues. Systematic trends are observed in the data regarding the temporal stability of magnitude estimations in Göttingen, as well as the apparent non-linearity of the instrument responsle in the case of the Wiechert seismograph in Zagreb. We were unable to clearly identify their causes – possible explanations include effects caused by the interaction of the seismometer's frame and mass, as well as local soil conditions, but nonhomogeneity of the reference catalogues cannot be ruled out. The results indicate that a careful re-examination and cross-checking of the reported magnitude figures for the earthquakes from the first half of the 20th century is required.