Which one of the three latest large earthquakes in Zagreb was the strongest – the 1905, 1906 or the 2020 one?

Following the damaging earthquake of 22 March 2020 (ML = 5.5, Mw = 5.3, Imax = VII EMS) in Zagreb, a question was raised whether this was the largest event after the Great Zagreb earthquake of 1880 (Imax = VIII MSK). The countercandidates are the events of 17 December 1905 and 2 January 1906, for which relevant earthquake catalogues mostly report larger or comparable magnitudes as for the earthquake of 2020, with their maximum intensities mostly within a narrow margin between VII and VII–VIII in various intensity scales. In order to resolve the question, we have (re)analysed all available macroseismic data for the two historical events, collected readings from station bulletins, and analysed available historical seismograms. Macroseismic proxy for the local magnitude (MmR) was estimated on the basis of modelled radii of isoseismals V EMS and VI EMS using the regressions derived for a set of 12 earthquakes in NW Croatia and the neighbouring areas. Macroseismic magnitude was found to be the largest for the 1906 event (MmR = 5.3), followed by MmR = 5.1 for the 2020 quake. Considering the magnitudes computed after Wiechert seismograms from the Göttingen (GTT) station, and from the amplitude/period readings reported from the German stations JEN and HOH for the earthquake of 1906, as well as the magnitudes calculated from broad-band records of the GTTG station and the stations of the Croatian network for the event of 2020, a unified local magnitude of ML = 5.3 is found for both events. The magnitudes of the 1905 earthquake were consistently the lowest of the three. Taking the uncertainties into account, the events of 1906 and 2020 should be considered approximately equal in size. However, the strongest shaking in the centre of Zagreb was caused by the 2020 event. It occurred on the reverse North Medvednica boundary fault, while the macroseismic epicentres of earthquakes of 1905 and 1906 lie practically on the trace of the nearby strike-slip Kašina fault. That Kašina fault could have been the source of the 1906 earthquake is also hinted at by the elongated region of the strongest shaking along its strike

Influence of parameters of an earth’s crust and upper mantle model on Rayleigh waves dispersion

In this paper we considered the influences of the changes of the crust and upper mantle earth model parameters on the Rayleigh waves group velocity dispersion. Final result of such analysis is necessary as the first step in the inversion of dissipation data for the earth interior. The partial derivatives of the group velocity with respect to the shear wave velocity b density r and the layer thickness d were calculated for the continental earth model PEM of Dziewonski et al. (1975), (Table 1). We have modified the model by adding a thin sedimentary layer on top of it. The Thompson-Haskel matrix method (see Appendix) was used for the calculation of theoretical phase velocity dispersion curves with and without taking earth sphericity into account (Fig. 1). Partial derivatives were calculated by the finite difference method (equations (1), (2)). Figures 2, 3 and 4 represent the partial derivatives curves with respect to b, r and d, respectively. The numbers on the curves denote the layers in which the parameter in question was changed. The conclusions of the analysis are the following: (i) Shear wave velocity shows the greatest influence upon the group velocity of Rayleigh waves. The influence of the layer thickness is more expressed, the thinner the layer is. (ii) Partial derivatives with respect to b have peaks for periods that correspond to wave lengths of 3.45-3.80h (h being the depth to the middle of the layer) except for the fourth layer where the wave length equals 3.15h. (iii) Changing the layer thickness results mainly in shifting the group velocity curves toward smaller or greater periods without changing the value of the neighbouring extreme

Seismology in Croatia, 2003–2006

Report submitted to the International Association of Seismology and Physics of the Earth’s Interior of the International Union of Geodesy and Geophysic

Seizmičnost Hrvatske u 1989.godini i potres od 6. prosinca u blizini planine Kamešnice

The seismicity of Croatia and its surrounding areas in 1989 was analysed on the basis of the earthquake catalogue consisting of 361 earthquakes. Its completeness threshold was estimated to be MLC ≥ 3.0. Seismically the most active was the coastal part of Croatia, where the strongest earthquake in 1989 occurred on December 6 with the focus beneath the hill-sides of the Kamešnica Mt. The fault plane solution for this event indicates the presence of a tectonic stress-field directed approximately SW-NE, which is compatible with the assumed anticlockwise rotation on the Adriatic microplate around the pole in Northern Italy, and the associated subduction of the Adriatic plate under the Dinarides. The aftershocks of the Kamešnica Mt. event were numerous, with hypocenters at depth up to 20 km. Macroseismic investigations confirm the frequently observed fact that seismic energy is much more efficiently absorbed perpendicularly to the direction of the Dinaric belt than along it

Fault plane solutions for earthquakes (1956-1995) in Croatia and neighboring regions

The study presents (re)evaluation of fault-plane solutions for 40 earthquakes that occurred in the period 1956-1995 in Croatia and neighboring regions. All events were analyzed in the same manner, using the best available velocity models and the most recently updated earthquake catalogue for that region. For the most important earthquakes our solution is briefly discussed and compared to the results of previous studies when they were available. The results are presented with all related data thus enabling estimation of each solution’s reliability