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REVISION OF LOCAL MAGNITUDES REPORTED IN THE CROATIAN EARTHQUAKE CATALOGUE FOR THE PERIOD 1908-1999

Croatian Earthquake Catalogue (CEC) contains basic information for over 25000 earthquakes that occurred in Croatia and in the neighbouring territories in the period 373 BC2006. The instrumental part of the catalogue starts in 1908 when the rst Wiechert seismograph was installed in Zagreb (ZAG) by A. Mohorovicic. All seismograms are stored and kept in the archives of the Department of Geophysics, along with notebooks containing calibration data. Mechanical Wiechert seismographs were the ocial ones until replaced with electromagnetic instruments in the early 1980-ies. Digital BB seismographs were installed in the year 2000. As ZAG is the only station in Croatia that has been continuously operating for the last 100 years, the ZAG local magnitude, M(ZAG), is reported in CEC for each event, along with the magnitude taken from various catalogues, bulletins, reports, etc. M(ZAG) is determined from ZAG records using local calibration function initially derived by D. Skoko (1969) on the basis of Wiechert seismograms. The same relation was also routinely used after switching to electromagnetic seismographs (Sprengnether), as well as after the station was moved to a new location. Recent investigations reveal that M(ZAG) shows signicant discrepancies from medians of magnitudes reported by other seismological institutions, especially at short epicentral distances, thus rendering CEC nonuniform and non-homogeneous with respect to magnitude. As CEC is the basic tool for seismic hazard analyses in Croatia it seems imperative to recalibrate ZAG magnitudes of the pre-digital era against a representative sample of magnitude estimates from European stations, as reported in the ISC and other available databases. Here we present results of regressions used to derive new local magnitude formulas, separately for Wiechert (1908-1982) and Sprengnether (1983-1999) seismographs. We also analyse the completeness and homogeneity of the revised catalogue, as well as the Gutenberg-Richter frequency-magnitude relation for the selected subcatalogues.

magnitude; Croatian earthquake catalogue

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