Probabilistic seismic hazard in terms of intensities for Bulgaria and Romania – updated hazard maps

Abstract

Since 2007 Bulgaria and Romania are members of the European Union. All member states have to introduce the European earthquake building code EUROCODE 8 (EC 8) in the coming years. Therefore, new seismic hazard maps have to be calculated according to the recommendations in EC 8. Here the authors present a novel approach to compute such hazard maps. We prefer to use the macroseismic intensity as hazard parameter because of two reasons: <br><br> - The irregular azimuthal attenuation pattern of the Vrancea intermediate depth earthquakes can be advantageously taken into account by using detailed macroseismic maps. <br><br> - The intensity is directly related to the degree of damage and is the original information in the historical earthquake catalogues. <br><br> The main base of our probabilistic analysis is the earthquake catalogue for SE-Europe (Shebalin et al., 1998) in combination with national and regional catalogues. Fore- and aftershocks were removed. Seismic source zones inside an area of about 200 km around Romania and Bulgaria were defined based on seismicity, neotectonics and geological development. For each seismic source the intensity-frequency relation was calculated and a maximum possible earthquake as well as a seismogenic depth was estimated. An appropriate attenuation law was assumed. To cope with the irregular isoseismals of the Vrancea intermediate depth earthquakes, a factor Ω was included in the macroseismic attenuation law. <br><br> Using detailed macroseismic maps of three strong intermediate depth earthquakes, Ω was calculated for each observation. Strong local variations of Ω are avoided by averaging within grid cells of 0.5 degree in longitude and 0.25 degree in latitude. <br><br> The contributions of all seismic sources, the crustal normal depth source zones and the Vrancea intermediate depth zone, were summed up and the annual probability of exceedance was calculated. The contribution of the Vrancea intermediate depth zone to each grid point was computed with the corresponding representative Ω of this point; a seismogenic depth of 120 km has been assumed. <br><br> Each final seismic hazard map is a combination of two maps, the one for normal depth source zones and the one for the Vrancea intermediate depth zone. This is illustrated for a recurrence period of 475 years. Additional hazard maps were calculated for different recurrence periods.