The Tbilisi earthquake of April 25, 2002 in the context of the seismic hazard of the Tbilisi urban area

In the present study we have compiled probabilistic seismic hazard maps of the Tbilisi area; for different parameters (macroseismic intensity, PGA, SA) and different probabilities. The hazard is dominated by the geometry of the seismic source zones that are situated in the immediate vicinity of the city. The northern part of the city, according to all maps, represents the area of highest hazard. Tectonic and seismic information indicates that the maximum expected earthquake in the Tbilisi area should not exceed M =.4.5. The accuracy of these maps was “checked” by the earthquake of April 25, 2002. It turned out that we did not consider any seismic source zone in the city itself because data about active tectonics and seismicity in the central part of the city were not sufficient. In general, the intensity hazard maps are in good agreement with the effects of the earthquake, the PGA hazard maps partly underestimate the ground motion and the SA hazard maps show quite a big difference. The seismicity on the Tbilisi territory is relatively poorly studied. The seismic source zones and their parameters need more precise estimations. The hypocentral locations and magnitude estimations have quite a low accuracy, especially for pre-Sixties events. The analysis of the historical seismicity is very important to study regions with moderate seismicity such as Georgia, but data are subject to large uncertainties and subjective choices.