Seismic microzonation with the use of GIS (Case study for Debrecen, Hungary) K. GRIBOVSZKI Seismological Observatory of Geodetic and Geophysical Research Institute, Hungarian Academy of Sciences, Meredek u. 18., H-1112 Budapest, Hungary, E-mail: kgribovs@ggki.hu G. F. PANZA Department of Earth Sciences, University of Trieste, Via E. Weiss 4., 34127-Trieste, Italy ABSTRACT: The aim of our study is to determine the design ground acceleration values at dif- ferent parts of Debrecen and to accomplish the seismic risk map of Debrecen using synthetic seismograms. Synthetic seismograms are computed by the so called ”hybrid technique” along 11 different profiles crossing the city. The hybrid technique consists of the modal summation method, followed by finite difference modelling. 11 independent computations have been per- formed using the same seismic source but different profiles. The seismic source has been lo- cated in the so called „Mobile Zone”, which is a seismically active fault system in Érmellék re- gion. The focal mechanism and the homogeneous and heterogeneous parts of the profiles are known from geophysical and geological data of the investigated area. As the results of the computations PGA grid maps of Debrecen for the 3 different components and the spectral acceleration (response spectra) charts of the synthetic seismograms for the transversal components came into existence. The seismic risk map of the city has been com- pleted from the SA charts and the grid map of the buildings in Debrecen with different number of floors by applying GIS tools. KEYWORDS: deterministic seismic hazard computation, hybrid method, seismic microzona- tion, GIS, seismic risk 1 INTRODUCTION Debrecen is the second largest city in Hungary after Budapest, with a population of about 300 thousand. It is located in the eastern part of the country not far from the Hungarian-Romanian country border. The Érmellék seismoactive region is the principal earthquake zone to be taken into account for microzonation purposes of the city. Visible macroseismic effects of the Ér- mellék earthquakes were also observed in the territory of Debrecen in 1829 and in 1834 corre- spond to the MKS intensity scale between 6º and 6.5º when in the Érmellék seismoactive region two devastating earthquakes occurred. Because of these high intensity values the determination of the ground motion with the microzonation map of Debrecen is necessary for the further seis- mic hazard (risk) analysis of this area. In the ideal case the seismic ground motion can be determined by comparing it against an earthquake of known intensity and known epicentral distance. In this case it is important to dis- pose of a wide data base of recorded seismic traces arranged from the viewpoint of similar source, propagation path and local site effects. In fact such a database is not available either for the Debrecen area, however the knowledge of the seismic source and wave propagation process – together with the known geological structure – makes it possible to write computer programs to calculate ground motions associated with a given earthquake scenario. By applying the so