0084 1 Istituto de Ricerca sul Rischio Sismico, Milano, Italy. 2 Servizio Sismico Nazionale, Roma, Italy. 3 Istituto de Ricerca sul Rischio Sismico, Milano, Italy. 4 Istituto de Ricerca sul Rischio Sismico, Milano, Italy. 5 Agenzia Nazionale per la Protezione dell' Ambiente, Roma, Italy. 6 Agenzia Nazionale per la Protezione dell' Ambiente, Roma, Italy. CRITERIA FOR A SEISMIC MICROZONING OF A LARGE AREA IN CENTRAL ITALY F PERGALANI 1 , R ROMEO 2 , L LUZI 3 , V PETRINI 4 , A PUGLIESE 5 And T SANO 6 SUMMARY The earthquake of 26 September 1997 in central Italy is one of the largest seismic events of the last 20 years in Italy. Two main events caused significant damage in a large area of Umbria and Marche Regions and site amplification phenomena were recorded even at large distances from the epicenter. After the emergency period, a detailed study of the surface effects was necessary for the post earthquake reconstruction, but in the way it should be carried out rapidly enough to allow urban planners to give instructions and codes to public administrators. Team of surveyors were trained to collect field information such as geologic and geomorphologic features and, where possible, pre-existing geotechnic or geophysic information. Such an amount of information was collected and analyzed with the aid of dynamic codes to calculate possible local site effects. A one-dimensional code, analyzing single soil columns, SHAKE [14], as well as two-dimensional codes working with finite or boundary elements, QUAD4 [9] and BESOIL [13], were used and the results are presented as response spectra or amplification coefficients. INTRODUCTION After the Umbria-Marche (Central Italy) Ms 5.9 Earthquake of 26 September 1997 the Italian Government decided that the amplification due to local effects had to be taken into account in repair and reconstruction. A working group, formed by researchers of the Servizio Sismico Nazionale (SSN) and the Istituto di Ricerca sul Rischio Sismico-Consiglio Nazionale delle Ricerche (IRRS-CNR), has been charged to define a procedure able to give the needed information on about 1000 villages in six months. The working group was also charged of the guide of the activity. Under this constraints the working group decided the following procedure [11]: 1. selection of 60 sample villages: the criterion was selecting them among those showing the highest degree of damage and representative of the main geologic and geomorphologic features of the area struck by earthquake, in view of the extrapolation of the results to the entire area; 2. collection of the basic geologic, geomorphologic and geotechnic data; 3. field surveys, which implied a geologic and geomorphologic survey at a detailed scale (1:5,000); 4. definition of the seismic input for the numerical analysis; 5. computation of site amplifications through one-dimensional and two-dimensional soil modeling, by finite and boundary elements methods; 6. definition of a set of standard local effect situations and of a table giving the values of the amplification factor for each situation.