Seismic microzoning of the area struck by Umbria–Marche (Central Italy) Ms 5.9 earthquake of 26 September 1997

Abstract

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 that 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 a way it should be carried out rapidly enough to allow urban planners to give instructions and codes to public administrators. A team of surveyors were trained to collect field information such as geologic and geomorphologic features and, where possible, pre-existing geotechnic or geophysic information. Information was collected and analyzed with the aid of dynamic codes to calculate the possible local site effects. A one-dimensional code, analyzing single soil columns, Shake [Schabel PB, Lysmer J, Seed HB. Shake, a computer program for earthquake response analysis of horizontally layered sites. College of Engineering, University of California, Berkeley, Report PB-220. 1972, p. 207], as well as the two-dimensional codes working with finite or boundary elements, Quad4 [Idriss IM, Lysmer J, Hwang R, Seed HB. Quad4, a computer program for evaluating the seismic response of the soil structure by variable damping finite element procedures. UCB EERC Report No. 73-16. 1973.] and Besoil [Sanò T. Besoil, un programma per il calcolo della propagazione delle onde sismiche. Technical Report SSN/RT/96/9. 1996, pp. 51.], were used and the results are presented as response spectra or amplification coefficients.