Seismic Zonation Methodologies with Particular Reference to the Italian Situation

Abstract

7.1. Introduction Experimental evidences of both large and moderate earthquakes show a high degree of variability of damage distribution. Since the mid of the fifties of past century, scientists realised the existence of a strong influence of soil characteristics and of near field source effects on the recorded values of strong motion. These observations constitute the premises of microzonation studies; the intention is to provide a tool to prevent damages by means of detailed assessing of design forces and land use planning in seismic areas. After the adoption of some over-simplified approaches in the fifties using microtremor analysis (Kanai, 1957), in the sixties Medvedev (1977), proposed a zonation method based on an empirical correlation between the seismic impedance ratio and the variation of macro seismic intensity. In the early eighties, the microzonation approaches became more sophisticated partly owing to the boost of nuclear power plants installations and the deployment of dense accelerometric arrays. Since then, the methods were implemented and nowadays Figure 7.1 well represents a scheme of microzonation methodology commonly adopted. As it is possible to see it encompasses the whole topics necessary to perform a microzonation that basically can be subdivided into three major items: (1) Evaluation of the expected input motion (2) Site effects analysis (3) Preparation of microzonation maps and recommendations for practical application