Regional and local seismic hazard assessment

Recent earthquakes such as the M-JMA 7.2 Hyogo-ken Nambu earthquake and the M 7.4 Kocaeli earthquake demonstrate once again the need to include detail ed soil investigation into hazard evaluation, that is the need of microzona tion. Seismic hazard assessment evaluated at a regional scale generally doe s not consider soil effects but only in a Limited way using an attenuation law that can be 'soft soil' or 'rock'. However, the relevant role of seismi c hazard in the assessment of seismic coefficients for the definition of th e actions in seismic codes must be properly considered. That is to say, the level of protection of buildings is proportional to a definite level of ha zard (generally considered to be the ground motion with 10% probability of exceedence in 50 years). When a microzonation is performed, this criterion cannot be ignored, therefore, a clear linkage must be established between h azard (regional scale) and microzonation. The crucial point is represented by the reference motion (or input motion) to be used for site effects analy sis, that must be compatible with the regional seismic hazard. In this pape r, three different approaches for reference motion evaluation are analysed: probabilistic; stochastic; and deterministic. Through the case history of Fabriano microzonation the three approaches are compared. It is shown that each approach presents advantages and disadvantages with respect to the oth ers. For example, the probabilistic approach (the reference motion is direc tly derived from the expected response spectra for a given return period) i s Linked with hazard, but produces an overestimation in short periods range , while the deterministic approach correctly simulates the wave propagation but it ends with a kind of conditional probability. Until now, clear crite ria to choose the right approach do not appear to exist and the expert expe rience is of fundamental importance. (C) 2001 Published by Elsevier Science Ltd.