A deterministic-probabilistic approach for seismic hazard assessment

A recurrent criticism of the deterministic approaches for Seismic Hazard An alysis (SHA) is that they make use of a controlling earthquake (usually the maximum to have occurred within a given time and space domain) instead of the frequency of earthquake occurrence. We propose a deterministic method f or SHA, in which, instead of a fixed controlling earthquake, the earthquake frequency is used. The method is based on the deterministic procedure, dev eloped by Costa et al. [Costa et al., 1993. J. Appl. Geophys. 30, 149-160]. This procedure computes synthetic seismograms over a regular grid of recei vers. The input for these estimations consists of available earthquake sour ces and structural models. The earthquake sources are modelled as follows: the seismogenic zones are divided into cells of dimension 0.2 degrees x 0.2 degrees and a point source is placed in the centre of each cell. The magni tude of the maximum observed earthquake in the cell and the available focal mechanisms are then used to assign the seismic moment and the double-coupl e orientation to the source. In this study, the rate of earthquake occurren ce is taken into account, using the recently computed [Molchan et al., 1997 . Bull. Seismol. Sec. Am. 87, 1220-1229] Gutenberg-Richter parameters in ea ch seismogenic zone in Italy. An annual probability of exceedance and an av erage return period are calculated for each event of a given magnitude. The average annual number of events producing ground motion exceeding a given value, and, therefore, an annual probability of exceedance, are then calcul ated at each receiver. The methodology allows us to estimate both the groun d motion due to large and rare events as well as that due to small and freq uent earthquakes, and the obtained results can be easily compared with the probabilistic studies (if available) for standard return periods. The appro ach for seismic hazard analysis proposed here has two outputs: a 'determini stic' (i.e. fixed ground motion due to a fixed event), and a 'probabilistic ' (i.e. annual average number of events, or probability of ground motion ex ceedance over a given time period). The method we propose overcomes one of the most recurrent criticism of the deterministic approaches for SHA, and a t the same time maintains their advantages: clear and realistic estimation of engineering parameters needed in the calculation of Seismic Risk. As an example, the 'deterministic-probabilistic' approach has been applied to est imate the seismic hazard for the region of eastern Sicily in Italy. (C) 199 9 Elsevier Science B.V. All rights reserved.