A deterministic–probabilistic approach for seismic hazard assessment

Abstract

A recurrent criticism of the deterministic approaches for Seismic Hazard Analysis (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 for SHA, in which, instead of a fixed controlling earthquake, the earthquake frequency is used. The method is based on the deterministic procedure, developed by Costa et al. [Costa et al., 1993. J. Appl. Geophys. 30, 149–160]. This procedure computes synthetic seismograms over a regular grid of receivers. The input for these estimations consists of available earthquake sources and structural models. The earthquake sources are modelled as follows: the seismogenic zones are divided into cells of dimension 0.2°×0.2° and a point source is placed in the centre of each cell. The magnitude of the maximum observed earthquake in the cell and the available focal mechanisms are then used to assign the seismic moment and the double-couple orientation to the source. In this study, the rate of earthquake occurrence is taken into account, using the recently computed [Molchan et al., 1997. Bull. Seismol. Soc. Am. 87, 1220–1229] Gutenberg–Richter parameters in each seismogenic zone in Italy. An annual probability of exceedance and an average 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 calculated at each receiver. The methodology allows us to estimate both the ground motion due to large and rare events as well as that due to small and frequent earthquakes, and the obtained results can be easily compared with the probabilistic studies (if available) for standard return periods. The approach for seismic hazard analysis proposed here has two outputs: a `deterministic' (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 exceedance over a given time period). The method we propose overcomes one of the most recurrent criticism of the deterministic approaches for SHA, and at 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 estimate the seismic hazard for the region of eastern Sicily in Italy.