A method to estimate intensity occurrence probabilities in low seismic activity regions

Abstract

The estimation of site intensity occurrence probabilities in low seismic activity regions has been studied from different points of view. However, no method has been definitively established because several problems arise when macroseismic historical data are incomplete and the active zones are not well determined. The purpose of this paper is to present a method that estimates site occurrence probabilities and, at the same time, measures the uncertainties inherent in these probabilities in low activity regions. The region to be studied is divided into very broad seismic zones. An exponential intensity probability law is adjusted for each zone and the degree of uncertainty in the assumed incompleteness of the catalogue is evaluated for each intensity. These probabilities are used to establish what may be termed 'prior site occurrence models'. A Bayesian method is used to improve 'prior models' and to obtain the 'posterior site occurrence models'. Epicentre locations are used to recover spatial information lost in the prior broad zoning. This Bayesian correction permits the use of specific attenuation for different events and may take into account, by means of conservative criteria, epicentre location errors. Following Bayesian methods, probabilities are assumed to be random variables and their distribution may be used to estímate the degree of uncertainty arising from (a) the statistical variance of estimators, (b) catalogue incompleteness and (e) mismatch of data to prior assumptions such as Poisson distribution for events and exponential distribution for intensíties. The results are maps of probability and uncertainty for each intensity. These maps exhibit better spatial definition than those obtained by means of simple, broad zones. Sorne results for Catalonia (NE of Spain) are shown.