Comparison of Two Methods of Seismic Risk Assessment in France

Abstract eng:
Two methods of seismic risk assessment are compared. The first one is based on historical data. It considers statistics of earthquakes of epicentral intensities I0 = V to IX and statistics of areas affected by intensities lower than or equal to I (I<=I0). The risk is estimated in terms of annual probability of occurrence of given damage degrees. Typical outputs are that, in average for masonry buildings, probability of a degree 2 damage is around 10-4 ; 10-5 for a degree 3. The second method is based on convolution of seismic hazard data and masonry building fragility curves. Seismic hazard is described in the form of 3 different maps of the metropolitan France. These maps are outputs of probabilistic seismic hazard assessments of France and provide PGA values for a 475 year return period (PGA values for other return periods are extrapolated on the basis of classical relationship between PGA and return periods). Fragility of masonries is described in the form of a classical log normal distribution of the probability of exceedance of a given damage versus the PGA (median value ‘a’ and variability ‘β’). A sensitivity study is carried out on the median value corresponding to damage degrees 2 and 3. For realistic values of ‘a’ and ‘β’, one of the maps leads to an tremendous overestimate of the risk, as compared to the historically observed risk, another one leads to an underestimate while the third one is in the middle. This approach could be used for reducing uncertainties in probabilistic seismic hazard assessment in low-moderate areas where sufficient historical data are available.

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