Methodology for updating magnitudes assigned to historical earthquakes: application to the La Tour-du-Pin 1889 earthquake

In the present work, a methodology is proposed to reassess the magnitude assigned to historical earthquakes thanks to a Bayesian updating approach. The initial information used to estimate the magnitude in the catalogue is supplemented by new data. The data is obtained with a combination of in situ observations and fragility characteristics of historical buildings. It is shown that no damage or low damage of buildings can be used to reduce the uncertainty on historical earthquakes. The methodology is applied to a case study in Southern France, the 1889 La-Tour-du-Pin earthquake. With the data and model considered, a small effect for the updating is obtained with an average value for the magnitude \(M_w\) going from 4.4 to 4.39 and an uncertainty \(\mathrm{err}_{M_w}\) going from 0.42 to 0.41. The impact of the choice of parameters of the Ground Motion Model (GMM) for this study is investigated. Indeed, collecting more information on the site in order to reduce uncertainty and using site-specific GMM could improve the accuracy of the estimations in future applications of the approach.