Implementation and validation of an approach for the estimation of the magnitude of historical earthquakes in Azores Islands

Eduardo Charters Morais, Tiago Miguel Ferreira, João M.C.Estêvão, Carlos Sousa Oliveira

In many regions of the globe, the seismic data associated with higher magnitude seismic events usable in the characterization of the seismic hazard is scarce. Therefore, seismologists have been estimating the magnitude and probable epicentral location of historical earthquakes using the historical records and intensity scales to map the observed damage. Another method, employed by earthquake engineers, uses fragility functions, a ground motion model and the Bayes’ theorem to estimate the probable magnitude of historical seismic events. Nevertheless, this method requires assembling an approach consistent with the local seismicity and building stock of the period of reference. This paper implements and validates an approach for the estimation of the magnitude of historical seismic events in Azores Islands, Portugal, that uses fragility functions. In order to carry out this study, data from the damage surveys, a vulnerability distribution and approximate epicentral positions of the 1998 Faial earthquake Mw = 6.0–6.2, as well as a specific attenuation model, are utilized in a calibration phase. The vulnerability index methodology is employed to derive fragility functions from a detailed vulnerability assessment. Afterwards, the approach is validated using analogous data corresponding to the 1980 Terceira earthquake Mw = 6.8–7.2. The different assumptions and results are discussed and compared. The results show that this approach can model expected magnitude values with accuracy: Mw = 6.01–5.76 for the 1998 Faial and Mw = 6.90–6.55 for the 1980 Terceira earthquakes. Additionally, the real PGAs may have been in several locations lower than those predicted by the selected ground motion equation.

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