Magnitude of Historical Earthquakes, from Macroseismic Data to Seismic Waveform Modelling: Application to the Pyrenees and a 1905 Earthquake in the Alps

Foreword Magnitudes of pre-instrumental moderate-size earthquakes (M similar to 5.5) strongly rely on the way macroseismic data are interpreted. In the first part of this paper, after recalling how macroseismic intensity is linearly related to magnitude, we apply a method based on the comparison between historical and recent earthquakes to estimate the moment magnitudes Mw of three earthquakes in the French Pyrenees (Bagneres-de-Bigorre (1660); Juncalas (1750); Arette (1967) and one earthquake in the Alps (Chamonix (1905)). In the second part of the paper we discuss these results in the light of two waveform modelling experiments related to the 1905 Chamonix earthquake, an event well recorded by a Wiechert instrument in Gottingen, and the more recent Arette (1967) earthquake by using WWSSN records. Our instrumental estimate for the Arette (1967) earthquake is 5.1 MW while we find 5.0 Mw from the macroseismic data. This confirms the rather low magnitude of this most destructive earthquake in continental France since 1909. For the Chamonix (1905) earthquake we find 5.5 Mw, a value close to our macroseismic estimate 5.6 Mw. This good agreement between our macroseismic and instrumental Mw is encouraging for future application of the differential rnacroseismic rnethod to historical earthquakes, such as the application presented here for the Bigorre (1960) and the Juncalas (1750) Pyrenean earthquakes.