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Magnitude of Historical Earthquakes,
from Macroseismic Data to Seismic Waveform
Modelling: Application to the Pyrenees
and a 1905 Earthquake in the Alps
M. Cara, P.-J. Alasset and C. Sira
Foreword
Magnitudes of pre-instrumental moderate-size earthquakes (M
5.5)
strongly rely on the way macroseismic data are interpreted. In the first part of this pa-
per, 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 M
W
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 M
W
while we find 5.0 M
W
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 M
W
, a value close to our macroseismic estimate 5.6 M
W
. This good
agreement between our macroseismic and instrumental M
W
is encouraging for fu-
ture application of the differential macroseismic method to historical earthquakes,
such as the application presented here for the Bigorre (1960) and the Juncalas (1750)
Pyrenean earthquakes.
∼
1 Introduction
Macroseismic observations are the only information available for estimating the
magnitude of pre-instrumental earthquakes when no fault rupture is observable at
the surface, as it is the case for most moderate-size earthquakes in Europe (magni-
tude
5.5). Macroseismic scales currently in use are twelve-degree scales that were
formerly based on the Mercalli Cancani Sieberg scale (MCS) (Sieberg 1932). It is
important to recall that the suggestion to extend the former ten-degree European
scales to twelve degrees is due to Cancani who suggested a quantitative approach
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