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difference of magnitude
M. Taking a difference of macroseismic intensity
I
=
2
2, a typical value in the application made in this paper, an error of 0.1 on “b”
would for example cause an error of 0.05 on
.
M. In addition to the error on “b”,
the uncertainty of the magnitude of a historical earthquake computed from (3) also
depends on the errors on
I and on the magnitude of the reference event.
2 Application to Historical Earthquakes in the Pyrenees
The Pyrenees is one of the most active seismic zones of France (Souriau et al. 2001).
In the present paper, we focus our attention on the western part of the mountain
range. Since the Lambesc (1909) earthquake in the South of France (M
w
7-
6.1 (Baroux et al. 2003)), this region of the Pyrenees has been visited by the most
damaging French earthquake, with a maximum intensity of VIII near the locality of
Arette. This is also where two large historical earthquakes occurred (Bigorre (1660)
I
max
=
5
.
VIII). In order to apply the differen-
tial technique described above, we choose two events as reference earthquakes,
one located near Lourdes (Argeles-Gazost (2006) M
L
=
VIII-IX, Juncalas (1750) I
max
=
=
4
.
9 ReNaSS, m
b
=
4
.
6
NEIC, M
w
5 from several independent sources) and another one near Arudy
(Arudy (1980) m
b
=
4
.
1 (Gagnepain-Beyneix et al. 1982)). The epicentre of the
Arudy earthquake is located about 35 km from both the macroseismic epicentres
of the Arette (1967) and Bigorre (1660) earthquakes (Fig. 1). The epicentre of the
Argeles-Gazost earthquake is located around 10 km from the Bigorre (1660) macro-
seismic epicentre.
=
5
.
Fig. 1
Epicentres of the Pyrenean earthquakes investigated in this paper
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