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with macroseismic data (M
w
=
5
.
0orm
b
=
4
.
9-5.1). Note also that teleseismic m
b
of M
5 earthquakes provide a good reference for computing the seismic moment
magnitude of this moderate-size Arette (1967) earthquake, while starting from local
magnitudes we find a significantly larger value M
L
=
∼
4.
Classical estimates of both the surface-wave magnitude M
SZ
and duration mag-
nitude M
D
can also be made for the Arette (1967) earthquake. By using the long-
period WWSSN records and a short-period record from a Mainka seismometer in
Bagneres-de-Bigorre, we find M
sz
5
.
2 (Alasset 2005).
This confirms the rather small magnitude we find above for the Arette earthquake.
One can thus conclude from these very different approaches that a magnitude 5.1
(M
w
,m
b
,M
sz
) is a quite well constrained value for the most damaging earthquake
that occurred in metropolitan France since the Lambesc (1909) earthquake. This
also confirms that the hypocentre of this earthquake should have been closer to
the surface than proposed by Rothe (1972) from his interpretation of macroseismic
data.
Estimating an instrumental magnitude for the Chamonix (1905) is much more
difficult. Quite many short period instruments recorded this earthquake in Europe
but very few long-period instruments were functioning at that time. Three horizontal
1-ton Wiechert seismometers recorded the April 29, 1905 earthquake (Strasbourg,
Gottingen and Uppsala). The records in Strasbourg have been lost and those in
Uppsala are of very small amplitude and clearly distorted by the solid friction of
the pen on the smoke paper drum. The two horizontal records made in Gottingen
are of high quality and the amplitudes of the seismograms are large enough so
that comparison with synthetics can be made. Another record is available from a
Rebeur-Ehlert long-period instrument in Uccle, Belgium, but the drum speed was
so small that no signal can be extracted from the record.
The only records we can rely on for the Chamonix (1905) are thus the two
horizontal Wiechert records from Goettingen observatory. Taking several plausible
focal mechanisms for this event based on tectonics hypotheses, we find a seismic
moment magnitude around 5.5 M
w
when modelling both the Love and Rayleigh
waves signals (Fig. 7). This result is in very good agreement with the magnitude
=
5
.
1
±
0
.
3 and M
D
=
5
.
E
s
t
-
W
s
t
N
r
t
h
S
o
t
h
Fig. 7
Fit of the two
horizontal component of the
Chamonix (1905) records
made in Goettingen with
synthetics. The observed
signals (
black line
)and
synthetics (
red line
)arelow
pass-filtered (cut-off
frequency 0.03 Hz). The focal
mechanisms used for
computing the synthetics
corresponds to a normal
left-lateral fault (strike 20
◦
,
dip 70
◦
, rake -70
◦
,
Alasset 2005)
6
6
4
4
2
2
0
0
-2
-2
-4
-4
-6
-6
1
0
0
2
0
0
3
0
0
1
0
0
2
0
0
3
0
0
t
i
e
,
s
t
i
e
,
s
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