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Ta b l e 1
Summary of the different magnitudes investigated in this study
Event
date
m
b
Instrumental M
w
Macroseismic M
w
Proposed M
W
4.6
(
a
)
4.5
(1)
Argeles-Gazost
17-11-2006
-
4.5
5.1
(
b
)
5.2
(1)
Arudy
29-02-1980
-
5.2
5.1
(a)
,4.9
(b)
5.1
(2)
5.0
(1)
Arette
13-08-1967
5.0
5.9
(a)
,5.7
(b)
5.8
(1)
Juncalas
24-05-1750
-
5.8
6.2
(
a
)
,6.0
(b)
6.1
(1)
Bigorre
21-06-1660
-
6.1
4.5
(3)
Vallorcine
08-09-2005
-
-
4.5
Epagny
15-07-1996
4.5
(
c
)
4.6
(4)
4.9
(3)
4.8
Grand-Bornand
14-12-1994
-
4.3
(5)
4.4
(3)
4.4
5.7
(3)
,5.6
(4)
,5.5
(5)
5.6
-
italic
: instrumental magnitudes m
b
and M
w
from different agencies and authors (
(a)
5.5
(2)
Chamonix
29-04-1905
-
(c)
and
(b)
(1)
(2)
USGS's PED catalogue;
Gagnepain-Beyneix et al. (1982);
INGV and Geoscience Azur,
this study;
(3)
SED, Geoscience Azur and INGV;
(4)
and
(5)
Braunmiller et al. 2005).
-
bold
: macroseismic magnitude computed from the reference event magnitude
(n)
.
Right column: final M
w
proposed in this study. The cumulative errors in
I,
b and the reference-
event M
w
cause an uncertainty on the macroseismic M
w
is estimated to
±
0.2.
earthquake that occurred in France in the XX century is much shallower than what
was previously thought.
There is another conclusion we can draw from Fig. 4 by comparing the two recent
Arudy (1980) and Argeles-Gazost (2006) earthquakes. Our macroseismic investi-
gation favoured a rather large magnitude difference
7 between the two
earthquakes, similar to the difference between their teleseismic body wave magni-
tude (
M
=
0
.
=
.
5), while the French catalogues published by BCSF show similar
values (4.9 M
L
for Argeles-Gazost (2006) according to ReNaSS and 5.0 M
L
for
Arudy (1980) according to Schlich and Hoang Trong (1987)). In addition to the well
known systematic discrepancy between M
L
and M
w
when looking at the catalogues
of several agencies in Europe (Braunmiller et al. 2005), this example shows that
there is no simple rule to convert M
L
into M
w
when using the BCSF catalogues
covering the last 25 years.
m
b
0
3 Application to the Chamonix April 29th, 1905
Earthquake in the Alps
The northwestern part of the Alps is another seismically active region of France (e.g.
Thouvenot et al. 1998) where we can test the differential macroseismic method. The
Chamonix earthquake of April 29
th
, 1905 is one of the poorly known earthquakes
of this region. Located near the triple border between France, Italy and Switzerland,
it is close to several M
6 earthquakes of the Swiss Alps. The catalogue issued by
the Swiss Seismological Service ECOS (Fah et al. 2003) contains four historical
M
w
>
∼
6 events at distances less than 60 km from the city of Chamonix. On Septem-
ber 8, 2005, an earthquake of magnitude 4.9 M
L
(4.5 Mw) occurred at proximity
of the macroseismic epicentre of the 1905 Chamonix event, near the locality of
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