Geoscience Reference
In-Depth Information
Ta b l e 3
Magnitude evaluation for the 18 Septembre 1692 earthquake (1) with the Boxer program
on the minimum/maximum intensity (I
min
/
I
max
) data for Belgian and Italian attenuation laws. RI
min
and RI
max
are the mean radii for the two datasets, respectively; (2) with the Ambraseys and Levret
et al. attenuation laws on the traced isoseismals of Fig. 5, considering a focal depth of 15 km. R is
the mean radius of the enclosed area of the considered isoseismal
Intensity I
min
Belgium
I
min
Italy
RI
min
(km)
I
max
Belgium
I
max
Italy
RI
max
(km)
Ambraseys
laws
Levret
et al. laws
R
(km)
V
=
6
.
05
M
=
6
.
75 205
M
=
6
.
08
M
=
6
.
78 213
M
=
6
.
22
M
=
6
.
23
252
VI
M
=
6
.
29
M
=
6
.
40 142
M
=
6
.
52
M
=
6
.
54 179
M
=
6
.
19
M
=
6
.
24
127
VII
M
=
5
.
26
M
=
5
.
81
18
M
=
6
.
15
M
=
6
.
49
62
M
=
6
.
12
M
=
6
.
04
45
et al. relationships by considering a depth of 15 km. Such a value seems coherent
with the large geographical extension of the effects of the 1692 earthquake and also
of the probable fault surface for an earthquake of this importance. For the Italian
relationships, the depth is implicitely fixed at 10 km.
The magnitude of the earthquake ranges from 5.3 (5.26) to 6.5 (6.52) when
using minimum or maximum intensity evaluations and the attenuation laws valid
for northwest Europe and France. The range of values obtained by BOXER are
excessive in the two directions because the reality is surely in between because con-
sidering only minimum (maximum) intensity evaluation minimizes (maximizes) the
real effects of the earthquake. Taking the average value gives M
4. Using
the isoseismals of Fig. 5 and the Ambraseys and Levret et al. laws, the evaluation
gives identical result: M
=
6
.
1
±
0
.
1.
These evaluations suggest a magnitude value in the range 6-6
1
=
6
.
1
±
0
.
4
.
Using attenuation laws for Italy provide magnitude values 0.5 higher as if the cor-
responding relations for Northwestern Europe are applied. This observation justifies
the choice to modify the attenuation laws in the BOXER software.
/
6 The Aftershocks of the 1692 Earthquake
The same day that the main shock occurred, two major aftershocks were felt from
Liege to Essen. Even the following days, shocks were still felt in the neighbour-
hood of Verviers and one major important shock on 28 October was even felt up
to Brussels. On 19 March 1694, another strong shock occurred. At the exception of
the fourth aftershock, only mentioned in Aachen, the other earthquakes have been
reported from different localities, indicated in Fig. 6. Table 4 provides the location
of the barycenters of the coordinates of the towns where the earthquakes have been
noticed and the distance of the farthest of these cities to this barycenter and to the
epicenter of the main schock.
There are not enough data to certify that these earthquakes are really originated
from the mainshock area. Aftershocks 1, 2 and 3 have been reported in distant
German towns whereas up to now, no mentioning has been found to the west in
the Belgian province of Brabant though the mainshock was here strongly felt. Per-
haps these earthquakes occurred more to the east in the Lower Rhine Embayment
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