Geoscience Reference
In-Depth Information
4 Epicentre
The macroseismic epicenter of an earthquake can be defined as the centre of the
area with maximal intensity. Considering the type of information at our disposal,
with a strong uncertainty on the intensity in many towns, which is expressed by a
range of values, the epicenter will depend on the choice of the epicentral intensity.
Fortunately, our extensive search of historical data allowed us to collect information
from many localities in the epicentral area.
We based our epicenter evaluation on two hypothesis by respectively considering
the barycenter of the epicentral area of I
=
VIII (4 localities) and I
=
VIII and
VII
VIII (15 localities). The obtained respective coordinates are very close to each
other: 50
−
83
◦
Long E. These loca-
tions are around the village of Dison, a few kilometers north of the city of Verviers.
Because of the uncertainties, the epicentre of the 1692 earthquake can be considered
to be located within the area of Verviers-Soiron-Herve-Montzen.
Of course, this conclusion could be revisited in the light of possible new discov-
eries in the archives.
62
◦
Lat N
87
◦
Long E and 50
62
◦
Lat N
.
−
5
.
.
−
5
.
5 Magnitude
The magnitude of historical earthquakes can be evaluated by means of the spatial
distribution of the evaluated intensities compared to that of recent earthquakes for
which a magnitude has been instrumentally determined.
A relatively simple way to evaluate the magnitude is to determine the radius of
the different isoseismals and to compare them to relationships established between
average macroseismic radii and magnitude. Ambraseys (1985) calculated laws valid
for northwestern Europe whereas Johnston (1996) established relationships with the
data from stable continental regions. Levret et al. (1994) developped also a relation-
ship based on historical earthquakes in France. Johnston (1996) considers estimation
of the seismic moment magnitude, M, of Hanks and Kanamori (1979). Ambraseys
(1985) established his laws by using instrumental earthquakes for which the mag-
nitude is defined as M
S
by the Prague formula (Vanek et al., 1962). The magnitude
provided by Levret et al. (1994) is similar to the one defined by Ambraseys (1985).
In the following of the text, M will be used for these three different evaluations of
the magnitude, because for the magnitude range considered here M
S
is relatively
equivalent to M.
With a smaller dataset, Camelbeeck et al. (2000) used the Johnston laws to es-
timate the magnitude of the 1692 earthquake suggesting a magnitude ranging from
M
0to6.5.
A more recent method to evaluate the magnitude for historical earthquakes is
that of Bakun and Wenthworth (1997). The source parameters are directly derived
from the individual intensity observations. Hinzen and Oemisch (2001) applied
this method to the location and magnitude of recent and historical earthquakes in
=
6
.
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