Geoscience Reference
In-Depth Information
Considering that most of the low-rise unreinforced masonry buildings commonly found
in these regions of Western Europe should be classified as vulnerability A and B (for the
less vulnerable part of the stock), the quantitative information of moderate or extensive
damage allows us to fix the intensity on the EMS-98 macroseismic scale: Intensity VI thus
corresponds to grade 2 or moderate damage in a few buildings (up to 10%) of vulnerability
class A and B. Intensity VII is associated with damage of grade 3 or extensive damage in
many buildings (more than 10%) of vulnerability class A. Few of these buildings suffered
grade 4 or complete damage. To reach intensity VIII, many of the vulnerability class A
buildings would have to suffer degrees of damage 4, and a few of them should collapse.
For historical earthquakes, there is no or little quantitative information on moderate
or more extensive damage. Therefore, in order to assign intensities, it is important to be
aware of the often poor information and the necessity to use intensity ranges, indicating
the uncertainty of the interpretation. Another significant problem is the lack of information
on the vulnerability of the buildings for which descriptions of damage are given in the
historical sources. Despite the great diversity of the materials used and the construction
methods during historical times, it is often supposed that the vulnerability class of traditional
houses ranges between classes A and B of the EMS-98 intensity scale, similar to that for
the low-rise unreinforced masonry buildings typical of the twentieth century. This may not
be totally correct, and is clearly an aspect that should be investigated more deeply when
studying past earthquakes. In the next section, we will present our evaluation of the intensity
based on the aforementioned hypothesis in the localities of the epicentral area of the M
=
5,
23 February 1828 central Belgium and M
, 18 September 1692 Verviers earthquakes.
For each of the studied earthquakes, we have compiled an inventory of the damage
caused to large buildings of the architectural heritage, mainly churches. For these churches,
occasion of the 1992 Roermond earthquake. They define slight damage as “fine cracks in
plaster, fall of small pieces of plaster and the loosening of pinnacles or comparable building
parts”, moderate damage as “small cracks in walls and vaults, cracks between church tower
and nave and the falling of pinnacles”, and heavy as “large and deep cracks in walls and
vaults, and damage to load-bearing parts”. We added a fourth grade, D4, when parts of the
church collapsed.
=
6
¼
8.5 Typical damaging earthquakes in Western Europe
In this section, we present the results of an investigation on the impact of some destructive
earthquakes that occurred in the study area. The list of seismic events studied is not
exhaustive. It includes seven earthquakes presenting typical characteristics of the seismic
activity of this part of Europe. This allows us to discuss the main characteristics of the
damage caused by moderate and large earthquakes in the area.
8.5.1 The 13 April 1992 Roermond earthquake
TheM
5.3, 1992 Roermond (the Netherlands) earthquake is one of the largest earthquakes
observed during historical times in the Lower Rhine Embayment (
Figure 8.1
)
. Its focal
=
