The impact of the earthquake activity in Western Europe from the historical and architectural heritage records - Intraplate Earthquakes - page 210

thick layer of soft Tertiary and Quaternary sediments in the Roer Valley graben in the region
of Roermond.
The second particularity is that, despite the small value of the maximum observed
intensity, the losses caused by this earthquake were evaluated at the time of the earthquake
as around 125 million EUR (Berz, 1994 ) . This may be due to the extent of the moderately
damaged area, which is directly related to the earthquake magnitude and focal depth, in a
region of dense population, industry, and insurance.
A significant part of the estimated losses, corresponding to 24%, is actually due to the
damage caused to churches. It is generally assumed (Grunthal et al ., 1998 ) that, even though
they may be better built, monumental buildings such as churches may be more likely to
sustain damage than ordinary buildings. This is probably true for slight damage, but in the
present case, moderate and extensive damage have been observed in some churches, even
in locations where houses suffered very slight or no damage.
This apparent difference in the behaviour of low-rise buildings and churches is essentially
linked to the fact that seismic wave energy in the high-frequency range, which corresponds
to the first mode natural frequency of houses, is largely attenuated before the waves reach
the Earth surface by a significant thickness of sediments covering the bedrock. To offset this,
at lower frequency the energy content of the seismic signal is less modified by propagation
through poorly consolidated sediments. As the Roermond earthquake source is large enough
to generate seismic energy at frequencies around 1 Hz, in the frequency band of the first
mode of vibration of large buildings such as churches, the seismic energy is sufficient to
cause damage to churches, even at large distance.
Reports and eyewitness accounts from the newspapers about the Roermond earthquake
can be found via the link: http://seismologie.be/cup2014.html .
8.5.2 The 8 November 1983 Liege earthquake
The earthquake that shook the region of Liege (Belgium) on 8 November 1983 at 0 h 49m
(UT) had unusual consequences considering its low magnitude, 4.6. Significant damage
left more than 1,000 people homeless, causing serious logistical problems.
Within an area of 10 km 2 around the locality of Saint-Nicolas ( Figure 8.3 c ), at the
earthquake epicentre, more than 16,000 buildings were damaged by this earthquake (De
Becker, 1985 ; Phillips, 1985 ; Plumier et al ., 2006 ) . The most apparent damage was the fall
of chimneys, bricks or ornamental features. The fall of these features caused damage to roofs
and many cars in the street, suggesting that if the earthquake had occurred during the day the
human consequences (two deaths and some dozen injuries) would have been more tragic.
Many walls were also intensively cracked and it was necessary to shore up many of them.
The worst structural damage was observed in older and poorly built structures. In the two
most affected localities, Saint-Nicolas and Liege, 129 houses were declared uninhabitable
and 37 were demolished. Well-constructed masonry buildings fared relatively well, with
damage usually restricted to chimneys. The damage in two churches in the epicentral area
was sufficient for the authorities to declare a temporary decree of un-inhabitability. There