The impact of the earthquake activity in Western Europe from the historical and architectural heritage records - Intraplate Earthquakes - page 225

magnitude 5.5-6.0 earthquakes. The 1938 Nukerke and 1828 Hannut earthquakes also
caused damage to churches, but their extent is more limited to the region of intensity
greater than or equal to VI, while the three smaller earthquakes with magnitudes around
4.5 caused damage to churches only in the limited regions of significant damage affecting
masonry buildings.
Note that the damage caused to churches during the 1938 earthquake was enhanced by
the thickness of the sedimentary cover, corresponding to a soil fundamental mode period
between 1.0 and 1.5 seconds, as discussed by Nguyen et al . ( 2004 ) . Our analysis differ-
entiates the damage caused to masonry buildings from that observed in large buildings
of the architectural heritage during the 1938 earthquake, permitting us to show that they
differ slightly by location. The damage to traditional houses is more important in the area
where the soil fundamental period is less than 0.3 second (or frequency higher than 3 Hz)
( Figures 8.3 and 8.4 ) . This difference can be explained by the progressive absorption of
the high-frequency seismic energy when the thickness of the sedimentary cover increases,
while the low-frequency cover is preserved and even amplified at the soil natural frequency.
This phenomenon also explains the low intensity in the epicentral area of the 1992 Roer-
mond earthquake by comparison to its magnitude (Meidow and Ahorner, 1994 ) . Hence, by
studying separately the two types of buildings, we provide information concerning two dif-
ferent frequency ranges in the response spectra, which is of high scientific and engineering
interest. These results suggest that it is fundamental to take into account the regional soil
properties in the studied part of Western Europe and that classical spectra that differentiate
the soil effects by their properties in the first 30 meters under the surface are not sufficient to
model earthquake strong ground motions in parts of the study area where the soil thickness
reaches several dozen to hundreds meters.
8.6.4 Risks from small and moderate earthquakes
Thanks to the outstanding study of the 1884 Colchester earthquake by Musson et al .( 1990 ) ,
we were able to compare the consequences of this seismic event, considered as the most
damaging earthquake during the last 400 years in United Kingdom, with six of the numerous
damaging past earthquakes in the region between the Lower Rhine Embayment and the
southern North Sea. The amount of moderate to extensive damage in the localities most
affected by this Colchester earthquake ranges between around 10% and 70% in an area
with a radius of less than 10 km. This is very similar to the observed damage during the
1983 Liege earthquake in Belgium. These two earthquakes warn us of the consequences of
such shallow small magnitude earthquakes that could occur everywhere in Western Europe.
The danger is particularly of concern if the event occurs in the vicinity of a large historical
city, such as Liege.
This is also the case for the three studied earthquakes with magnitude between 5.0 and
5.3. Of course, for thesemoderate earthquakes, the focal depth plays a role in the significance
of the damage and associated geographical extent. Typical effects from a deep earthquake
are well illustrated by the Belgian 1938 earthquake. With an estimated focal depth of around