Earthquakes and geological structures of the St. Lawrence Rift System - Intraplate Earthquakes - page 79

Geological effects can provide evidence of prehistoric earthquakes. The area around
Charlevoix is the site of numerous prehistoric earthquakes that caused submarine slumps
and liquefaction (Doig, 1986 , 1998 ; Tuttle et al ., 1990 ; Filion et al ., 1991 ; Ouellet, 1997 ;
Tuttle and Atkinson, 2010 ; Locat, 2011 ) . From their data, Tuttle and Atkinson ( 2010 )
could not document precise return periods. In the Ottawa River valley, landslide scars are
interpreted as traces left by M > 6 prehistoric earthquakes (Aylsworth et al ., 2000 ; Brooks,
2013 ) . Between Quebec City and Trois-Rivieres, where current seismicity is of very low
level, no such prehistoric earthquake evidence was found (Tuttle and Atkinson, 2010 ) . In
Charlevoix, in the region covered by seismic surveys, no evidence for a coseismic rupture
beneath the surface of the St. Lawrence River could be found (Lamontagne, 2002 ) .
Earthquakes of the St. Lawrence valley also damaged man-made structures. In past
earthquakes, damage was more common in buildings with unreinforced masonry elements
(URM; i.e., masonry without steel reinforcement) that rest on thick clay deposits (Bruneau
and Lamontagne, 1994 ) . For the 1988 Saguenay earthquake, 95% of all damage was
associated with soft soils (53% with clay, 24% on multi-layer, and 18% on sand). It was
also found that damage to buildings built on sandy foundations was restricted to 150 km
epicentral distance, whereas for clay foundations damage existed up to 350 km distance
(Paultre et al ., 1993 ) . It is now recognized that urban and industrial developments in the
SLRS have taken place on soils capable of amplifying earthquake ground motions. The
presence of non-upgraded old buildings coupled with the population growth, particularly on
sensitive soils, make earthquakes a significant natural hazard along the SLRS. Fortunately,
despite the damage to buildings and earth movements, only two earthquake-caused deaths
are known in Canada (Lamontagne, 2008 ) .
In conclusion, the seismic risk in the SLRS is far from being negligible considering the
seismic hazard, the low attenuation of Lg waves, the presence of unconsolidated deposits
capable of amplifying ground motions and subject to mass movements, and a number of
aging buildings built prior to modern building codes.

4.3 Seismic zones of the SLRS

Each year, approximately 350 earthquakes are recorded in the SLRS by the Canadian
National Seismograph Network (CNSN). In the SLRS, the CNSN has had a detection
completeness of about M 1.5 since around 1980. On average, of these 350 earthquakes,
about one or two exceed M 3.5, ten will exceed M 3.0, and about twenty will be reported
felt ( M 2.0). A decade will, on average, include two events greater than M 4.5, which

is the threshold of minor damage. On a yearly basis, most earthquakes occur in three main
seismic zones, namely the Charlevoix, Western Quebec, and Lower St. Lawrence ( Figure
4.1 and Figure 4.2 ) .

4.3.1 Charlevoix

Based on historical and current earthquake rates, the Charlevoix Seismic Zone (CSZ)
is the zone with the highest seismic hazard in continental Eastern Canada ( Figure 4.3 ) .