Earthquakes and geological structures of the St. Lawrence Rift System - Intraplate Earthquakes - page 76

Figure 4.1 Seismicity of the St. Lawrence Rift System together with some geological features. The
shaded area is the SLRS as we define it for discussion purposes. Earthquakes of magnitude larger
than 3.0 recorded between 1980 and 2012 inclusively together with significant earthquakes of Eastern
Canada are shown (Lamontagne et al ., 2008a ) . Seismic zones are: WQSZ, Western Quebec Seismic
Zone; CSZ, Charlevoix Seismic Zone; and LSLSZ, Lower St. Lawrence Seismic Zone, as defined
by Basham et al .( 1982 ) . Other acronyms are: OBG, Ottawa-Bonnechere Graben; SG, Saguenay
Graben; A, Canadian Shield; B, St. Lawrence Platform; C, Appalachian Orogen; LL, Logan's Line;
S, Superior Geological Province; G, Grenville Geological Province; P, Pessamit. The boundaries of
Figure 4.2 are shown.
to the short time window of observation (i.e., since the 1600s). It was also assumed that
these semi-quiescent areas could become active if longer periods, such as tens of thousand
of years, were considered. The other source model assumed that the seismic zones active
in the past will be the likely sites of future sizeable earthquakes.
In this chapter, we describe the current knowledge of the seismically active areas of the
SLRS ( Figure 4.1 ) and examine possible contributing factors to the earthquake activity.
We are going to use the expression “St. Lawrence Rift System (SLRS)” not necessarily to
express a possible relationship between the earthquakes and the rift faults. We use SLRS
as a convenient way to refer to the vast region along the St. Lawrence River, including the
Ottawa and Saguenay river valleys.