Earthquakes and geological structures of the St. Lawrence Rift System - Intraplate Earthquakes - page 73

earthquakes ( M < 4.5). A good example of this is the earthquakes of theWQSZ,
which tend to concentrate in a well-defined NW-SE alignment with no obvious
geological control, except perhaps a hypothetical hotspot track. Two local fac-
tors can lead to the occurrence of SLRS earthquakes: weak faults or enhanced
stress levels. We propose that local conditions, concentrated in a few seismic
zones, can alter these factors and lead to the occurrence of earthquakes, espe-
cially those with M < 4.5. At a continent-wide scale, the correlation between
the SLRS and earthquakes is appealing. We suggest, however, that pre-existing
faults related to the SLRS do not explain all features of the seismicity. Seis-
micity is concentrated in more active areas, some with conspicuous normal
faults and some with suspected weakening mechanisms, such as intense pre-
fracturing (e.g., due to a meteorite impact), the passage over a hotspot, or the
presence of intrusions and lateral crustal density variations.
4.1 Introduction
The St. Lawrence Rift System (SLRS), which includes the Ottawa-Bonnechere and Sague-
nay grabens, is located well inside the North American plate ( Figure 4.1 ) . Its earthquake
activity was first described in written accounts from the early 1600s and recorded by seis-
mograph stations since the late 1800s. Table 4.1 provides an overview of the most important
earthquakes of the SLRS together with a summary of the impact on the natural and man-
made environments. Some prehistoric-events have also been recognized. This knowledge
defines seismic zones with sustained seismic activity (Charlevoix (CSZ), Western Quebec
(WQSZ), Lower St. Lawrence (LSLSZ)) and others with a lower (“background”) seismic
activity. Outside these areas, most of the Canadian Shield and bordering regions have had a
very low level of earthquake activity for hundreds of years. Since major population centres
are located in the SLRS, including the metropolitan areas of Ottawa, Montreal, and Quebec
City ( Figure 4.1 ) , its seismic hazard is of great interest for the protection of the public and
infrastructures critical to the Canadian economy.
Through time, there have been multiple hypotheses to explain the earthquake activity. In
the 1960s, the prevalent view was that the numerous epicentres lined up with Logan's Line,
which is the main boundary between the Appalachian thrust belt to the southeast and the
Precambrian Shield and overlying Ordovician sedimentary rocks to the northwest ( Figure
4.1 ; Milne and Davenport, 1969 ) . Following the work of Leblanc et al . (1973, 1977) it
became obvious that earthquakes occurred on faults within the Precambrian Shield. The
largest of these faults were normal faults created during the opening of the Iapetus Ocean
(Kumarapeli, 1985 ) . From the concentrations of hypocentres that were dipping similarly to
the normal faults in the Charlevoix Seismic Zone, this hypothesis has become one of the
two seismic zoning models in Eastern Canada (Adams et al ., 1995 ) . In the seismicity-rift
model, it was assumed that the mild activity outside the recognized active zones was due