Earthquakes and geological structures of the St. Lawrence Rift System - Intraplate Earthquakes - page 83

River valley and was the site of the 1935 M 6.2 Temiscaming earthquake. In the year 2000,
an M 4.7 earthquake reactivated one of the local faults (Bent et al ., 2002 ) , adding support
to the correlation between the earthquake epicentres and the normal faults of the Ottawa-
Bonnechere graben (Adams and Vonk, 2009 ) . These earthquakes appear to reactivate faults
smaller than the conspicuous ones near Lake Temiscaming (Bent et al ., 2002 ) .
The majority of WQSZ earthquakes, mostly smaller than M 4.5, occur in an elongated
NW-SE zone within the Grenville Geological Province with most focal depths varying
between 7 and 25 km (Lamontagne et al ., 1994 ; Ma and Atkinson, 2006 ) . Although
northwest-trending structural features are known, correlating these with the epicentral
trend is uncertain because of the thrust sheets that make up the uppermost crust in this area
of the Grenville Province. The mid-crustal hypocentral depths of many earthquakes, the
east-west trend of the fault planes of some earthquakes, and variations in regional focal
mechanisms all suggest reactivation of deep structural features that may not have a surface
expression. It is possible that at the eastern end of this active band an anorthosite body may
act as a stress concentrator (Lamontagne et al ., 1994 ) .

4.3.4 Background seismicity

Outside the seismic zones described above, other areas along the SLRS are much less active
( Figures 4.1 and 4.2 ) . On a yearly basis, very few earthquakes are recorded in these areas
and very few significant earthquakes are known (one notable exception being the 1988 M
5.9 Saguenay earthquake; Table 4.1 ) . This earthquake occurred at lower-crustal depth
(29 km) in the Canadian Shield, outside the seismic zones defined by the recurring activity
(North et al ., 1989 ) . Possibly due to its focal depth, very few aftershocks have been recorded
and only four M 3 earthquakes were recorded there between 1988 and 2013. Despite

the differences in orientation between the two nodal planes of the focal mechanism and the
graben fault at the surface, the proximity of the earthquake epicentre to faults of the Sague-
nay graben suggested a relationship between the two (North et al ., 1989 ) . Other authors
warned of the difficulty in linking a lower-crustal depth earthquake with the numerous
faults of the Precambrian basement that have unknown fault extensions at depth (Du Berger
et al ., 1991 ) . This earthquake suggests that moderate to large earthquakes can occur in areas
outside the zones defined by recurring seismic activity and that many faults of the whole Pre-
cambrian Shield could be near failure. This suggestion is supported by the numerous cases
of reservoir-triggered seismicity in areas almost devoid of naturally occurring earthquakes
(Lamontagne et al ., 2008b ) . Another noteworthy event is the moderate 1997 M 4.9 Cap-
Rouge earthquake, which occurred at a depth of 22 km with a focal mechanism consistent
with the reactivation of a mapped SLRS fault ( Table 4.1 ; Nadeau et al ., 1998 ) .

4.4 The St. Lawrence Rift System

Kumarapeli and Saull ( 1966 ) proposed that the series of normal faults along the St. Lawrence
valley indicated rifting similar to the East African rift system. Although it is now recognized
that the St. Lawrence Rift System (SLRS) is not an active rift, this hypothesis provided a