Earthquakes and geological structures of the St. Lawrence Rift System - Intraplate Earthquakes - page 82

S Hmax. This reorientation and the maximum plausible stress difference required to cause
fault reactivation can be argued to support the presence of high pore-fluid pressures along
faults (Lamontagne and Ranalli, 1996 ) . On the other hand, Hurd and Zoback ( 2012 ) believe
that high pore-fluid pressures are not necessary to explain the majority of preferred focal
mechanisms in eastern North America.
There are indications that the region itself may have sub-areas with different rheological
properties due to the presence of the meteor crater and its faults. Larger events concentrate
at both ends of the seismic zone outside the meteor impact ( Figure 4.3 ; Stevens, 1980 ;
Lamontagne, 1999 ) . Recently, geomechanical modelling showed that the weakening of the
rift faults produces a stress increase in the region of the crater bounded by faults, leading
to low-magnitude events within the crater and large events outside it (Baird et al ., 2009 ;
2010). If this hypothesis is true, the local seismicity would be caused by local conditions
rather than by more regional conditions.

4.3.2 Lower St. Lawrence

Located in the estuary of the St. Lawrence River, the Lower St. Lawrence Seismic Zone
(LSLSZ) experiences about 60 events with magnitude

2.0 yearly but, unlike the CSZ,
only two earthquakes were near magnitude 5.0 in the last 100 years (in 1944 and in 1999;
Lamontagne et al ., 2003 ; Figure 4.1 ) . Most earthquakes occur under the St. Lawrence
River with hypocentres in the Precambrian mid to upper crust, between 5 and 25 km depth,
similar to the CSZ. Adams and Basham ( 1989 ) suggested that the seismicity was restricted
to the regional normal faults, uniquely found offshore. Focal mechanisms show, however,
variable fault plane orientations with most earthquakes clustering along or between the
geologically mapped and geophysically inferred Iapetan faults (Lamontagne et al ., 2000 ) .
Mazzotti and Townend ( 2010 ) note that this seismic zone contains evidence for a local
rotation of S Hmax for the generally NE-SW-oriented S Hmax.

4.3.3 Western Quebec

The Western Quebec Seismic Zone (WQSZ) includes the Ottawa Valley from Montreal
to Temiscaming, as well as parts of the Laurentian Mountains (Basham et al ., 1982 ;
Figure 4.2 ) . This seismic zone includes historical earthquakes as large as M 6.2 and
frequent low-level seismic activity. Earthquake epicentres define two sub-zones: a mildly
active one along the Ottawa River, including a more active cluster in the Temiscaming
region, and a more active one along the Montreal-Maniwaki axis.
The first band of seismicity, which parallels the Ottawa River, includes the epicentres of
moderate earthquakes: M 6.2 near Temiscaming in 1935; an M 5.6 near Cornwall-Massena
in 1944 and possibly a magnitude of about 5.8 near Montreal in 1732 ( Figure 4.2 ; Leblanc,
1981 ) . The diffuse group of earthquakes in the first band appear to correlate with a zone of
Paleozoic or younger normal faults along the Ottawa River, called the Ottawa-Bonnechere
graben (Forsyth, 1981 ) . The Temiscaming area is more active than the rest of the Ottawa