Earthquakes and geological structures of the St. Lawrence Rift System - Intraplate Earthquakes - page 85

Table 4.2 Main characteristics and differences of the earthquake zones of the SLRS
(see Table 4.1 for a description of the earthquakes)

Common characteristics of SLRS active areas Differences/anomalies along the SLRS

99% of earthquakes occur at less than 25 km
depth

Some larger events occur near or deeper
than 25 km (1988 M 5.9 Saguenay,
29 km; 1997 Cap-Rouge M 4.7, 22 km;
2010 Val-des-Bois M 5.0, 22 km)

Earthquakes occur in areas where paleo-rift
faults are present (CSZ, WQSZ, LSLSZ)

Part of the earthquake activity is also present
where no SLRS faults are present
(WQSZ)

No surface rupture known for any of the large
events
Proximity to a large water body (CSZ, LSLSZ)
with tides

No large water bodies (WQSZ)

Earthquakes occur over a vast region (WQSZ,
LSLSZ)

Concentrated activity (CSZ)

Protracted aftershock sequence for moderate
earthquakes

Very few aftershocks (1988 Saguenay at
29 km focal depth)

Presence of a meteor impact structure (CSZ)

and southeastern boundaries of the intact Iapetan margin and its potentially seismogenic
faults for seismic zoning purposes.
A seismotectonic model of the earthquakes of the St. Lawrence Valley must consider
the similarities and differences of the seismic and weakly seismic zones ( Figure 4.2 ) .
Table 4.2 presents an overview of the similarities and differences between the various
seismically active regions of the SLRS. In the 1970s, focal mechanisms showed that
microearthquakes represented the reverse-faulting reactivation of pre-existing faults in the
Precambrian basement (Leblanc et al ., 1973 ; Leblanc and Buchbinder, 1977 ) . Later studies
with larger magnitude earthquakes confirmed this (see Hurd and Zoback [2012] for a
review). The hypothesis that connects the SLRS faults and the current seismicity was
mainly derived from the concentration of CSZ earthquake hypocentres along southeast-
dipping trends, similar to SLRS faults (Anglin and Buchbinder, 1981 ; Anglin, 1984 ) . By
extension outside the CSZ, it was hypothesized that all earthquakes along the St. Lawrence
valley represented reactivation of similar faults. This SLRS connection was also made at
the time when all worldwide M

7.0 continental earthquakes were shown to correlate
with rifted cratonic areas (also called “extended crust”; Johnston and Kanter, 1990 ) . This
rift-faults-seismicity connection was also used by Adams and Basham ( 1989 ) in discussing
the seismicity of the SLRS.
Outside the CSZ, the best correlation between the normal faults of the SLRS and
seismicity can be found in the Temiscaming region, where the 1935 M 6.2 earthquake