Earthquakes and geological structures of the St. Lawrence Rift System - Intraplate Earthquakes - page 72

4
Earthquakes and geological structures of the
St. Lawrence Rift System

MAURICE LAMONTAGNE AND GIORGIO RANALLI

Abstract

The St. Lawrence Rift System (SLRS), which includes the Ottawa-Bonnechere
and Saguenay grabens, is located well inside the North American plate. Most
historic and the some 350 earthquakes recorded yearly occur in three main
seismically active zones, namely Charlevoix (CSZ), Western Quebec (WQSZ),
and Lower St. Lawrence (LSLSZ). Outside these areas, most of the Canadian
Shield and bordering regions have had a very low level of earthquake activity.
In the SLRS, moderate to large earthquakes (moment magnitude, M , 5.5 to 7)
are known to have occurred since 1663, causing landslides and damage mostly
to unreinforced masonry elements of buildings located on ground capable of
amplifying ground motions. Most earthquakes in these seismic zones share
common characteristics such as mid- to upper-crustal focal depths, no known
surface ruptures, and proximity to SLRS faults. Variations also exist such as
vast seismically active regions (WQSZ and LSLSZ), the presence of a large
water body (CSZ and LSLSZ), and absence of SLRS faults near concentra-
tion of earthquakes (WQSZ). The CSZ is the best studied seismic zone; there,
earthquakes occur in the Canadian Shield, mostly in a 30 85 km rectangle

elongated along the trend of the St. Lawrence River with local variations in
focal depth distribution. Faults related to the SLRS and to a meteor impact
structure exist, and earthquakes occur along the SLRS faults as well as in
between these faults. Overall, the SLRS faults are probably reactivated by the
larger earthquakes ( M

×

4.5) of the twentieth century (CSZ in 1925; WQCSZ
in 1935 and 1944; Saguenay in 1988) for which we have focal mechanisms.
We propose that caution be exercised when linking historical events that have
uncertain epicentres with SLRS faults. Similarly, SLRS faults should not neces-
sarily be considered to be the reactivated structures for most small to moderate