Introduction - Intraplate Earthquakes - page 3

Accurate determination of the nature and configuration of seismogenic structures respon-
sible for the seismic hazard of a region is particularly important, especially when it includes
highly populated locations. Such is the case in Eastern Canada where the metropolitan areas
of Ottawa, Montreal, and Quebec all lie in the St. Lawrence Rift System (SLRS), the most
active seismic region in eastern North America. Lamontagne and Renalli address this issue
in Chapter 4. The NE-SW-trending SLRS, which extends for more than 1,200 kilometers,

is a failed paleo-rift and attached to it are the NW-SE-oriented aulacogens, the Ottawa-
Bonnechere and the Saguenay grabens. The structure of the SLRS is further complicated
by a Devonian age, 50 to 60 kilometers broad meteor impact structure lying over the SLRS
and centered around Charlevoix. The impact structure has further shattered the rocks in the
SLRS and produced cross faults.
The authors use an integrated approach in assessing the seismic hazard for this region -
critical to the Canadian economy. Starting with historical accounts of European settlers in
the seventeenth century, there is evidence of several M > 6.0 earthquakes in the SLRS.
Instrumentally recorded seismicity identified three main seismic zones; the Charlevoix
seismic zone along the SLRS, the northwest-trending Western Quebec seismic zone, and
the Lower St. Lawrence seismic zone. Outside these zones there is low-level background
seismic activity. Most of the hypocenters are located in the middle to upper lower crust,
although a deeper event (

29 km) did occur in the Saguenay graben in 1989. Traditionally,
the seismic hazard has been associated with larger events in the rift structure. However,
the authors argue that moderate events with M > 4.5 also can pose a seismic hazard
because of low attenuation, thick unconsolidated post-glacial marine clay soils that amplify
ground motions, and the presence of old buildings in these zones. They suggest that most
of the moderate earthquakes are associated with local causes rather than with the rift
structures and such causes should be considered in evaluating the hazard. They identify
these as locally weakened faults, elevated pore pressures, and local variations in the stress
level.
In the past decade, as newer and better seismicity data began to accumulate, ques-
tions emerged regarding the stationarity of seismicity in intraplate regions. A common
assumption is the practice of seismic hazard analysis carried over from studies of inter-
plate regions. In most intraplate regions, the historical data and sporadic paleoseimological
investigations are inadequate to accurately describe the spatiotemporal pattern of past seis-
micity needed to assess the assumption of stationarity of seismic sources. In Chapter 5, Liu,
Wang, Ye, and Jia address this question in their study of the intraplate earthquakes of North
China.
On a continental scale, North China is one of the most seismically active regions in
the world, with a historical record of more than 3,000 years. These records document
more than 100 large (M > 6) earthquakes in the past 2,000 years, including the world's
most catastrophic M 8.3 1556 Xuaxian earthquake with a death toll of 830,000 victims.

Liu et al . describe the spatiotemporal pattern of the historical seismicity and, using the
results of intensive geological, geophysical, seismological, and GPS investigations in the
past decades, relate the significant earthquakes to specific structures. They show that no
M 7.0 event ever ruptured the same fault segment twice in the past 2,000 years, and