Tuesday, February 21, 2012

Seismicity in the Saint Lawrence valley

Canada east of the Cordillera, extending north from the US border to the Arctic Ocean, comprises about two-third of the stable craton of the North American plate. Much of this large area appears to be substantially aseismic, although it contains several zones of significant seismicity and a few other regions of lower-level seismicity.

Although less seismicity is observed in eastern North America than the plate boundary to the west, the Saint Lawrence valley located in eastern Quebec (Figure 1) is still prone of substantial intra-plate seismicity and earthquake hazards. This area has a large range of intra-plate earthquake patterns, from zones with large earthquakes (M = 6 – 7) to zones with very little background seismicity (Adams and Basham, 1991). Even though seismicity is seen all over the Saint Lawrence valley, three main foci of earthquakes, namely the Charlevoix seismic zone, the Lower Saint Lawrence seismic zone, and West Quebec Seismic Zone can be easily recognized.

Geological setting of the Saint Lawrence valley

Figure 1. Geological setting of the Saint Lawrence valley (modified from Mazzotti et al. 2005)

Located about 100 km to the downstream from Quebec City, the Charlevoix seismic zone (CSZ) is the locus of the strongest earthquakes with five M ≥ 6 events in the past 350 years (NRCan, 2011). CSZ was clearly delineated after two field surveys of Geological Survey of Canada (GSC) in 1970 and 1974 as an active zone about 30 by 85 km, elongated along the Saint Lawrence river, and enclosing the towns of Baie-St-Paul, La Malbaie and La Pocatière. Because most of the earthquakes in this area occur under the Saint Lawrence river, between Charlevoix County on the north shore and Kamouraska County on the south shore, this region is also referred to as the Charlevoix-Kamouraska seismic zone (Figure 2). Occurrence of more than 200 earthquakes per year makes CZS the zone of highest seismic hazard in eastern Canada.

The locus of earthquakes in the Charlevoix seismic zone

Figure 2. The locus of earthquakes in the last five years in Charlevoix seismic zone (data source: Earthquakes Canada website)

Located in the same direction to the CSZ about 400 km from Quebec City, the Lower Saint Lawrence seismic zone (LSZ) is another seismically active region of eastern Canada. LSZ has experienced lower seismic activities than CSZ in terms of the number and the magnitude of earthquakes. Only two events are known to have exceeded the magnitude of 5.0 during the past 70 years. While the LSZ has had five earthquakes of magnitude 4.0 or larger between 1977 and 1997, the CSZ has had eight events during the same period. About 60 events happen per year in the LSZ, most of them under the Saint Lawrence river, within a triangular zone defined by the towns of Baie-Comeau, Sept-Iles, and Matane (Figure 3). This area is also named "Lower Saint Lawrence-Quebec North Shore" seismic zone, as the most earthquakes occur under the Saint Lawrence river, between the regions of the Quebec north shore and the Lower Saint Lawrence.

The locus of earthquakes in the Lower St. Lawrence seismic zone

Figure 3. Lower St. Lawrence seismic zone (data source: Earthquakes Canada website)

In contrast to CSZ and LSZ, the area between Quebec City and Montreal does not show high seismic activity. To the west and north west of Montreal, however, there is another tectonically active area named West Quebec Seismic Zone (WSZ). The WSZ constitutes a vast territory that encloses the Ottawa valley from Montreal to Temiscaming, as well as the Laurentians and the eastern Ontario (Figure 4). Earthquakes are apparently concentrated in two sub-zones within this area: one along the Ottawa river and the other along the more active Ottawa–Maniwaki axis. WSZ was shocked by four M ≥ 5.0 earthquakes during the past 300 years (NRCan, 2011).

The locus of earthquakes in the West Quebec seismic zone

Figure 4. West Quebec seismic zone (data source: Earthquakes Canada website)

The seismo-tectonic setting of the Saint Lawrence valley makes it an interesting area for testing the relationship between crustal strain, paleo-tectonic structures, and earthquake locations and magnitudes in an intra-plate environment. In particular, LSZ provides a good opportunity to study the present day crustal strain rates in regions of high, medium, and low past earthquake activity (Mazzotti et al., 2005).

Adams, J., and Basham, P. (1991). The seismicity and seismotectonics of eastern Canada. Neotectonics of North America, 1, 261-276.

Mazzotti, S., James, T. S., Henton, J. A., and Adams, J. (2005). GPS crustal strain, postglacial rebound, and seismic hazard in eastern North America: The Saint Lawrence valley example. Journal of Geophysical Research, 110(B11), B11301. doi:10.1029/2004JB003590

NRCan. (2011). Earthquake zones in Eastern Canada. Natural Resources Canada. Retrieved September 5, 2011, from http://earthquakescanada.nrcan.gc.ca/zones/eastcan-eng.php

No comments:

Post a Comment