Since the 1960s geologists know that the Earth's surface is segmented into a number of tectonic plates that are in constant motion relative to each other at rates which are typically about several centimetres per year. These plates generally move as rigid blocks, however, at their edges there is a zone (often called a plate boundary) where they rub against each other and deform, causing earthquakes and other geophysical phenomena. Seismic activities are at the highest rate at the plate boundaries (Figure 1). Different with plate boundaries, large earthquakes within stable plate interiors are an evidence that significant amounts of elastic strain can accumulate along geologic structures far from plate boundary faults, where the vast majority of seismic energy is released (Calais et al., 2006).
Figure 1. Global earthquake epicentres between 1973-2011 with magnitude greater than 3 (data source: USGS 2011)
The Saint Lawrence valley located in eastern Quebec, Canada, is one of the active intra-plate regions in North American tectonic plate, characterized by a large number of intra-plate earthquake patterns, from zones with significant earthquakes to zones with very little background seismicity (Figure 2). Although causes of earthquakes in eastern Canada are not well-understood, there are some different explanations about the intra-plate tectonic activity in the Saint Lawrence valley. Rondot (1968) suggests a volcanic origin for the Charlevoix semi-circular structure due to its location on the margin of the Saint Lawrence valley tectonic system, but the high rate of the event and the violence of the shock resembles rather of a meteorite origin. According to the author, the tectonic activity is related to the Devonian meteoric impact that occurred 350 MYBP in this region. Reactivation of Iapetan normal faults during the Jurassic rifting and opening of the North Atlantic Ocean is another explanation proposed by Lemieux et al. (2003). A third explanation given by Mazzotti et al. (2005) is related to the GIA of the lithosphere after the Wisconsin ice age toward its former position of the isostatic equilibrium.
Figure 2. Geological setting of the Saint Lawrence valley (modified from Mazzotti et al. 2005). Circles show seismically active areas.
Since the early 1970s, the Saint Lawrence valley has been under ongoing monitoring of the earthquake hazard by a number of institutions and researchers, among them, the Earth Physics Branch now a part of Geological Survey of Canada (GSC), and the Geodetic Survey Division (GSD) of the Natural Resources Canada (NRCan). While the former institution concentrated on the Charlevoix Seismic Zone (CSZ), the latter one started an extensive program to study the geodynamics of eastern Canada.
CSZ is a well-known area within the Saint Lawrence valley due to occurrence of many large historical earthquakes during the last four centuries. However, details of the small scale earthquakes and geographical extent of the area were incomplete, mainly because of the large errors in the determination of the epicentres (Buchbinder et al., 1988). The first field experiment was conducted in 1968 in order to determine the extent and rate of the micro-seismicity (Milne et al., 1970), albeit with very few instruments and essentially inconclusive results. A larger experiment undertaken in 1970 (Leblanc et al., 1973) located the CSZ boundary to the south-west and an even larger survey in 1974 (Leblanc and Buchbinder, 1977) found the boundary to the north-east. The earthquake potential of the CSZ led GSC to conduct two seismic field surveys in 1970 and 1974. These two surveys clearly delineated CSZ to be 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 Pocatiére (NRCan, 2011).
In 1974, the Earth Physics Branch began an extensive geophysical monitoring program in CSZ. Many parameters including micro-seismicity, seismic travel times, electrical impedance, vertical movement, horizontal movement, tilt, gravity change, and strain through water well level changes, were studied with the goal of developing the capability of predicting the earthquakes in the area. Although no clear precursor was detected in the months before the largest seismic event (M = 5.0) that occurred during the observation period (in 1974), the multi-parameter experiment provided new insights into the structure and the mechanics of this active region (Buchbinder et al., 1988).
Mazzotti et al. (2005) have studied the amplitude, pattern, and origin of the crustal deformation in the Saint Lawrence valley using GPS observations. The research is based on the analysis of the observations of sixteen GPS stations from the Canadian base network (CBN), surveyed three or four times between 1994/1996 and 2003. The GPS stations have shown coherent south-eastward motion of 0.6 ± 0.2 mm yr-1, relative to the North American plate, and uplift of 2.6 ± 0.4 mm yr-1. The average value of the horizontal strain rates are mostly ESE-WNW shortening at (1.7 ± 1.0)×10-9 yr-1. Aythors have also measured a coherent pattern of isostatic adjustment at the rate of 4-5 mm yr-1 uplift in the north-west, decreasing progressively to 0-1 mm yr-1 in the south-east. Based on these measurements, they have concluded that the shortening rate across CSZ is about twice as big as the regional average, and the motion is consistent with the lithosphere GIA.
In another study that was published by Lamothe et al. (2010), the first-order precise levelling data has been used along with GPS measurements in order to quantify the local horizontal and vertical deformations in CSZ. The study is based on the analysis of repeated levelling lines of the Canadian first-order vertical network measured between 1909 and 1991 in CSZ, as well as the first-order GPS network along the Saint Lawrence valley surveyed in two campaigns of 1991 and 2005. The results from the precise levelling analysis showed that the pattern of relative uplift is increased toward north-west, perpendicular to the direction of the Saint Lawrence river. The GPS observations showed a coherent horizontal velocity field toward the east and south-east, and a coherent vertical velocity field compatible with the previous GPS campaigns and GIA models.
According to predictions of the GIA models (e.g., ICE3G/VM1), the hinge line between crustal uplift and subsidence is located near, or slightly south of the Saint Lawrence valley (Tushingham and Peltier, 1991). This prediction is validated by a number of researchers. The uplift pattern in the Saint Lawrence valley found by Mazzotti et al. (2005) is consistent with the model. A similar location of the hinge line is found by Sella et al. (2007) based on analysis of motions of three hundred and sixty GPS sites in Canada and the United States (US). According to the latter study, the hinge line is also consistent with data from water level gauges along the Great Lakes, showing uplift along the northern shores and subsidence along the southern ones.
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