The Charlevoix Seismic Zone Alexander Martell

What is the Charlevoix Seismic Zone? Located about 100 km downstream of Quebec City Region of highest seismic activity, and seismic hazard, in Eastern Canada Lies in stable interior of North American plate, and causes of seismic activity are not well-understood compared to seismically active regions near plate boundaries Earthquakes thought to be tied to a complex regional stress field, concentrated in areas of crustal weakness [Natural Resources Canada]

Since 1977, earthquake activity monitored by a network of 7 local seismograms Approximately 200 earthquakes a year, with magnitudes ranging from -1.0 to 5.0, and hypocenters at depths of 0 km to 30 km Earthquakes trace out a roughly elliptical region with long axis parallel to the St. Lawrence river, most occurring below the river itself Most of the earthquakes are magnitude 3.5 or less, but we know of at least 5 large events between magnitude 6.0 and 7.0 that have occurred in the CSZ (in 1663, 1791, 1860, 1870 and 1925) [Lamontagne et al. (2004)]

The 1925 Charlevoix-Kamouraska Earthquake Occurred on February 28, 1925, at about 21:20 Eastern Time Estimated magnitude of 6.2 Estimated maximum intensity of VIII on the Mercalli Scale Only one of the large CSZ events to have been recorded by seismographs Felt as far as 1000 km from it’s epicenter (red ellipse on figure to right) Aftershocks for several weeks of magnitudes ranging from 3.7 to 5.0 [Lamontagne (2012)]

Red line shows the estimated orientation of the fault ruptured by the Charlevoix-Kamourska event Believed to have been caused by large horizontal compressional forces Significant damage to many stone structures in surrounding towns and villages, especially churches No fatalities, perhaps due to the sparse population density at the time In some regions, including what is now a densely developed part of Quebec city, ground motions thought to have been amplified by deep deposits of soft soils and clays

Possible Mechanisms Three important faults cut through the CSZ, but they are difficult to link with earthquake activity. Moreover they seem bound regions of high activity rather being regions of high activity themselves Lamontagne (1999) reasons that the high incidence of small earthquakes is due to regions of crustal weakness, possibly combined with high pore fluid pressure Large events are interpreted as reactivation of one of the dormant Iapetan faults [Lamontagne et al. (2004)]

Type of Faulting and Lack of Surface Rupture CSZ earthquakes mostly represent strike-slip to reverse faulting on planes of varied orientations Interestingly, no apparent surface rupture in the CSZ, including at the bottom of the St. Lawrence River as verified by seismic reflection profiles This indicates that the unusual CSZ seismic activity is geologically recent Theorized that it began just a few thousand years ago at end of most recent glaciation period [Mazzotti and Townsend (2010)]

State of Stress in the CSV Mazzoti and Townsend (2010) carried out an analysis of the state of stress in 10 different seismic zones across North America, comparing data from borehole measurements (0.5 km to 2.0 km) and from Bayesian analysis of earthquake focal mechanics (1.0 km to 30 km) Compared the azimuths of the maximum horizontal compressive stress from focal mechanisms (SHS) and from borehole measurements (SHB), and found that SHS is rotated about 32° clockwise relative to SHB in the northwest of the CSZ, and about 47° clockwise in the southeast of the CSZ, significant to within a 90% confidence interval. In general borehole- and earthquake-determined stress orientations are similar and can be jointly accounted for, so Mazzotti and Townsend considered various explanations for this discrepancy. Perhaps due to post-glacial rebound stresses becoming amplified in zones of weakness

Future Seismic Hazard - A future large event in the CSZ has the potential to do great damage, in particular to parts of Quebec City that are now densely populated and situated on soft soils and clay that can amplify any earthquake ground motions - The Insurance Bureau of Canada commissioned a report released in 2013 which estimated that a large CSZ event could cause up to 61 billion dollars’ worth of damage, and that there was a 5 to 15% chance of such an event occurring in the next 50 years

References Lamontagne, M. 2012. The Charlevoix-Kamouraska earthquake of 1925; Geological Survey of Canada, Popular Geoscience 96, poster. doi:10.4095/29157 Lamontagne, M. 1999. Rheological and geological constraints on the earthquake distribution in the Charlevoix Seismic Zone, Quebec, Canada. Ph.D. Thesis, Carleton University, Ottawa, Canada. 362 p. Lamontagne, M., Beauchemin, M., and Toutin, T. 2004. Earthquakes of the Charlevoix Seismic Zone, Québec. CSEG Recorder 29(8). Mazzotti, S., and Townsend, J. 2010. State of stress in central and eastern North American seismic zones. Lithosphere 2(2):76-83. Canada. Natural Resources Canada. Earthquakes Canada. Earthquake Zones in Eastern Canada. Natural Resources Canada, n.d. Web. 9 Apr. 2016. "Major Earthquake Could Cause $75B in Damage, Study Warns." CBC News. CBC, 29 Oct. 2013. Web. 9 Apr. 2016.