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Understanding the Charlevoix Seismic Zone: Help Given by New Gravity Data.
M. Lamontagne, R.V.Cooper, J.Halpenny and G. Ranalli
Between 1977 and 1995, the Geological Survey of Canada has recorded some 1500 earthquakes in the Charlevoix Seismic Zone, located some 100 km downstream from Quebec City. The majority of these events are located under the St. Lawrence River where little is known about fault locations. Because faults sometimes separate geological blocks with contrasting
densities, efforts were made to complete the regional gravity coverage over the St. Lawrence River where no gravity data existed. A GSC gravity survey was conducted on the St. Lawrence River in August 1994. It has successfully demonstrated
a new, lower cost marine survey technique. Using a 34 foot launch, over 1000 km of profiles were measured in the river between Quebec City and Rimouski. New GSC software used with GPS navigation and an improved gravity meter controller made
it feasible to use a launch rather than a large survey ship. This greatly reduced costs and allowed measurements in shallow areas inaccessible to larger vessels. The results have been added to the National Gravity Data Base and provide a clearer understanding of the Charlevoix meteor impact structure and the accompanying earthquake zone, especially in the underwater areas where most earthquakes occur. The 5000 points of gravity data will be used together with remote sensing data and other geoscientific information. The Bouger anomaly map shows abrupt changes that correspond to block movements related to the impact crater.
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