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 Earth Sciences Sector > Priorities > Canadian Hazards Information Service > Earthquakes Canada |
Revisiting the geological structures of the Lower St. Lawrence Seismic Zone, Quebec:
Insights from magnetic, gravity and seismic data
Lamontagne, M., Keating, P
2000 AGU Fall Meeting Abstract
The Lower St. Lawrence Seismic Zone (LSZ) is located in the estuary of the St. Lawrence River, some 400 km downstream from Quebec City. It is one of the four seismic zones of eastern Canada and is defined by the clustering of magnitude < 5 earthquakes. Generally two magnitude 3 earthquakes occur yearly, whereas magnitude 4 events occurevery 3-4 years. On March 16, 1999, a magnitude mN 5.1 earthquake occurred in the LSZ under the St. Lawrence River. This event, the largest event ever recorded in the LSZ, generated a renewed interest in the regional tectonic features. Geologically, the LSZ is located at the Appalachian Front, a region where nappes were thrust over the Precambrian rocks and early Paleozoic sediments. Earthquakes occur in the Precambrian basement, below sedimentary sequences up to 8 km thick.
Since most LSZ earthquakes occur under the St. Lawrence River, the geological structures of the basement must be derived from available geophysical information (magnetic, gravity and seismic). Onshore magnetic data were acquired along north-south flight-lines spaced 800 m apart. Offshore, flight-lines spaced at 1600 m and their orientation follows Loran C navigation hyperbolas, which are roughly oriented north-east in the area of interest. Gravity station spacing averages about 15 km on land. Offshore data spacing varies greatly as some data were acquired with a sea bottom gravity meter, and others with a surface gravimeter along north-south and east-west ship tracks over the Sept-Iles intrusion. Potential field data in the region are dominated by the signals associated with this 75 km diameter layered mafic intrusion. Faults and depths-to-basement interpreted from seismic profiles bring additional constraints to our interpretation.
The solutions of the Euler deconvolution of the magnetic data show deepening of the Precambrian basement towards the south-east under the Appalachian nappes. The deepening is mostly related to normal faults, generally oriented east-west or ENE-WSW. These normal faults of regional extent are consistent with the model of Iapetan rifting of the Precambrian basement. Some magnetic lineaments cross-cut the Sept-Iles intrusion and could be associated with normal or reverse faulting.
In the LSZ, earthquakes occur within areas bounded by large normal faults. Very rarely do clusters of earthquakes match the positions of large normal faults. The March 16, 1999 earthquake, for instance, does not correlate with any gravity anomaly, but locates on the northern flank of a 200 nT amplitude rectangular magnetic anomaly (10 x 4 km). Hence, the event may occur near an off-shoot of the Sept-Iles mafic intrusion, within 20 km of a large normal fault.
