The estuary of the St. Lawrence River between Baie-Comeau and Sept-Îles is an area where 50 to 100 earthquakes are detected yearly. This study defines the major lineaments of the Precambrian basement and compares them with mapped faults where possible, and examines their possible correlation with the local earthquakes. Onshore, aerial photographs, remote sensing images, and field mapping are used to identify the geological structures of the Precambrian basement, while offshore, they are interpreted from potential field (magnetic, gravity) and a published synthesis of seismic reflection profiles. The Precambrian basement dips towards the southeast under the Appalachian nappes with normal faults with kilometre-scale throw and east–west or ENE–WSW strikes. Onshore, a system of normal brittle faults with a complex history of movements crosscuts both the Precambrian basement and the overlying Ordovician sedimentary cover. Most earthquakes occur beneath the St. Lawrence River at focal depths between about 7 and 25 km that place them well within the Precambrian Shield. In the Lower St. Lawrence Seismic Zone, the trend of the normal faults that changes from SW–NE to mostly east–west, and lateral density anomalies possibly enhance the local stress level. It is also suggested that local faults could be weak because of crustal fluids at depth, possibly under hydrostatic pressure, or to fault gouge, which leads to a lower coefficient of friction. It is possible that the region was intensely fractured by the emplacement of the Sept-Iles layered igneous complex.

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