Lamontagne, M. Keating, P., Perron, G. and Adam, E. 1997. Offshore Faults of the Charlevoix Seismic Zone, as Interpreted from Seismics, Magnetics and Gravity. Abstracts of the 69th annual meeting of the Eastern Section of the Seismological Society of America. Ottawa, Ont., Seismological-Research-Letters. 69. (1). p. 80.
AbstractIn the Charlevoix Seismic Zone (CSZ), most earthquakes occur in the Precambrian basement under the St. Lawrence River. There, the potentially seismogenic faults are masked by a thick sequence of Appalachian rocks and by a series of valleys filled with Quaternary sediments up to 600 m thick. To define the position and extent of the basement faults better, proprietary seismic, magnetic and recentlyacquired offshore gravity data were interpreted. Contrasts in acoustic impedance (seismics), in magnetic susceptibility (magnetics) and in density (gravity) are used to map the PrecambrianAppalachian interface at depth. In addition, the solutions from the Euler deconvolution of the magnetic field are used to estimate the depth to the magnetic basement. A series of riverparallel normal faults (possibly related to the St. Lawrence paleorift system) are interpretated from the gravity and magnetic fields. Another series of normal faults at high angle with the river axis also exists. As most seismic profiles, acquired in the early 70's, do not clearly image the deeper structures, one of these seismic lines was reprocessed using modern techniques. The new migrated section, relieved of the strong waterbottom multiples, shows reflectors, some as deep as 15 to 18 km. On this profile, a few normal faults are suggested by the discontinuous reflectors near the Appalachian front. The fault system that emerges from these geophysical indicators is one of regional normal faults parallel to the river axis crossed by local complexities, possibly introduced by the Charlevoix impact structure. Although one of these regional normal fault appears seismogenic; most regional faults are inactive or at best, constrain the seismic activity. One highlyfaulted area in the impact crater is also highly seismically active.