Abstract

Seismoacoustic profiling of 34 lacustrine basins within the St. Lawrence Valley in southern Québec shows the presence of faulting, convolution, diapirs, and gravitational slumping beneath, within, and at the surface of the Holocene deep-water–sediment zone. In small, deep lakes in the highlands of the Charlevoix-Tadoussac region, a thin (2–6 m) Holocene organic sedimentary unit, which directly overlies over a seismo-opaque substratum, exhibits considerable deformation. These subaquatic sediment disruptions are exclusively restricted to lakes located within a radius of about 100 km of the well-known Charlevoix seismic zone.

According to the present seismicity of the region and the spatial distribution of the slump frequency, the minimal-triggering threshold for earthquakes was found to be as low as 0.17 g. These results indicate that the Charlevoix seismic zone has occupied the same area throughout at least the Holocene, and it appears that no major seismic shock has occurred over the past several thousand years outside of the zone.

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