Abstract

Earthquake-induced liquefaction features have been found in Holocene and Late Wisconsin sediments exposed along three rivers in the Charlevoix seismic zone. On the basis of their stratigraphic position and radiocarbon age constraints, the liquefaction features are thought to have formed during three or more earthquake episodes centered in Charlevoix during the past 10,000 years, including at least two prehistoric episodes approximately 5,000 and 10,000 years ago. The spatial distribution of liquefaction features coupled with liquefaction potential analysis suggests that the Charlevoix earthquakes were of moment magnitude (M) ≥ 6.2. Liquefaction features have not been found in similar sediments exposed along eight rivers in the Quebec City–Trois Rivieres area, 70 to 150 km from Charlevoix in the St. Lawrence River Valley. The apparent absence of liquefaction features in the Quebec City–Trois Rivieres area suggests that few, if any, large earthquakes have occurred here during the same time period. The geologic record of earthquakes may be incomplete in both areas due to fluctuations in Holocene sea level. Nevertheless, the rate of large earthquakes has apparently been much higher in the Charlevoix seismic zone than in adjacent areas of the St. Lawrence for thousands of years. These findings suggest that seismicity is localized in Charlevoix and that the presence of Iapetan rift faults that underlie the St. Lawrence Valley of southeastern Canada may not, in itself, indicate earthquake potential. These results may have important implications for other Iapetan rift faults in the eastern United States, as well as seismic source zone characterization and hazard assessment throughout eastern North America.

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