Abstract

Eastern Canada has experienced at least five earthquakes of estimated Richter magnitude 6 or greater during the last 350 years. The epicentres are usually under the St. Lawrence River, some 100 km east of Quebec City. Historical records of the earthquake of 1663, possibly the largest of these events, describe high levels of silting in streams for up to several months. A silt horizon in normally organic-rich lake sediments has been found in lakes with separate drainage systems and is interpreted as the 1663 event on the basis of rough sedimentation rates obtained from observation of 137Cs fallout of the 1950's and the effects of a dam on one of the lakes. Two other silt layers correspond reasonably well to the next largest earthquakes of 1791 and 1860 + 1870 (combined). Two deep layers were found that by extrapolation of the assumed 1663 layer yield dates of about A.D. 1060 and 600 and are believed to represent prehistoric earthquakes, though possibly not as large as that in 1663. Chemical analyses of the cores show that organic material, Ti, and especially K are very useful for identifying these layers and others that are not visible in the cores.

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