Effects of strong seismic shaking in lake sediments, and earthquake recurrence interval, Témiscaming, Quebec

Abstract

The magnitude 6.3 Timiskaming earthquake of 1935 resulted in discoloration in small lakes in the epicentral region. Sonar profiles for Lac Tee have shown that organic-rich gyttja has been removed from the sides and redeposited in the deep parts of the lake. Lake sediment cores from deep basins contain a 2–3 cm uppermost layer, which is interpreted as the normal accumulation of sediment since 1935. This is followed by a 20 cm chaotic zone of black gyttja mixed with partly tabular fragments of a previously formed silt layer, underlain by a dense 1–2 cm thick silt horizon. The latter is interpreted as having formed by rapid settling of the denser silicate portion of the resuspended sediment. This 20 cm zone is insufficient to account for the abnormal thickness of gyttja in the basins, so events of this kind would have had to have occurred repeatedly since deglaciation. The only major prehistoric event revealed by these ~100 cm cores is at a depth equivalent to about 1500 years ago, based on the amount of sediment deposited since 1935. A minor silting event occurred about 400 years ago. On this basis, the recurrence interval of magnitude 6 or greater earthquakes is longer at Témiscaming than at Charlevoix, Quebec (75 years historically), consistent with the present relative frequency of small earthquakes in these two regions.