Abstract

Impoundment of McNaughton Lake was initiated on March 29, 1973 and full load of 25 × 109 m3 and maximum depth of 191 m were first achieved in August 1976. The reservoir lies principally in the Rocky Mountain Trench, a major structural boundary between the Rocky Mountain Thrust Belt to the east and the Eastern Metamorphic Belt to the west. Although the regional seismicity is moderate, with an average of about one earthquake with Richter magnitude ML ≥ 3.0 every 2 years in the period 1963–1972, a magnitude 6 earthquake occurred adjacent to the present reservoir site in 1918.A monitoring program initiated in January 1973 has located 212 earthquakes with M ≥ 1.5; the largest ML = 4.8. Most of the seismicity was confined to four sequences. The first swarm started prior to initiation of impoundment. The next two sequences occurred in the same area 17 km into the Rocky Mountains from the central section of the reservoir. The final sequence, consisting of the ML = 4.8 earthquake and its aftershocks, had a preferred epicentre 4 km east of the Rocky Mountain Trench near the northern section of the reservoir. Comparison of the characteristics of the earthquake sequences with data from reservoirs where seismicity has been induced indicates that the observed seismicity at McNaughton Lake is unlikely to have been induced.The regional seismicity pattern from 1973 to 1978 was similar to that of the historical record: distributed earthquakes in the Rocky Mountains and in the adjacent Monashee and Cariboo Mountains of the Eastern Metamorphic Belt, no earthquakes within the Rocky Mountain Trench, and the Selkirk Mountains continuing to be almost aseismic. Both the regional seismicity and swarms yield b values near 0.65. Inclusion of the historical data with the regional seismicity gives b = 0.74 ± 0.08.

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