At 22h37m02s UTC on May 14, 1978, a magnitude (ML) 4.8 earthquake occurred in the Canadian Rockies near McNaughton Lake, the large reservoir behind the Mica Dam in eastern British Columbia. The data suggest it was not a reservoir-induced earthquake.
The earthquake was within a seismic array monitoring the reservoir and consequently, the epicenter is well constrained in a northwest-southeast direction. The preferred epicenter is 52.65°N, 118.89°W, slightly east of the Rocky Mountain Trench, with a shallow focal depth of about 10 km. There were no foreshocks but a normal aftershock sequence. The focal mechanism, the first for the eastern Canadian Cordillera, indicates predominantly right-lateral strike-slip faulting along the strike of the mountains with a significant thrust component. The moment is 4 ± 2 × 1023 dyne-cm with a low stress drop (<10 bars). A well-developed Lg phase was recorded to the south of the earthquake. The isoseismals are elongated in a north-south direction and the rate with which intensity attenuates in a southerly direction is comparable to that for eastern North America.