An analysis of the 1985 Nahanni earthquakes

Abstract

Abstract Two large magnitude earthquakes occurred in 1985 near the North Nahanni River in the northeast Cordillera, Northwest Territories, Canada, on 5 October ( M S 6.6) and 23 December ( M S 6.9). These events were unprecedented for this region of the Cordillera, and the December event is the largest earthquake ever known in the eastern part of the Canadian Cordillera. This report presents detailed studies of the Nahanni earthquakes, their source parameters, macroseismic effects, aftershock sequences, triggered rock avalanches, their relationship to the geology of the epicentral area, and their seismic hazard implications. Parameters calculated here for these two major events are Parentheses indicate other studies. The earthquakes occurred within a small relatively undeformed plateau, the Mackenzie Plain, in the Foreland Fold Belt created along the northeastern Cordillera during the Columbian or Laramide Orogeny. That fold belt is now subjected to northeast-southwest horizontal compressive stresses, possibly very similar to the original Laramide stress regime, and the fault plane implied by the seismic mechanisms is very similar to the Laramide faults in the area. However, no surface fault break has been found, and none of the mapped Laramide faults in the epicentral area appears to be involved in the aftershock activity. It is thus believed that the Nahanni earthquakes occurred on a blind Laramide thrust fault under the Mackenzie plain. The large magnitudes of the Nahanni earthquakes show that the present seismic zoning for building code purposes in the eastern Cordillera north of 60°N needs reassessment and suggest that the zoning in the eastern Cordillera in British Columbia and Alberta may also need revision. The felt areas of the earthquakes show that regional anelastic attenuation in the eastern Canadian Cordillera is significantly anisotropic and much lower along strike than across strike. The felt areas also appear to scale with seismic moment rather than m b magnitude. The thrust mechanisms, regional stress regime, shallow focal depth, and high-velocity sedimentary rocks in the focal region suggest that the Nahanni earthquakes and particularly the near-field strong ground motion recorded for the second event are ideal engineering design data for critical facilities in eastern North America.