1985 Nahanni earthquakes

Geology and impact

The earthquake sequence is commonly considered to consist of three major events over the span of almost three years: a 6.6 tremor on October 5, 1985, a 6.9 on December 23, 1985, and a 6.2 on March 25, 1988 UTC. All three events occurred within just several kilometers of each other.

The epicenters of the mainshocks and aftershocks lie approximately 44 km (28 mi) west of the Camsell Bend of the Mackenzie River in the foreland fold and thrust belt of the northeastern American Cordillera. This part of the Cordillera is dominated by horizontal crustal compression (shortening) in a northeast direction, forming a large amount of thrust faults and folds. This region is relatively sparse of intense seismic activity—the Nahanni earthquakes generated much interest among seismologists due to being the only recorded earthquakes M6.0 in this area. Between October 5, 1985 and March 25, 1988, 323 aftershocks were recorded, occurring in a roughly 60 km by 20 km zone. The characteristics of aftershocks suggest that the rupture plane was a west-dipping low-angle thrust fault. The earthquakes are interpreted to have occurred on unmapped faults in the vicinity of the Iverson Thrust Fault.

The ground motion of the mainshock of December 23 was exceptionally intense, reaching a peak ground acceleration of more than 2 g and a peak ground velocity of over 40 cm/s. Due to the remote location of the epicenter, no structural damage was reported. The earthquake was felt 115 km (71 mi) away in Wrigley, where residents noted the effects of strong ground motion. A massive rockslide was triggered by the 6.6 event on October 5, around 7 km (4 mi) northeast of the epicenter. The rockslide displaced a volume of approximately 7.6 million cubic meters of earth in the Mackenzie Mountain foothills.

References

References

1. "M 6.9 – Northwest Territories, Canada". United States Geological Survey.
2. (1988). "Engineering implications of the 1985 nahanni earthquakes". Earthquake Engineering & Structural Dynamics.
3. [http://earthquakescanada.nrcan.gc.ca/historic_eq/20th/nahanni/nahanni85_e.php Natural Resources Canada: Rock and Roll in the N.W.T.: The 1985 Nahanni Earthquakes] {{webarchive. link. (2009-02-10)
4. [http://earthquakescanada.nrcan.gc.ca/historic_eq/20th/signif_e.php Natural Resources of Canada: Significant Earthquakes in Canada] {{webarchive. link. (2008-10-13)
5. Ristau, Johannes. (1999). "Applications of Synthetic Aperture Radar Interferometry in the Study of the Nahanni Earthquake Region". University of Manitoba.
6. (1990). "A fault model for the Nahanni earthquakes from aftershock studies". Bulletin of the Seismological Society of America.
7. (1991). "Application of airborne C-SAR and SPOT image data to the geological setting of the Nahanni earthquake area". Canadian Journal of Remote Sensing.
8. (2010). "Canada’s Earthquakes: ‘The Good, the Bad, and the Ugly’". Geoscience Canada.
9. (2018). "The 1985 earthquake-triggered North Nahanni rockslide, Northwest Territories, Canada: The co-seismic movement of a sedimentary rock mass conditioned by residual strength". Engineering Geology.