The MacKenzie Mountains, Northwest Territories, earthquake sequence of 1953 to 1957

The northern Canadian Cordillera is a region of intense, poorly understood seismicity. Between 1953 and 1957, five moderate to large earthquakes (M5.8-6.4) struck the remote northern MacKenzie Mountains. We utilize historical seismograms, felt reports, and other information to define the epicentres, focal depths, magnitudes, and focal mechanisms of this previously unstudied earthquake sequence. The 1953 and 1955 earthquakes were felt (up to intensity V) to distances of 230-280 km. Magnitudes (Mw) were determined at 6.3 and 6.4, respectively. Epicentres were relocated using path corrections obtained from well-recorded modern earthquakes (M4.5-5) in this region. The relocated epicentres are distributed in an east-southeast-westnorthwest direction over <^>-45 km. The 1955 mainshock and largest aftershock are separated by <^>-20 km, with the aftershock located north-northeast of the mainshock. The 1957 (M5.8) earthquake is located a similar distance and direction from the 1956 (M6.1) earthquake, suggesting that it may be an aftershock of that event. Focal mechanisms and depths were determined using body wave modelling. Depths ranged from 9 to 15 km, and focal mechanisms show thrust faulting along either a north-northeast-plane or a south-southwest-dipping plane. The orientation of the east-southeast-west-northwest striking nodal plane is consistent with the distribution of epicentres and the regional geology. The north-northeast-south-southwest striking, shallow-dipping P-axes are consistent with the regional stress field. This study reveals numerous similarities with the 1985-1988 Nahanni earthquake sequence, including shallow, thrust faulting, earthquakes over several years, and likely caused by stresses transmitted from the plate boundary (700+ km to the west), activating favourably oriented thrust faults.