Abstract

On 19 October 1990 at 07:01 UT, a magnitude mbLg 5.0 earthquake occurred approximately 9 km southwest of Mont-Laurier, Québec, Canada (latitude 46.47° N and longitude 75.59° W). This earthquake is the largest to have occurred in the Western Québec Seismic Zone since the 1944 magnitude 5.6 Cornwall-Massena earthquake. The Mont-Laurier earthquake was widely felt up to distances of 500 km and caused some minor damage in the epicentral region [intensity Modified Mercalli (MM) V]. During the week following the earthquake, a field survey recorded 23 aftershocks (magnitudes were all less than ML 1.5). Analysis of the mainshock and its aftershocks indicates focal depths in the range 10 to 12 km. Of the two east-west oriented nodal planes given by the predominantly thrust mechanism of the mainshock, the steep northerly dipping one probably represents the fault plane, as indicated by the trend of aftershock hypocenters.

The last 10 yr of recording have confirmed that most western Québec earthquakes, including the Mont-Laurier earthquake, occur in a northwest-southeast-trending zone inside the Central Metasedimentary (CM) Belt with most focal depths varying between 7 and 25 km. Although some northwest-trending structural features are known, correlating these with the epicentral trend is premature. The mid-crustal hypocentral depths of many earthquakes, the east-west trend of the fault plane of the Mont-Laurier earthquake, and variations in regional focal mechanisms all suggest reactivation of deep structural features, which may not have a surface expression. Inside the Central Metasedimentary Belt, most earthquakes occur along the eastern side, where the activity seems to correlate with the Labelle Shear Zone and the Morin anorthosite body, which may act as a stress concentrator. The western side of the activity does not end with the Central Metasedimentary Belt, a fact that implies that even though the factors controlling seismicity lie predominantly within the Central Metasedimentary Belt, the adjacent geologic domains are also seismogenic.

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