THE MONT-LAURIER, QUEBEC, EARTHQUAKE OF 19-OCTOBER-1990 AND ITS SEISMOTECTONIC ENVIRONMENT

On 19 October 1990 at 07:01 UT, a magnitude m(bLg) 5.0 earthquake occu rred approximately 9 km southwest of Mont-Laurier, Quebec, Canada (lat itude 46.47 degrees N and longitude 75.59 degrees W). This earthquake is the largest to have occurred in the Western Quebec Seismic Zone sin ce the 1944 magnitude 5.6 Cornwall-Massena earthquake. The Mont-Laurie r earthquake was widely felt up to distances of 500 km and caused some minor damage in the epicentral region [intensity Modified Mercalli (M M) V]. During the week following the earthquake, a field survey record ed 23 aftershocks (magnitudes were all less than M(L) 1.5). Analysis o f the mainshock and its aftershocks indicates focal depths in the rang e 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 t rend of aftershock hypocenters. The last 10 yr of recording have confi rmed that most western Quebec earthquakes, including the Mont-Laurier earthquake, occur in a northwest-southeast-trending zone inside the Ce ntral 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 tren d 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. Th e western side of the activity does not end with the Central Metasedim entary Belt, a fact that implies that even though the factors controll ing seismicity lie predominantly within the Central Metasedimentary Be lt, the adjacent geologic domains are also seismogenic.