Abstract

On July 4, 1981, a magnitude 3.3 earthquake shook the area near Cornwall, Ontario and Massena, New York. Within the next four days 17 additional earthquakes occurred ranging in size from magnitude 3.3 to 0.5. All of the events above magnitude 2.5 have well determined hypocenters located to within ± 0.6 km of 45°06.5′N and 74°36.6′W and are 16 ± 3 km deep. Fault plane solutions determined for the two largest events (both m = 3.3) show reverse faulting with some strike-slip motion on north or west striking planes. Most of the earthquakes in northern New York and southern Quebec and Ontario have similar fault plane solutions. Intensity data gathered from the first event reveal a maximum intensity of V surrounding the instrumentally located epicenter. All the isoseismals are elliptical with major axes that trend northeast-southwest, and they are elongated to the southwest with respect to the July 4 epicenter. The trend of the isoseismals is roughly parallel to the regional strike of the nearby Grenville structures. The extent of groundshaking is apparently determined by structural fabric rather than by the geometry of faulting. We believe that the earthquake sequence in July 1981 is genetically related to earthquakes clustered along a NNW band of seismicity extending from the Adirondacks in New York to just north of the Baskatong reservoir in Quebec. Nearly all fault plane solutions from earthquakes in the band show reverse faulting on NNW striking planes. The alignment of nodal planes with the seismicity trend suggests that earthquakes in this region of relatively high seismic activity are releasing stress along a NNW zone of weakness.

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