The North Gower, Ontario, Earthquake of 11 October 1983: Focal Mechanism and Aftershocks

Abstract An earthquake near Ottawa (45.20°N, 75.75°W, focal depth 12 km) of unusually large size for the region's seisrhicity (mb(Lg) = 4.1) provided good P-wave first-motion data for a focal-mechanism solution. The maximum intensity was V(MM) and the area of perceptibility in Canada about 80,000 km2. The first and largest recorded aftershock occurred nine minutes after the main shock with magnitude mb(Lg) = 1.7. Two further small aftershocks ware recorded by a field network. The mechanism is thrust faulting with a predominantly horizontal pressure axis trending 154°. Thrust mechanisms have been found for other earthquakes in southeastern Canada and the northeastern U.S., but orientations of their stress axes are different, and so the North Gower earthquake may reflect a local and not a regional stress field. The nodal planes have strike 71°, dip 75°, and strike 221°, dip 17°. The spatial distribution of aftershocks suggests the gently-dipping nodal plane could be the fault plane. There is uncertainty about the seismotectonics of the region, and the orientation of neither nodal plane correlates with known geological features. More earthquake fault-plane solutions are required to interpret the seismotectonics and the stress regime.