Abstract

Earthquakes are common in Atlantic Canada, but instrumentally determined magnitudes have rarely exceeded 5 on the Richter scale. One exception is the Grand Banks earthquake of 1929 which had a magnitude of 7.2. However, it is probable that the Richter scale overestimates earthquake magnitude in Atlantic Canada. A recalculation of magnitudes for many of the non-instrumentally recorded earthquakes, using the various empirical relationships between felt area and magnitude, rather than maximum intensity and magnitude, shows a reduction in magnitudes for historically reported earthquakes. Because of the paucity of available instrumentation - there has been only one three-component seismograph in the region for a part of the time - the detailed pattern of epicentres and focal depths is poorly known in Atlantic Canada. Attempts to correlate epicentres with known geological features have generally failed because of the uncertainty in the epicentral positions. Nevertheless, several hypotheses have been advanced to explain the earthquake pattern in Atlantic Canada: 1 ) movement on faults, 2) glacial rebound, 3) association with igneous intrusions. An analysis of these explanations, shows that the five major known episodes of faulting all happened between Precambrian and early Tertiary times and are not at present active. Furthermore, glacial rebound in detail cannot be related to the distribution of earthquakes. Association with igneous intrusions is a working hypothesis, but recent evidence from Quebec and New England suggest a mid-crustal focal depth for those earthquakes for which a good solution is available. It is therefore suggested that earthquakes are not associated with the intrusions as such, but possibly only with deep channels that led the magma to the upper crust. The repeated motion on these deep magmatic conduits is provisionally attributed to the cooling of the existing thermal high that underlies New England and Atlantic Canada.