A study of the source parameters of some large earthquakes of northeastern North America

Waveforms of body and surface waves for six moderately large north eastern North American earthquakes are analyzed for source information. The earthquakes studied are the 1925 Charlevoix, Quebec, earthquake (MS ≃ 6.4), the 1935 Timiskaming, Quebec, earthquake (MS ≃ 6.0), the 1939 Charlevoix, Quebec, earthquake (mb ≃ 5.4), the 1940 Ossipee, New Hampshire, earthquakes (ML ≃ 5.3 and 5.1), and the 1944 Cornwall, Ontario-Massena, New York, earthquake (M ≃ 5.1). The focal depths for all of the earthquakes for which determinations could be made (1925, 1935, 1939, and December 20, 1940) were about 8 to 10 km. The waveforms of the 1925, 1935, 1939, and 1940 events are consistent with thrust mechanisms, although there is some ambiguity in the strike direction for each of the events. The favored focal mechanism for the 1925 and 1939 events had northeast-southwest nodal planes. For the 1935 event the available data suggest that the earthquake occurred on a northwest-southeast oriented fault plane. The strike of the nodal planes for the December 20, 1940, event is quite uncertain, but a north-south orientation is preferred. No body waves that could be modeled were collected for the December 24, 1940, or 1944 events. The stress drops for the events scatter widely but are comparable to those from other intraplate earthquakes. There is a difference in source complexity from event to event, with the December 20, 1940, event having had one simple source, while the 1935 shock had three separate energy releases. The source time functions found for the 1935, 1939, and December 20, 1940, earthquakes from the short-period teleseismic body waves resemble those for some shocks from California. The focal mechanisms found here suggest that the minimum principal deviatoric stress is vertical and that the horizontal principal deviatoric stresses, both compressional, are controlled primarily by plate-driving forces.