A comparison of geological data shows that most of the earthquakes of western Quebec lie within or near the eastern and western boundaries of a Grenville (ca. 1000 Ma) central metasedimentary belt and in the area of the younger fault zone along the Ottawa River. Although most of the earthquakes have occurred within terrain of Grenville age or older, the younger rift zone is characterized by the larger earthquakes (Temiskaming, M6.2, 1935; Cornwall–Massena, M5.7, 1944). The earthquakes also correlate with gradient zones around two prominent anomalies in the smoothed Bouguer gravity field.The pattern of aeromagnetic anomalies and drainage channels reflects Preeambrian structural trends and indicates the central metasedimentary belt continues beneath Paleozoic sediments to Lake Ontario. The strikes of magnetic, gravity, and drainage features north of Lake Ontario are parallel to features mapped to the south of the lake. Geological data and geophysical trends suggest that a cataclastic zone is continuous along most of the eastern border of the metasedimentary belt. A prominent, positive regional aeromagnetic trend and some cataclastic evidence characterize the western border.Although the seismicity may reflect the adjustment of existing structures to stresses from regional density variations, continental deglaciation, and interplate forces, the reason for the greater seismic activity in the Grenville age terrain than along the younger fault zones remains unclear.