series of three large earthquakes that occurred near New Madrid in southern Missouri on Dec. 16, 1811 (magnitude from 8.0 to 8.5), and on Jan. 23 (magnitude 8.4) and Feb. 7, 1812 (magnitude 8.8). There were numerous aftershocks, of which 1,874 were large enough to be felt in Louisville, Ky., about 180 miles (300 km) away. The principal shock produced seismic waves (seismic wave) of sufficient amplitude to shake down chimneys in Cincinnati, Ohio, about 360 miles (600 km) away. The waves were felt as far away as Canada in the north and the Gulf Coast in the south. The area of significant shaking was about 38,600 square miles (100,000 square km), considerably greater than the area involved in the San Francisco earthquake of 1906. Subsequently it was discovered that North American continental earthquakes, such as the Missouri shocks, produce greater shaking than do comparable shocks along the Pacific coast. In one region roughly 150 miles long by 37 miles wide (240 km by 60 km), the ground sank 3 to 9 feet (1 to 3 metres) and was covered by inflowing river water. In certain locations, forests were overthrown or ruined by the loss of soil shaken from the roots of the trees.