Audubon was working in Missouri and out riding when the 1811 New Madrid earthquake struck. When Audubon reached his house, he was relieved to find no major damage, but the area was shaken by aftershocks for months. The quake is estimated by scholars to have ranked from 8.4 to 8.8 on today's Richter Scale of severity, stronger than the San Francisco earthquake of 1906 which is estimated at 7.8. Audubon writes that while on horseback, he first believed the distant rumbling to be the sound of a tornado,