Quote from this link
..http://quake.usgs.gov/prepare/factsheets/NewMadrid/
"Earthquakes in the central or eastern United States affect much larger areas than earthquakes of similar magnitude in the western United States.
For example, the San Francisco, California, earthquake of 1906 (magnitude 7.8) was felt 350 miles away in the middle of Nevada, whereas the New Madrid earthquake of December 1811 (magnitude 8.0) rang church bells in Boston, Massachusetts, 1,000 miles away. Differences in geology east and west of the Rocky Mountains cause this strong contrast."