Seismologists know that earthquakes in the eastern United States have larger felt areas than earthquakes with similar magnitudes in the western United States. The corollary that earthquakes in the East should have larger damage areas has been proven untrue, according to Arch Johnston, Center for Earthquake Research and Information, Memphis, Tenn., and T. C. Hanks, U.S. Geological Survey, Menlo Park, Calif. They presented their findings at the AGU Spring Meeting.
Their studies show that ratios of damage and felt areas for eastern and western North America range from a factor of 100 to a factor of 10, with little distinction between low and high intensity earthquakes. Johnston and Hanks show that the ratio is much lower than previously thought, and that it decreases from a high of 7 for felt areas to a factor of 2–3 for intensity 6, and approaches unity at intensity 7. Earthquake intensity is measured on a scale of 1–12, which gives felt and damage information. Intensity 1 represents an earthquake that is barely felt, and 12 an earthquake that causes substantial destruction. Damage is evident following earthquakes with intensities of 6 and 7.