Earthquakes can be measured in terms of either the amount of energy they release
   (magnitude) or the degree of ground shaking they cause at a particular locality
   (intensity). Magnitude is calculated from the record (wave amplitude) made by an
   earthquake on a calibrated seismograph. The magnitude scale is logarithmic. An
   increase of one in magnitude represents a tenfold increase in the recorded wave
   amplitude. However, the energy release associated with an increase of one in
   magnitude is not tenfold, but about thirty fold. For example, approximately 900
   times more energy is released in an earthquake of magnitude 7 than in an
   earthquake of magnitude 5.

    Earthquakes can affect hundreds of thousands of square kilometers; cause damage
   to property measured in the tens of billions of dollars; result in loss of life
   and injury to hundreds of thousands of persons; and disrupt the social and
   economic functioning of the affected area. Although earthquakes in the United
   States occur most frequently in states west of the Rocky Mountains, devastating
   earthquakes have also occurred in the Midwest and East. All 50 states have some
   degree of risk from earthquakes.