Huge plates are constantly moving from the earth's surface. As they slide past each other along fault lines, stress points form, limiting movement and building up enormous energy potential. When this pressure is discharged, an earthquake occurs.

Its intensity is measured using the Richter scale. Each increase of one whole number on the Richter scale means a tenfold increase in strength. In 1964 an earthquake in Alaska measured 8.4 on the scale, making it the most powerful earthquake in the last hundred years in the United States.*