The scientific theory that the Earth's outer shell is composed of several large, thin, relatively strong plates that move relative to one another. Movements on the faults that define plate boundaries produce most earthquakes
U.S. Department of the Interior,U.S. Geological Survey, Putting Down Roots in Earthquake Country (Your Handbook for the San Francisco Bay Region), Glossary, Denver, CO, USA 2006. http://pubs.usgs.gov/gip/2005/15/#glossary, http://bhs.idaho.gov/pages/Preparedness/Hazards/PD F/Putting%20Down%20Roots%20in%20EQ%20Country.pdf, (ND/ Erthq - Brit) {897}