Plate tectonics is the over-lying theory presently used by most Earth Scientists to describe motion within the outer-most layer of the solid Earth (also know as the lithosphere). Individual plates of varying size move about the surface of the Earth at varying speeds. Where plate pull apart, slide by each other or collide, there is tectonic activity manifested as earthquakes. The great majority of seismicity on the planet occurs at plate boundaries, although intra-plate seismicity can occur as well when stresses build up in the plate. For instance the Mew Madrid Fault zone of the mid-western USA is an example of a intra-plate seismic belt. In general, the deepest plate boundary earthquakes are at plate collision (or subduction) zones, and the shallowest are at divergent margins.
