10:35 AM EST on Wednesday, January 19, 2005

By TERRI BENNETT / Chief Meteorologist

Earthquakes are predominately caused by movements of the Earth's thin outer surface called the crust. The crust is made up of about 15 irregularly shaped plates that move about relative to each other.

The boundary between plates is called a fault. Along the San Andreas Fault the Pacific Plate is moving northward along the North American Plate at a rate of as much as 2 inches per year. This is known at a strike-slip fault where the movement is in opposing horizontal directions.

A normal fault is one where two plates are moving apart from one another and one plate slides downward relative to the other. Plates collide and are compressed along a thrust fault where one plate is forced up and over another.

Most tsunamis are caused by earthquakes on the ocean floor along a normal or thrust fault where the ocean floor is shifted in a vertical or up and down direction. The resulting rapid rise or fall of ocean water above and along the fault is what causes a tsunami. I’ll have more on tsunamis Thursday.