The ground that you walk on, drive on, and play on may seem rock solid, but our earth's surface is in fact a dynamic grid of slowly moving sections, known as tectonic plates.  This motion is normally incredibly slow.  For example, North America and Europe are drifting apart at the rate of just five centimeters per year.

The movement of the tectonic plates causes stress beyond a critical threshold, then a portion of the crust will give way, causing sudden and violent shifting, or an earthquake.  The more observations that are performed, the better prepared we can be when the Big One actually occurs.

It is the fear that Californians have of a future catastrophic earthquake.  Many believe that it will be around the los Angeles area, and will kill 1,000's of people while destroying numerous buildings.  The state of California has spent millions of dollars on research that will predict the actual “Big One”, but no one really knows for sure when or if it will happen at all.

"Looking east from Kawashima,"
Hanshin Expresessway, Kobe,
Japan earthquake, January 17, 1995.
This image was taken from:
National Information Service for
Earthquake Engineering; Northridge
Collection, Earthquake Engineering Research Center,
University of California, Berkeley.