Although we are still years away from predicting earthquakes, researchers are discovering an increasing number of pieces to the puzzle of seismic motions.

One of these was recently announced by researchers at the Yale and Washington universities. According to their findings, strong earthquakes, similar to the Sumatra one, may be caused by the build up of sediment on top of subduction zones. This new theory could pave the way to a new method for predicting severe ground motions.

Subduction zones are the boundaries where two tectonic plates collide, one plate pushes over and one pushes under the other. The most severe earthquakes in recent history, in Indonesia in 2004, Alaska in 1964, and Chile in 1960 occurred at subduction zone faults.

Seismologists have long known that the motion of the plates at subduction zones can be smooth and steady in some areas, and sticky and unsteady in other areas," said Mark Brandon, professor of geology and geophysics at Yale and senior author on the paper appearing in the February issue of the journal Geology.

The Earth's lithosphere is laced with about 32,000 miles of subduction zones, and the motion along the margins of the zones averages a slip of about two inches (5 centimeters) per year. However, where the margins stick and then rip apart into earthquakes, displacements have been as much as 65 feet (20 meters), over many hundreds of miles.

Past research has shown that as a subducting plate slides beneath an upper plate, stress builds where the plates meet and stick. The upper plate's warp creates a basin which fills with sediment that empties from nearby rivers.

It appears that the most severe subduction zone earthquakes occur in areas where such sediment-filled basins are found.

"This phenomenon is analogous to a mayonnaise jar in the refrigerator. The lid opens easily if you use it every day. But if you open the jar infrequently, adhesion will make it difficult to open. A sharp tap on a counter breaks the adhesion and the lid opens with a quick spin. In the Earth, the earthquake marks the break down of adhesion on the subduction zone," Brandon said.