Re: What causes the shifting of the plates of the earth, causing earthquakes?

Led by Dr. Seiya Uyeda of Tokyo University, geophysicists have come to beleive that the driving force behind plate motion is "slab pull." Slab pull is the force, exerted on a plate by the portion of the plate that is sinking back into the mantle at subduction zones. As the dense oceanic lithosphere subducts into the mantle, it pulls on the rest of the plate. The key observation was that the greater the length of subducting edges around a plate, the faster the plate moves, in general. Many school science textbooks still say, incorrectly, that the plates move due to convection of the mantle. Convection plays a very small role in driving the motion.

The plates move on a soft (but not liquid) zone in the mantle at about 70- 100 kilometers (~40-60 miles) depth, known as the asthenosphere. The softening, caused by the interplay of temperature and pressure with mineral physics, creates a weak zone allowing the plates to creep along relative to the underlying mantle.

For lots of wonderful information about plate tectonics, including descriptions of how and why the plates move, it is hard to beat the online book This Dynamic Earth at the USGS web site:
http://pubs.usgs.gov/publications/text/dynamic.html