This theory claims that the earth crust, more precisely the lithospere, which emcompasses the Crust and the Mantle's upper part, down to the depth of 100 km, is broken in a determined number of hardplates, which shift themselves in horizontal movements and can be represented as rotations with respect to the axis that is through the center of the Earth.
These movements occur because the Lithospere, which is lighter and colder, practically “floats” on the hotter and denser material, partially molten, which exists at the top of the Asthenosphere. It is within this viscose part (the first 200 km of the Asthenosphere), that convection currents are generated, presumadly, the mechanism that accounts for the movement of the plate tectonics.
Source: Tassiani, C. (2000)
The plates slide or crash into one another at a variable speed of 1 to 10 cm/year. In the regions in which they collide or rub against each other, there is an intensification of the rock-deforming stresses and, periodically, the strongest earthquakes happen at such points.
It is precisely on the tectonic plates' limits, throughout narrow and continuous strips, that the majority of the Earth's whole seismicity is concentrated. It is also close to the plates' edges that the molten material (magma), at the top of the Asthenosphere, comes up to the surface and burst through crackings or channels to form volcanos. Although earthquakes and volcanos are usually born nearby the plates' limits, under exceptional circumstances, superearthquakes may take place in the plates' middle.