earthquakes

[largo]

If an earthquake is caused by the movement of the earths plates, why is there no earthquake on any of the plates other edges as it moves?

[Clanad]

Thumbing through my well worn copy of "Rocky Mountain Geology" from a college course, I'm reminded that the tectonic plates that cover the entire earth are constantly moving, as you're probably aware. During this movement along the zones where the plates come into contact (commonly known as "faults") they sometimes get "stuck" against each other. The plates can move in opposite direction from each other (one north, one south for example) or one can dive under another (subduction) or they can even move directly apart as in the Mid-Atlantinc Ridge all of which can result in the "earthquake" when the tension between the plates suddenly lets go in a rupture of the fault.
Therefore, in relation to your question, the quake is centered only where or very near to the point of disturbance between the plates...

[jomifl]

The rock that tectonic plates are composed of is plastic on a world scale so one part of a plate can move or stay still relative to another part with the rock between deforming to make up the discrepancy. As Clanad implied not all parts of a plate 'stick and slide' to give large earthquakes but slide relatively smoothly with frequent tiny quakes that are un-noticeable to all but seismologists.

[Old_Geezer]

I'd have thought there would be a risk of earthquakes there. But the major disturbances would be where 2 plates are vying for the same space, forcing one up, and one under. But that's basically a guess on my part.

[DTCwordfan]

because in really active plates, the other end is pushing out the lava - for example the mid Atlantic rift that runs through Iceland - we are becoming more distant to out American cousins by some 3cm a year.

Most people do not know that the Appalachians, the Lewisian rocks of the very NE tip of Scotland and the Norwegian mountains were all the same mountain range (they know this from composition and the orientation of iron which in a liquid solution will crystallise out to magnetic north.

Another one is that the Lakes and N Wales were as far apart as we are from the States today - (maybe it's a shame it is not like that today) - plates themselves have changed a lot over the billions of years.