Earthquakes occur when two tectonic plates slip past each other. (or under/over each other)
Now, I imagine the contact between these plates as a plane, since both plates are 3D objects, pressed up against each other.
Then one plate moves, relative to the other one.
Surely the whole plane of contact experieces the jolt.
If the two plates are to remain the same shape and size, the focus can't be located in one particular spot.
If two carpets are overlapping, and I pull one out from under the other one, there is friction across the entire plane of contact between the two carpets. (all right, I know the analogy is a constructive margin, and earthquakes occur at destructive margins, but surely the principle is the same whichever way the plates are moving?
How come the focus is non dimensional, (a single point) rather than two dimesional, and the epicentre is non dimensional rather than one dimensional?