What would be the radius spread of an earthquake magnitude of 7.8?

Question

e.g Suppose it is to hit [X] city at 7.8 magnitude and [Y] city is about 30 miles away, what would be the magnitude of the earthquake that [Y] city would feel?

Answer 1

It depends upon the depth of the focus, as well as the material the S and P waves are moving through.  

And no matter what, the earthquake would be listed as a 7.8.

Answer 2

About 500 k/m radius it will affect..You will feel the shaking of about 300 mm.But these calculations are only approximate. It depends upon several factors.