Biggest variable is the fact it's a wave, so the further from the epicentre the weaker it is, and also anything that affects normal propagation of the wave, which would stop it spreading like a ripple on a pond, means that you have a something called an "interference pattern" set up, in some areas the wave cancels itself out, in some places it doubles, or even triples in strength. Very scarey Same applies, really, huge distances and interference patterns Link