> The other question, can zombies chain their 50 mile radius alerts?
> 
> 
> The reason I thought they should be somehow time capped is, assuming zombies move aimlessly unless they have a reason to go to a specific location, there are a whole lot of zombies wandering the ocean floor who could at any point walk up the continental shelf and blunder into you. This is only a humerous problem if they can't chain their alerts.

No chaining, otherwise the entire landmass of North America would eventually come after you in one sighting. The zombies caught up in the call radius, simply change direction and head towards their comrades. Once at the battle/massacre sight, they'll then make a call, with the others, but it'll be locked in the initial fifty mile radius. 

However, if a band of dead coming in to aid their decaying friends stumbles into the living enroute, then they'll start yelling for help from there. So there could be a chain, but the reinforcements would have to encounter the living on the way in order to scream for help. If that makes sense?

Basically, they have to sense a life force before they call for help. If they're just going to aid a request for help, then they haven't actually sensed a life force, they're just going somewhere because they're being asked to. Therefore, no screaming, no chain. :)