Yes, the further you are from the epicenter the more time there is for a warning, but it doesn't rely on having a detector right at the center. That's because there are two sets of seismic waves that travel at different speeds. P waves arrive first; S (shear) waves cause most of the damage. Of course, electronic signals travel much faster than either.
In the large earthquake in Tokyo in October, I felt the P wave, then got the alert on one of my phones, then the s-wave hit and I got the alert on my other phone (different provider). Interesting seeing the race, and I wonder how optimized notification systems are.
