Unfortunately this is too early to be truly efficient, especially in the most seismic hazard prone areas of the near field environment. Otherwise the more you have of well distrubuted stations, efficient communicarions and management the closer you are to hopefully getting some results In the relatively less exposed far and intermediary fields.
A quick solution is to have a look on these items to choose what is closer to your ideas . https://www.google.com/search?q=optimized+earthquake+early+warning+system&ie=UTF-8&oe=UTF-8&hl=fr-jo&client=safari Good luck.
This is a progress. The efficiency is strongly dependent on the Seismic Source-City distance and the degree of organization of the responsible authority, also on the degree of awareness of the population.
It should be in a tomographic pattern optimization and dependent on source receiver geometry. The denser it is better is the resolution. But there are other constraints as mentioned above.