This newspaper article explains how the system could work — if ever funded ...
There was a minor shaker in Oregon last week and the event provided an opportunity to explain how the earthquake early warning system might work
As noted in the article, other countries have fielded such a system already — we (United States) are waiting for the BIG ONE to motivate us to do something like this — right after the BIG ONE hits and we are at the point where we want to be seen as doing something in reaction to the disaster that will likely kill and injure many people.
While the machine-to-machine (M2M) is mentioned in the article, I think this is the most valuable aspect of the warning system, since we Americans are not conditioned to take rapid action in response to a warning that we receive. We like to "mill around" before taking action.