One thing that they can't afford - False Alerts.
The system has to be foolproof, and only for major quakes.

I thought the warning time for Los Angeles was closer to 20-30 secs. That's a pretty good warning. If they have sensors at Palm Springs, the warning system will give downtown L.A. about 20-30 secs. But the warning needs to be triggered by multiple sensors, all going off at the same time.

7 secs is enough time to dive for cover.