Yesterday, on the anniversary of the 1989 Loma Prieta earthquake, they tested out an early warning system hooked up to the system that drives Amber alerts. This will deliver warning of an impending earthquake, tens of seconds before an earthquake hits (in supported areas, like SF) due to the propagation speed differences between an earthquake's P-wave and S-waves.

(Japan has had such a system in place since the 90's, and will stop bullet trains and surgeries just before the earthquake hits.)