Another issue is how to get alerts out to the general public. Software developers in Japan have created cellphone and computer apps that can receive and display signals from that country's early warning system. In this country, television and radio stations and mobile-phone companies also have voluntary systems for broadcasting emergency messages, although it's not clear that the phone companies are ready to blast text messages at the speed and volume demanded by a major quake.
Assuming those issues are resolved, the alerts could reach at least some people a few seconds or more before the shaking begins (the farther from the epicenter, the more advanced the warning). That's enough time not just for automated controls in buildings and transit systems to react but also for people to take cover. Again, however, notifying the public won't be worth much without an effective campaign to educate people about what to do.