Just over 30 years ago, the Loma Prieta earthquake shook Northern California, killing 63 people and injuring almost 4,000 more.

It got Californians dreaming about an earthquake early warning system. Even a few seconds of lead time would allow people to stop what they’re doing, drop to the ground, and take cover, or pull over if driving.

To build this system, three large technological challenges had to be overcome.

First, the state installed hundreds more seismometers and electronically connected them.

Next, it had to write a complicated computer algorithm that could use data from just a few of these seismic stations to instantly calculate the epicenter of the quake and predict where its shock waves would travel, how quickly, and at what magnitude.

The system could do this by reading fast-moving compression waves, which arrive well before the more dangerous shear waves—the ones that cause the surface to jump and buckle.

Finally, they needed a way to get this information instantly out to citizens. So, on the 30-year anniversary of Loma Prieta, California launched a smartphone app called MyShake.

Alerts can go immediately to any smartphone, even using the phone’s location data to calculate how much shake the user would experience, and when.

The system can now provide minutes of warning. For instance, people in LA would know 2 minutes before shear waves from San Diego reach them. And that’s plenty of time to save many lives in the next big quake.