The phone alerts many Venezuelans saw before this week's earthquakes were not a glimpse of the future. They were the result of Google's Android Earthquake Alerts System, which turns phones into a distributed seismic network. Outside parts of the United States that use ShakeAlert, Android phones can use their accelerometers to detect motion that may indicate an earthquake. When enough phones near the source send similar signals with coarse locations, Google's server estimates what is happening and sends alerts to users who may feel shaking. The warning can arrive first because electronic messages travel faster than the damaging seismic waves moving through the ground.

The social fact is sharper than the technology. In countries without dense public warning infrastructure, a private phone network can become the first alarm of a national disaster. That may save seconds, and seconds can be enough to move away from glass, leave a staircase, or drop and cover before stronger motion arrives. It also means that an emergency function once associated with the state now depends on operating systems, device settings, network coverage, battery charge, and a company server. The alert on the screen is small, but behind it sits a new form of public safety built from private sensors in millions of pockets.