Earthquakes usually strike without warning, leaving people no advance notice to take cover. When a temblor occurs, it sends seismic P waves through the ground that a Google app called MyShake can detect with a network of 1,300 U.S. Geological Survey sensors.

All smartphones have accelerometers that can pick up earthquake signals. When triggered, the phone sends a message to a detection server with location data to piece together where the earthquake is occurring. When four sensors are triggered simultaneously and the data meets the right criteria, the system determines that stronger S waves, which can cause damage and hurt people, may be imminent. The Federal Emergency Management Agency interprets that data and sends out alerts via the ShakeAlert system. 

During a recent 4.8 magnitude Bay Area earthquake, more than 1 million Android users received messages seconds before the event. Robert de Groot, a member of the ShakeAlert team, says, “One of the things we’re trying to do is build an earthquake early warning industry.” Equipping phones to pick up signals is a cheaper and quicker solution than planting larger sensors 10 feet underground in earthquake-prone areas.