How does an earthquake alarm work? - Home

An earthquake alarm provides an early warning by detecting an earthquake's soundwaves before the shear wave hits your area. The alarm will detect a quake that is even many miles away! A loud alarm will then go off.

When an earthquake occurs, it creates P-waves & S-waves in the earth's crust. The P-waves travel faster than the S-waves. The earthquake alarm is sensitive to the P-waves. The alarms sounds when it detects P-waves. The S-waves are the waves that typically cause damage. The amount if time between the sounding of the alarm and the arrival of the S-waves is depenednt on the distance from the alarm to the quake epicenter.

If you're really close to the epicenter, you'll get a few seconds warning. Further away and you'll get more time.

http://www.earthquakestore.com/earthquake-alarms-more.htm