Known as MyShake, it can sense an earthquake even when the cell device is being carried in a pocket or a bag.
The researchers want users to download the app, in the first instance, to help test and improve its capabilities.
But ultimately the idea is that recruited phones will be part of a network that not only gathers data but also issues alerts.
Destructive ground motions take time to move out from the epicentre of a large tremor, meaning people at more distant locations could receive several seconds' vital warning on their phones.
