  Public earthquake early warning systems have the potential to reduce
individual risk by warning people of an incoming tremor but their development
has been hampered by costly infrastructure. Furthermore, users' understanding
of such a service and their reactions to warnings remains poorly studied. The
smartphone app of the Earthquake Network initiative turns users' smartphones
into motion detectors and provides the first example of purely smartphone-based
earthquake early warnings, without the need for dedicated seismic station
infrastructure and operating in multiple countries. We demonstrate here that
early warnings have been emitted in multiple countries even for damaging
shaking levels and so this offers an alternative in the many regions unlikely
to be covered by conventional early warning systems in the foreseeable future.
We also show that although warnings are understood and appreciated by users,
notably to get psychologically prepared, only a fraction take protective
actions such as "drop, cover and hold".
