An earthquake early-warning (EEW) system is a set of capacities to detect and characterize a significant earthquake, estimating the distribution of shaking and distribute the information to communities and organizations, enabling individuals to prepare and act appropriately and in sufficient time to reduce the possibility of loss and protect life.
OpenEEW is a project that features a set of core EEW components; hardware, software and know-how, that will place effective EEWs to be within the reach of many underserved communities around the world.
The project includes schematics for low-cost sensors, detection in the cloud, alerting apps, provisioning apps, and a dashboard for monitoring.
The alerts are sent in real-time to people before the shaking or the strong part arrives.
One of the purposes for building low cost, time of flight, gravimeters arrays is for earthquake early warning. at https://hackaday.io/project/164550
I put some notes about gravitational earthquake early warning at GravityNotes.Org