The newly updated MyShake app detects seismic events using peoples cell phones, then sends notifications to people nearby 5-10 seconds before the shock waves arrive.
Edit: I'm also not sure how much battery it uses... hopefully the battery-saver mode won't use much?
https://en.m.wikipedia.org/wiki/Earthquake_Early_Warning_(Ja...
[0] https://www.theverge.com/platform/amp/2019/10/17/20919639/ca...