How would something like this work without connectivity?

In Japan you can get early earthquake notifications, before the seismic waves arrive, via area broadcast.

https://en.m.wikipedia.org/wiki/Earthquake_Early_Warning_(Ja...

I didn't say without connectivity, I just said without phone data connectivity. Email would be best, SMS could work too.

Email doesn't work without phone data

Sure it does? It goes to my computer.

Ah geez. I was assuming you wanted the alerts to be sent to your mobile while you were away from home.

And your assumption was right. The original premise was "how can I get notified within seconds after tremors occur" and not "please send an email for me to read when I happen to use a desktop PC".

Some people spend most of their day on their laptops and try to limit phone time. People in this thread are all assuming everyone does things they way they do, but not all people are the same.

Email delivery can easily be delayed 1-2 minutes. I’m not sure it’s the best method for an early warning system.

I usually get my emails instantly, but if they get delayed, then they get delayed. At least a chance of getting a notification quickly is far better than nothing.

A mix of technologies might help: say 10 cellphones in a given area detect a tremor, they start communicating each other via mesh WiFi (long range BT, LoRa if available etc.) their data and if one of them is connected to the Internet, it packs all data and send it to a central server where it is matched with other (groups of) cellphones data and evaluated if an alert is to be sent. If yes, it can use the same path (central server->Internet->connected phone->phones nearby) plus broadcasting an alert to other media sources, TV, radio etc. All it would need is one cellphone to be connected at a time, which in non rural areas should be almost the norm today. Also, the amount of data sent and received would be trivial so it wouldn't impact connectivity costs.

I agree, having an API or some such would be nice; I don't usually carry around a phone indoors when I have massive screens available to me, and also put my phone in airplane mode while sleeping.

The system only works because people run the app on their phones to produce the data.