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.