> That means LoRa and cellular network won't help either... That's good to know.

It does. If anyone is buried under the rubble "why don't they just tweet/SMS/mesh-network they are stuck" might be the first question someone would have. A simple answer is that the nearest GSM tower is also under rubble. But that's not enough. Even if the nearest GSM tower is 100% fine (or their next LoRa mesh peer is also fine), you need to be less than 12-15 inches of concrete (in the most optimal/reflective way) to have any signal at all. At the end of the day you're up against physics. 3 questions physics mandates:

1. What is the wavelength you're transmitting?

2. How far is your receiver?

3. What is your medium?

The keyword "Antenna Theory" or "Antenna Design" is a good rabbit hole for this sort of thing. There is this funny saying among radio enthusiasts "everything is an antenna if you squint hard enough"[1].

There is also something that cooks call "The Human Element"[2] when they're trying to evaluate the appeal of some foods to people. The "Human Element" in an "under the rubble" situation is that you want something that a normal human being would have on them when the sudden situation of an earthquake happens and they find themselves under a rubble.

I don't know what that is. I live in a place that is pretty far removed from earthquakes as a major problem. Geologically speaking, we get a "major earthquake" every 500 years or so. However, that's an average and it's been 600 years for us but the max has been 3,000 years and the min has been 200 and the mean means nothing to the individual. However, a device of some sort that can emit low frequency, low bandwidth[3], long range message seems like a useful thing. I don't know if that would be the type of device every human would have "on their body" all the time. But maybe??

[1] Everything that's electrically conductive is an RF antenna. If you get buried in the right place next to a 100 feet long rebar, you could (theoretically) send a signal that anyone in 100 mile range could hear provided you had the right equipment of course.

[2] In cooking, it is this elusive thing that explains how some cultures/people love certain tastes or textures that other cultures don't. Slimy foods in Middle Eastern or Japanese/Asian cultures vs Western Pallet for example.

[3] A device that needs to send "S.O.S" needs maybe 1byte/second or even less. You can't communicate any meaningful information in a seismic-pulse-signal type device, but "SOS" is certainly doable.

If someone is under rubble and needs to signal anyone, if they have the use of their hands, best thing they can do is get something solid and tap the SOS morse code on something. I thought that's what they did during the Kursk submarine accident but apparently that wasn't correct.

Anyway, SOS morse code will carry pretty well through concrete or steel or whatever.

TIL about Antenna Theory/Design.

Yeah the human element is the strictest here. From the little research/thinking I did, BLE felt like the most "commonly available" tool we have. But I'm wondering if manufacturing a cheap wearable could also be an option.

I definitely need to do a longer/deeper dive for this project. I overlooked it in the past, thinking people don't need it. But it seems like I just dropped the ball in distribution previously.