I didn't get any notification on my phone. How does it try to figure out whether you should get one? Do you need to turn on location services, have a cell tower connected, or is it detecting pre-waves (I really know nothing about earthquakes) using the accelerometer?
There are different systems. The more critical system in the USA, WEA is a mandatory (FCC rules) central system that most consumer devices and consumer networks take part in. The "last mile" technology varies by network type and version.
The GSM lineage (including 4G, LTE, 5G) is implemented using cell broadcast which is a protocol data unit that solves for the one to many problem. Extensions added geo-fencing & geo-targeting on the client devices (device compares its idea of location to the incoming message), and also cell based routing optimizations network side. The cell broadcast part is technologically available ~everywhere as a result of broad GSM deployment, but whether it is integrated into an alert system is a jurisdiction issue. Sadly few countries (in the global scope) have the kind of power of the US or the EU to gain access to things like extended on device features.
The app based systems are largely based on some form of location reporting and push messages which are far less reliable - one to one messaging systems (like Apple/Google push notifications) are physically impossible to scale for real time sub-second alerting in dense populations, given the bandwidth constraints of radio.
CB is expensive at the radio tower, and can disrupt normal communications (potentially including emergency services communications) as such it's use is more limited than other solutions, for example in todays event it was only used in an area of about a 25km radius. To put this in some context, many folks commenting here, including myself still in the "felt it" areas were within a ~90km radius (SF is about 85km away from the center).
Notably, these earthquake alerts do not go through the WEA system or an app. Or at least they don't follow the normal w e a path from the cell tower. They go through the operating system itself which is why they are received on Android and not iPhone. Apple has not integrated the functionality into their operating system. With iPhones you can get third-party apps however.
You can always hang the sense of it and pass the time :-)
I wasn’t in the area, but I believe WEA would have been sent in the inner ~20km based on the system report, which is sjc and the surrounding area just up to the southern tip of the bay.
An expert may have more information, but I think it is a broadcast message from all towers in a given region. That is, every phone that is attached to this tower gets a message.
