If it was a big deal, beyond the opt-in SF text alerts, there's a thing called Wireless Emergency Alerts. It's geographically targeted. It's basically like Amber Alerts going to your phone except for imminent emergencies.

There has been some controversy about the timing and use of WEAs for evacuations for the big wildfires in California recently, but I imagine it's hard to balance alarm fatigue with the need to warn people.

The state can broadcast emergency messages, it doesn't need to send them individually to some database of cell phone numbers.

Alert About Missile Bound for Hawaii Was Sent in Error, Officials Say https://news.ycombinator.com/item?id=16140761

How do amber alerts work? When I lived in SF I got SF-related alerts. I don't know enough about how they work - but it sounds like tech that could be re-used for major emergency alerts.