They do detect underground blasts, and they would probably detect a surface-level explosion as well. The Air Force Technical Applications Center looks at some of that data to determine if a nuclear bomb went off

https://en.wikipedia.org/wiki/Air_Force_Technical_Applicatio...

Seismic waves only propagate at up to ~14 km/s. If New York is bombed, it would take ~5 minutes (I think) for the wave to reach Los Angeles. Twitter would be much faster (assuming Twitter survives the attack).

> Would earthquake monitoring systems pick up a nuclear blast?

Yes, but the operator probably wouldn't identify and publish that it was a nuclear blast “within a minute” as the OP desires.

Interesting thought! My guess is that it would depend on the size of the blast. I can imagine a scenario where Putin uses one (or a few) dial-a-yield type warheads turned all the way down. The point isn't to blow stuff up, it's to clearly communicate "I am willing to use nuclear weapons".

In that scenario, would a few kt be enough to set off sensors / be distinguishable from noise? I don't know.

Still, a very good idea and worth looking in to.