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).
This is also an interesting point! I assume there's a network of monitoring stations all over the world which are hooked up to the internet & other modes of speed-of-light communication.
https://www.ldeo.columbia.edu/~richards/my_papers/Richards&K...
https://www.ctbto.org/verification-regime/monitoring-technol...
I can't figure out how to get their data in real time and set up push alerts, though...!