Update: In the comments on this post, a reader has raised a point that I need to check out. It is possible that the apparent tremor signal is actually an echo of the major quake in Indonesia today, superimposed on the signals from local activity.

Correction: A lesson in caution: the wavy signal looks like harmonic tremor, but it must have been a seismic echo from Indonesia. All stations in the network recorded it at similar strength, indicating it was not a local event. I should have looked at them before jumping on the “harmonic tremor” notion. This was a false alarm — and the Yellowstone swarm has been alarming enough as it is.