Learning more about seismology and volcanos now that I live close to two of them.

A harmonic tremor is a sustained release of seismic and infrasonic energy typically associated with the underground movement of magma, the venting of volcanic gases from magma, or both. It is a long-duration release of seismic energy, with distinct spectral lines, that often precedes or accompanies a volcanic eruption.

You mean, like this:

The above is the last 24 hours at the seismograph just to the north west of Lassen, which erupted in 1915.

Or this:

On the northwest flank of Shasta this has been going on since I started looking last year.  Often, those burps go red on the graph because of the size of the movement.  I got a geologist once to tell me straight out that those bumps are likely magma moving through a restriction.

I'd be curious to know what plans they have at the big reservoirs around that mountain should it erupt and melt one or two of the seven glaciers sitting on the slopes.   If that should happen in the winter when the 14,000 foot tall mountain is covered in snow, the flood down the Sacramento River could be biblical.

Probably won't happen in my lifetime, but for sure there's magma slurping around under both Lassen and Shasta all the time - sorta normal for those two.