Harmonic tremors and spectrograms

I've been following Mary Greeley's YouTube channel for some time and I finally came around to look a little closer into those mysterious spectrograms she's parading in her videos.

The source of those is Unavco, or more precisely this webpage here: https://www.unavco.org/instrumentation/networks/status/pbo/bsm, where bsm stands for borehole strain meter, which is an instrument that measures the deformation of the borehole due to whatever forces.

More precisely four data streams are produced

• CH0: raw strain,
• CH1: processed strain,
• CH2: environmental pressure and
• CH3: pore pressure.

I don't know exactly what those are, but there seems to hold a linear relation

CH0 = a CH2 + b CH3, where a, b are real numbers which vary from borehole to borehole.

Anyway, so you get a data plot like the one from yesterday for Anza, California,

and by the power of short interval Fourier transformation, the like WinAmp uses to visualise the frequencies occurring in the ballet (Coppélia) I'm listening to right now, you get a spectrogram like the following one, where dark blue denotes Fourier coefficients close to 0 and violet Fourier coefficients close to 1, never mind the actual normalisation.

I take it you notice the little red quarter circle around 18:30 universal time. Now, this Mary would call magma intruding and the funny thing is that it actually might be so, although not in a sense quite as direct as Mary suggests. Because what the quarter circle actually signifies is that around 18:30 there started a harmonic oscillation of a low frequency, i.e. around 0.05 Hz.

To understand this better, let us look at the borehole at Yellowstone Lake. First the spectrogram

and now the actual seismogram from https://www.isthisthingon.org/Yellowstone/daythumbs.php

See that (in the line with 19:00 on the right)? [Sorry, couldn't help it.]

Well, that is a harmonic oscillation for good a half of an hour. So what this says is that the strain in the borehole oscillates just like the needle of the seismograph. It also says, of course, that this oscillation isn't a local one at Yellowstone. In fact it shows on all strainmeters that Unavco operates, including this one in Bamfield, British Columbia.

Somewhat mysterious perhaps, what would let the whole continent vibrate from 18:25 to 19:00 UTC yesterday?, but there it is.

Postscript from March 27, 2018. It has happened again, yesterday from 10:30 to 11:00 UTC. The spectrogram from Yellowstone Lake

There can be no question now that the source of these vibrations is the New Ireland earthquake from March 8, 17:39:50 UTC, and the New Britain earthquake from March 26, 9:51:00 UTC. I do find it slightly peculiar though that the waveform should be transmitted as is. A muffling effect? A low-pass filter? Funny enough that you can identify earthquakes from New Guinea in these spectrograms.