Harmonic Tremors are Really Cool

Those of us who celebrate the earth and its processes are having a real good time in Iceland (late November 2023). Please don’t think me insensitive: while the situation is surely fraught with peril and disruptive for the residents of Grindavik, those of us living half a circumference away are learning ever more about how the earth does what it is compelled to do. (Click here for an earlier post about the 6th Law of GeoFantasy.)

We’ve already covered some of the basics that relate to volcanoes and eruptive characteristics; including a handful of similarities and a world of differences:

Click here for an index to blog posts related to volcanoes.

Click here for an Introduction to Volcanism.

Click here for a discussion of mafic vs. felsic igneous compositions (and follow the links).

Click here for an index to blog posts related to earthquakes.

Iceland sits smack-dab on the Mid-Atlantic Ridge — a major plate boundary where the earth’s crust is separating (called a Zone of Divergence), with Europe and Africa going one way, the Americas going the other, and the widening Atlantic Ocean forming between these drifting continents. Click here for a brief overview of global tectonics (and, again, follow the links as needed).

What is happening now in Iceland is additional evidence that the currently accepted tectonic models are, for the most part, on the right track. I could go on and on about the things we are learning and the concepts that are being confirmed, but I’d like to focus this post on a single volcanic mechanism that is near and dear to my comfort zone: Harmonic Tremors.

(Before we start: One of the givens in the earth sciences — and one of the few factoids that nearly everyone would agree with — is that while it’s feasible to break a solid, it is essentially impossible to break a liquid (you can shatter ice but not water). This is why most earthquakes happen at relatively shallow depths, where the lithosphere is somewhat cooler and the rocks more brittle. Click here for an Introduction to Earthquakes, and the Brittle-Ductile Transition Zone.)

I first woke up to harmonic tremors — and how they relate to impending volcanic eruptions — in 1980 during the ramp-up to the explosive eruption of Mt. St. Helens in southwestern Washington. One of the major peaks of the Cascade Mountain Range and sitting above the Cascadia Subduction Zone, the reality that it was (is) an active volcano should have come as no surprise.

But in large measure it did! When I took my undergraduate courses in the early 1970s, the paradigm shift of plate tectonics was just beginning to gain traction. When discussing the volcanic peaks that form the High Cascades, I vividly remember Dr. Leper telling us newbies that Mt. Lassen was active (it had last erupted in 1917, with active hydrothermal vents still stinking up the environment at the Sulfur Works and Bumpass Hell), Shasta was dormant (he cited reports of a possible small eruption in 1786), and everything north of California (including St. Helens) was extinct. Oops! (I promise a more detailed look at St. Helens in a future post.)

Anyway, the build-up to the main event (18 May 1980) went on for several months. The preliminary festivities included numerous minor ventings of steam and ash as the volcano cleared its throat, as well as literally thousands of small earthquakes, most of them less than three on the Richter scale (click here for an earlier post about how the Richter Scale of Earthquake Magnitude works).

This type of earthquake activity was noted at many places, and by 1985 — after the massive eruption and lahar (volcanic mudflow) of Nevado del Ruiz in Columbia buried the town of Armero, killing greater than 23,000 residents — volcanologists with the United States Geological Survey reportedly first started calling them “harmonic tremors”.

But no matter where the volcano occurs, the precursor quakes are generally of low magnitude and relatively shallow. The weird thing, however, is that over time the focus of the quivers often seems to move upwards in the crust… and directly below the summit of the volcano.

So how does this relate to harmonic tremors and volcanoes? Well, as usual it’s pretty simple. At some depth below every active volcano is a magical place called a magma chamber. This is where the lava lives, and while the magma chamber may start at depth within the lithosphere, in order to erupt at the surface it has to move upwards through the crust. While in motion, the stresses shatter the surrounding rock, leading to swarms of small mini-quakes.

We may not be able to feel them here on the surface, but the sensitive seismographs that monitor such tectonic insolence surely can detect them, and they always indicate the same thing: magma is in motion and striving to become lava. (Don’t sweat the terminology: lava is just magma that made it the whole way to the top and has erupted from a volcanic vent.)

Put it all together and it’s pretty obvious: the magma chamber is where the earthquakes are not happening. And over time, as the magma moves ever closer to the surface and the eruption looms, so does the “hole” in the earthquake pattern also move upward. It is literally possible to chart the depth of the magma chamber — and therefore how close the volcano may be to erupting — by mapping where the harmonic tremors are not.

So how does this relate to Iceland today? Well, I have yet to see anyone call them harmonic tremors, but the thousands of small, shallow earthquakes beneath the southwest corner of the island sure seem to check all of the critical boxes, at least to my way of thinking.

And while the frequency of the tremors seems to have dropped a bit over the past several days, it may be important to note that this is an active plate boundary, and even if the activity of the past month doesn’t lead to an imminent eruption, one will surely come in the N.T.D. future.

Like I said at the top: pretty cool, huh?