Teaching Experiment: Earthquakes 1

Just as an exercise, I wanted to share some of my education in the mechanics of earthquakes. I’ve never seen it offered anywhere else this way, particularly in public education. It seems it always has big gaps in it, so no one remembers much because they really didn’t get it. Much of this is based on my research into the Bad Friday Earthquake in Alaska in 1964 (it’s a play on “Good Friday,” the day it happened). My family moved to Anchorage just a short time after that, and it intrigued me all through my adult life. I really wanted to understand the devastating aftermath that we saw when we arrived there. It turns out it was one of the best studied earthquakes ever, because it was about the first time the full array of modern equipment was available and listening.

That has to do with a little bit of history regarding nuclear weapons. This was not long after a string of treaties established a global network of seismographs so that the treaty partners could keep an eye on each other, to make sure no one was cheating on the nuclear test ban. It turns out that, not long after all that equipment was in place, one of the biggest earthquakes in history hit Alaska. It wasn’t just a shake or shock in one spot, but a vast stretch of fault-line (some 600 miles) shifted all at once, lasting four and half minutes. Somehow, the US and state governments, along with international academia, mobilized a small army researchers to go out and survey the damage with the best technology available at the time. I’ve seen the product of this research that was published; it’s a large bookshelf of volumes in loose-leaf binders that would take countless hours just to read, assuming you could understand it. I don’t believe any earthquake since then has received so much attention, in part because that one in Alaska established a new and very broad baseline against which other earthquakes are now understood.

This will be one bite at a time, building from what my experience tells me is more-or-less common knowledge. I’ll try to find images I can use, or link to, but I’ll rely more on what is likely a unique approach to explaining it. First, let’s take a quick review of a basic physics principle, because it’s critical for something later. I’m going to oversimplify, so humor me.

You’ve heard of sound waves. It’s common for them to be illustrated in profile, or as waves rippling across the surface of water. Those can be misleading, in the sense that it doesn’t get at the actual physical nature of what it is. A sound wave is just a label plastered on an event well understood in physics: it’s a compression wave. Smack two objects together. The impact will compress the molecules, at least on the surface of the two objects (depending on how hard the material is). Let’s keep this simple and pretend it’s two fist-sized stones of granite. The molecules won’t compress much; it’s movement down on a molecular level. But the rebound will affect, say the air molecules around it. They’ll be jostled by the by the impact and rebound. They will in turn jostle the adjacent molecules, and those will jostle others.

This jostling isn’t random, but takes on a pattern. It erupts outward from the source, spreading by jostling in a spherical compression wave. It’s that molecular compression and rebound propagating through the air. When the compression wave hits your eardrums, it registers as sound. Of course, in the open air, the intensity of the compression wave reduces over distance (the fancy term for that is “inverse square law” — see this for an illustration of that).

Now try the same stunt under water; pop those two rocks together. You still get a compression wave of molecules jostling each other, but now it’s liquid water instead of air. It’s a little slower. If you happen to do this with part of your torso in the water, you’ll feel it before your ears hear it as the concussion propagates out of the water up to your ears. Or, if your head is underwater, you’ll hear it when you feel it. But you’ll feel the greatest impact of this compression wave in your stomach. It may even make you briefly nauseous, because your guts don’t like that kind of thing. But your stomach is a relatively large cavity of differing density from the rest of your body, so it registers there like it does with ears in the air. The difference in the rate of travel is why it’s unpleasant.

Air and water tend to be uniform in density for these experiments. The earth’s crust is not uniform like that. What if I could somehow slam those two stones together while they are inside solid ground? Once again, don’t think of it as a ripple across the surface of water. It’s not a vertical shift; it’s a compression wave that shifts molecules by briefly compressing them. It propagates outward because they snap back and pass it on. The compression wave propagates outward in a sphere until it meets some resistance. In the ground, that typically means a change in soil composition or a body of water, or something else. The compression waves can be bounced, and can be focused by rebounding off harder materials, intensifying the effects. It’s not just a simple matter of losing intensity gradually as it would be in the air. A compression shock wave is a critical element in understand the damage earthquakes do.

About Ed Hurst

Disabled Veteran, prophet of God's Laws, Bible History teacher, wannabe writer, volunteer computer technician, cyclist, Social Science researcher
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3 Responses to Teaching Experiment: Earthquakes 1

  1. Jay DiNitto says:

    So do the compression waves lose energy by basic friction? That would explain why they travel farther through air than solids or liquids.

    Thanks for this, Ed.

    Like

  2. Ed Hurst says:

    The compression waves dissipate, losing their intensity over a growing area. But you are right to envision an underlying molecular friction. But with something on the scale of earthquakes, the vast range of frequencies and intensity sees them travel farther through the earth’s crust than they would in the air. Water is slowest because the liquid state offers a very poor conductor, a high molecular friction. Solid stone transmits very efficiently with low friction. Shock waves in the earth tend to be faster than sound in the air.

    Correction: I double checked. Compression waves travel faster in water than in the air, but their transition from water to air slows them down. It’s that shock waves from an earthquake slow down when they hit water because the are so very fast in stone — “330 m/s in air, 1450 m/s in water and about 5000 m/s in granite” (says Wikipedia).

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  3. Jay DiNitto says:

    Seems like things “change” depending on scale. Sound (or compression waves) travels better through solid rock, but banging two rocks together above ground would give the layman the impression that vibrations roam free in the air but stop (or merely bounce) when it hits solid surfaces. We’re much more exposed to the atmospheric sound, so we tend to think of sound working in a small range of ways.

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