From Trapped Winds to Springy Rock: How We Learned What Shakes the Ground

A few years ago I was standing in my kitchen, waiting for the kettle, when the floor gave a short, sharp thump. A second or two later the cupboard doors began to sway and the spoons chimed in the mug. It was a small tremor, nothing to worry about, and it was over almost as soon as I'd noticed it. But I stood there holding a tea bag and wondering why it had come in two parts. A thump, a gap, then a sway.

A neighbour who knows far more about this than I do explained it over the fence a few days later. The thump and the sway were different kinds of wave, travelling at different speeds from the same source. I've been following that thread ever since, and this article is where it has led me so far.

The short answer to the title is this. An earthquake is rock that has been slowly squeezed or sheared for years, centuries or longer suddenly slipping along a crack called a fault. The slip sets off waves in the ground, and those waves are what we feel as shaking. Everything else is detail, but the detail is where the interesting part lives, and so is the history. People got to this simple picture by a long and winding route.

What did people think made the ground shake?

For most of recorded history, the explanations were guesses made with the best ideas available. Aristotle, writing in ancient Greece, thought that winds trapped in caverns under the earth were forcing their way out and shaking the ground. It's a sensible idea if you've watched a lid rattle on a pot. Versions of it lasted a very long time.

A real shift came in the eighteenth century. After a great earthquake in 1755 that was felt across a wide area of Europe, an English clergyman and natural philosopher named John Michell set out to think carefully about what had happened. In 1760 he published a paper arguing that earthquakes were waves moving through the rock itself, a bit like ripples running along a shaken carpet. He also saw that if the waves took time to travel, then the time they reached different places could, in principle, point back to where they started.

Michell was right about the waves and wrong about the cause. He blamed steam produced when water met underground fires. I find that comforting rather than embarrassing. His careful reasoning about how the shaking travels was sound, even though his idea about what starts it was not. Both turned out to be separate puzzles, and it took until the twentieth century to put the second one right.

In the nineteenth century, an Irish engineer named Robert Mallet took up the study in a more systematic way. He gave us the word "seismology", and he tried to measure how fast shaking moves through rock. Then, in the 1880s, John Milne and some colleagues working in Japan built and used instruments that recorded ground motion as a wiggling line on paper. Once you can see the shaking drawn out in time, you can start to measure it.

How does stress build up along a fault?

The answer that finally clicked came from looking at the ground after a big earthquake. In 1906, a large earthquake along the San Andreas Fault in California left fences, roads and field boundaries visibly broken and shifted sideways, in some places by several metres. A geophysicist named Harry Fielding Reid studied this, along with land surveys carried out in the same region decades earlier. Surveyors had fixed the positions of marker points on either side of the fault, then measured them again later.

What Reid saw in the comparison was this. In the years before the earthquake, the land on opposite sides of the fault had been creeping in opposite directions, but the fault itself hadn't moved. The rock near it was being bent. Then the fault slipped, and the bent rock sprang back toward a relaxed shape. He called this elastic rebound. Reid wasn't the only person thinking along these lines, but his name is the one most tied to it, because he laid out the whole cycle so clearly.

You can feel the idea with things from around the house. Tie a stretchy band or a thick rubber band to a heavy book on a table, and pull the other end very slowly. For a while, nothing happens except that the band stretches. The friction between book and table is holding the book still, and the band is quietly storing energy. Then, suddenly, the book lurches forward and the band goes slack. That cycle of sticking, stretching and slipping is called stick-slip, and it's a good small model of a fault. (It's a model, not the real thing. The real fault is kilometres of rock, and it doesn't slip as neatly. But it's the right first picture.)

The big question Reid couldn't answer was why the land was being pulled in the first place. For that, people needed plate tectonics, which became widely accepted in the 1960s. The idea is that Earth's outer shell is broken into large plates that move slowly, at roughly the speed fingernails grow. Where two plates press or slide past each other, the rocks at their edges get stuck, and the strain builds.

New Zealand sits on one of these edges. The Pacific and Australian plates meet here, and a good share of the motion along the South Island is taken up by the Alpine Fault, where the rock slides sideways and is also pushed upward, which is part of why the Southern Alps exist. Scientists have found evidence in the layers of the ground that this fault has ruptured many times before. The slow squeeze never stops, which is why stress keeps accumulating.

What happens in the seconds when a fault slips?

Rock is not very stretchy, but it doesn't have to be. Over a long distance and a long time, even stiff rock can store a lot of elastic energy, just as a long steel ruler can bend a little under a steady push.

A fault doesn't slip all at once along its whole length. It starts at one small spot deep underground. That spot is called the hypocentre (or focus). The point on the surface directly above it is the epicentre. People often treat the epicentre as "where the earthquake is", but it's only the surface address of where it started.

From the hypocentre, the slipping spreads outward across the fault surface, a bit like a tear running through fabric. This rupture moves at roughly two to three kilometres a second, which is fast, but not instant. That has a useful consequence. A small earthquake finishes in a blink because only a small patch of fault slips. A large one can last half a minute or more, because the rupture has to travel along tens or hundreds of kilometres of fault. A hundred kilometres at about three kilometres a second is roughly 33 seconds.

Here is a misconception worth clearing up. Many of us picture an earthquake as the ground splitting open. In reality, the fault is a place where two blocks of rock slide past each other, and in most earthquakes the slip never breaks the surface at all. What you feel as shaking isn't the fault itself. It's the energy that escapes from it. Only part of the stored energy becomes waves. A good deal goes into breaking rock and warming the fault.

Why do you feel a thump first, then a sway?

Now back to my kitchen. The sudden slip pushes and shears the rock around it, and that disturbance spreads outward as waves. There are three main kinds, and they travel at different speeds.

• P waves (the "primary" waves) are compression waves. The rock is squeezed and stretched along the direction the wave is travelling, the same way sound moves through air. They are the fastest, and they arrive first as a sharp thump or jolt.
• S waves (the "secondary" waves) are shear waves. The rock is shaken from side to side, at right angles to the direction of travel, like a rope flicked sideways. They arrive second, and they tend to be the strong rolling or swaying part.
• Surface waves travel along the top of the ground, and they are slower still. They arrive last and often shake the ground for longest.

Why is one faster? It comes down to what the rock has to do to carry the wave. A P wave involves squeezing the rock, and rock resists being squeezed and resists being sheared, so it springs back strongly and quickly. An S wave involves only shearing, and rock springs back less stiffly in that mode. Stiffer springing back means a faster wave. In the standard equations, P speed depends on the rock's stiffness against squeezing and shearing combined, while S speed depends only on its stiffness against shearing, and both are divided by density. In ordinary crustal rock, S waves travel at a little over half the speed of P waves.

A second consequence is worth noticing. Liquids can be squeezed but they cannot be sheared in this way, so S waves cannot travel through liquid. That fact turns out to be very useful, as we'll see.

Surface waves are the reason shaking often feels like it builds and lingers. They are trapped near the top of the ground, so their energy isn't spread out in all directions the way a P or S wave's is. They're slow and long, and they tend to have the biggest motion.

How does the gap between the waves tell you where the earthquake was?

This is the part that made me sit up as a reader of history, because it's Michell's old idea made precise. The P wave outruns the S wave, so the farther away the earthquake, the bigger the delay between them. The gap is a ruler.

Here is a worked example with round numbers. Typical speeds in the crust are around 6.0 km/s for P waves and 3.5 km/s for S waves. They vary from place to place, so treat these as illustrative, not exact.

Suppose the earthquake is 100 kilometres away.

• P wave travel time: 100 ÷ 6.0 ≈ 16.7 seconds
• S wave travel time: 100 ÷ 3.5 ≈ 28.6 seconds
• Gap between them: about 11.9 seconds

Turn it round, and you can estimate distance from the gap. Each second of delay corresponds to about 8.4 kilometres, because (6.0 × 3.5) ÷ (6.0 − 3.5) = 8.4. So a gap of 12 seconds suggests roughly 100 kilometres, and a gap of 6 seconds suggests roughly 50 kilometres.

One instrument gives you a distance but not a direction. It tells you the earthquake lies somewhere on a circle around the instrument. Draw circles from three or more instruments and they meet near one place, and that's the source. It's the same logic as finding your position from distances to three landmarks. Modern networks do this automatically, in seconds.

The person who first read these wave types clearly in the wiggles of a seismograph was R. D. Oldham, around 1900. He saw that distant earthquake records showed an early set of wiggles, then a later set, then a long, large set, matching P, S and surface waves. Before that, the lines on paper were a smudge of shaking. Afterwards, they were a message with a structure.

What did earthquake waves reveal about Earth's inside?

This is my favourite twist. Having learned to read the waves, scientists realised they were also a kind of torch shone through the planet. Nobody can drill to the centre of the Earth, but earthquake waves go there for free.

In 1906, Oldham found that waves from distant earthquakes behaved as though something at the centre of the Earth was slowing or bending them. He argued there was a core. Later work showed that S waves don't make it through the outer part of that core. Since S waves can't cross liquid, the outer core must be liquid. In 1936, a Danish seismologist named Inge Lehmann noticed faint P waves that appeared where theory said there should be none, and concluded that inside the liquid there is a solid inner core.

I find this astonishing. The same physics that explains the thump and sway in my kitchen, a push-pull wave and a side-to-side wave, was used to find a molten layer and a solid centre thousands of kilometres below anyone's boots. The outer core begins about 2,900 kilometres down.

Does a bigger number mean more shaking?

Two more bits of care are worth taking, because the words get mixed up.

Magnitude describes how much energy the earthquake released at the source. In 1935, Charles Richter devised a scale for this, working with Beno Gutenberg, and today scientists mostly use a refined version called moment magnitude. The scale is logarithmic. Each whole step up means the recorded ground motion is about ten times larger, and the energy released is roughly 32 times greater.

Intensity describes how strongly the shaking was felt at a particular place. It depends on distance, depth, and the ground you're standing on. Soft, loose ground can shake longer and harder than solid rock a short way off. So a single earthquake has one magnitude but many intensities. If a friend two suburbs away says it felt much stronger than it did for you, they are probably not exaggerating.

Can scientists predict when an earthquake will happen?

Not the exact timing, no. Despite everything above, this is where I like to be honest. Reid's picture of stress building and then releasing suggests a cycle, and for some faults scientists can estimate that large earthquakes tend to recur over spans of decades to centuries. But real faults are not clocks. The strain is spread unevenly, the rock is a patchwork, and the point at which a fault finally gives way depends on details that we can't measure deep underground.

What scientists can do is describe the odds. They can say how likely a damaging earthquake is in a region over a span of years, and they can map where the ground will probably shake hardest. Those are useful, and they help people decide how to build and prepare. But "next Tuesday at three" is beyond us. Researchers are still working out whether there are any reliable warning signs, and I'm still learning how much is known and how much isn't. I'd rather say that than pretend otherwise.

If you'd like to understand your own patch of ground better, one practical step is to look up the faults near where you live, which regional geological surveys map in plain language. Knowing the neighbourhood's geology is a gentle way to turn a surprising thump into something you recognise.

Something to wonder about

What I keep coming back to is how quiet the whole process is until the last moment. Beneath the lawn, the road and the kitchen floor, rock is bending, very slowly, and has been for longer than anyone has been counting. It stores that bending like a held breath. Then a few seconds of slip send out waves that are fast enough to outrun each other, and by the time a mug chimes on a shelf, the pattern of those waves can say how far away it began.

And here is the question I can't quite put down. Michell worked out in 1760 that the shaking travels, and it took people the best part of two centuries to learn why it starts. What is the part of the story that we are still holding the wrong way round today? I don't know, but I suspect that someone, somewhere, with a patient instrument and a good question, is about to find out.