The Earth's outer shell is cracked into giant plates of rock, and they're always trying to slide past each other — really slowly, slower than your fingernails grow. But where two plates meet, friction can lock them in place. So instead of sliding, the rock just bends and bends, storing up energy like a stretched rubber band, for years. Then one day it can't hold any longer, the rock snaps free all at once, and the ground shakes. Build up the strain in the simulator and trigger the slip yourself.
Most people think an earthquake is the ground suddenly deciding to move with no cause. In fact the cause builds silently for years to centuries as a locked fault bends and stores strain, then releases it all at once in seconds, which is exactly why the timing is so hard to forecast.
What's actually happening
It's tempting to imagine an earthquake as the ground suddenly deciding to move, an event with no warning and no cause. In fact the cause has been building, silently, for a very long time. The Earth's crust is broken into huge plates that creep across the surface at about the speed your fingernails grow — a few centimetres a year. Where two plates meet, they don't always glide smoothly past each other. Friction can weld them together along the fault, and that's where the trouble starts.
While the fault is locked, the plates keep being pushed from behind. The rock can't slide, so it does the only thing it can: it bends, storing the strain like a steel ruler you're slowly flexing. This goes on for years, decades, sometimes centuries — the rock quietly loading up with energy, the surface barely moving, everything looking calm. Then the stress finally exceeds what the friction can hold. The fault gives way all at once, the bent rock snaps straight, and the two sides lurch into the position they should have reached gradually. That sudden snap, geologists call it elastic rebound, flings the stored energy outward as seismic waves, and those waves are the shaking you feel.
The cruel part is how the energy scales. The magnitude scale is logarithmic, so the numbers hide how violent the jumps are. Each step up the scale is about 32 times more energy released: a magnitude 7 isn't a bit stronger than a magnitude 5, it's around a thousand times more energetic. That's why a small quake is a curiosity and a large one levels cities, even though they sit only two numbers apart. And it's why prediction is so hard: the years of strain build invisibly, and the release, when it comes, is over in seconds.
The magnitude scale is logarithmic, so each step up is about 32 times the energy, which is why a quake two numbers larger levels cities.
- 1Put a book on a table and loop a rubber band from the book to your finger.
- 2Pull your finger away slowly and steadily. The band stretches and stretches while the book stays stuck — that's strain building on a locked fault.
- 3Keep pulling: the book suddenly jerks forward in one quick slip. You've just felt elastic rebound — slow loading, sudden release.
Common questions
The magnitude scale is logarithmic, so each whole step up releases about 32 times more energy. A magnitude 7 is not slightly stronger than a magnitude 5 but around a thousand times more energetic, which is why the headline number is so deceiving.
The strain builds invisibly. A fault can sit locked and loading for centuries with the surface barely moving and no visible sign, and then the release is over in seconds, so the timing of the eventual slip is extremely hard to forecast.
It is the sudden snap when a locked fault finally gives way. The bent rock springs straight, lurching the two sides into the position they should have reached gradually, and that release flings the stored energy outward as the shaking you feel.