What Causes an Earthquake?

Imagine you're sitting quietly when the floor suddenly begins to shake. A glass rattles on a shelf. A hanging light swings from side to side. Outside, the ground itself seems to be moving.

We usually think of the ground beneath our feet as completely solid and still.

But Earth is not as still as it looks.

Far below us, enormous pieces of Earth's outer layer are moving incredibly slowly. Most of the time, we don't notice them at all. Sometimes, however, pressure builds between rocks until they suddenly move.

The result is an earthquake.

Earth Has Layers

To understand earthquakes, we first need to look inside our planet.

Earth has several major layers. At the centre is the inner core, surrounded by the outer core. Above that is the enormous rocky mantle, and at the surface is the thin crust.

The crust and the rigid uppermost part of the mantle together form a layer called the lithosphere.

But the lithosphere isn't one unbroken shell.

It's divided into huge pieces called tectonic plates.

These plates fit together across Earth's surface rather like the pieces of an enormous, uneven jigsaw puzzle. Continents and oceans sit on top of them.

And the plates are moving.

How Can Solid Ground Move?

Tectonic plates move extremely slowly, usually only a few centimetres each year.

That's roughly comparable to the speed at which your fingernails grow.

It doesn't sound very impressive, but tectonic plates are enormous. When pieces of rock thousands of kilometres across slowly push, pull or slide past one another, tremendous forces can build.

The edges where tectonic plates meet are called plate boundaries.

Some plates move apart.

Others move towards each other.

Some slide sideways past one another.

Many earthquakes occur around these boundaries, although earthquakes can also happen within a tectonic plate.

Rocks Can Get Stuck

If tectonic plates are always moving, why isn't the ground shaking constantly?

Because rocks don't always slide smoothly past one another.

Their surfaces can be rough and irregular. Along cracks in Earth's crust called faults, sections of rock can become stuck because of friction.

The plates continue trying to move.

The rocks resist.

Stress gradually builds.

For years, decades or sometimes much longer, the rocks may bend or deform slightly as more energy is stored.

Eventually, the stress becomes greater than the friction holding the rocks in place.

Then something dramatic happens.

The rocks suddenly slip.

Energy that had been stored in the stressed rock is released, and the surrounding ground begins to shake.

That's an earthquake.

Imagine Bending a Ruler

Here's a simple way to picture the idea.

Hold a flexible plastic ruler at both ends and gently bend it.

At first, the ruler bends while you add more force. Energy is being stored in it.

Keep bending it and eventually it may suddenly spring out of your hands or snap back towards its original shape.

The movement happens very quickly.

Rock deep underground can behave in a somewhat similar way. Tectonic forces slowly deform rock and build stress. When the rock finally slips along a fault, some of that stored energy is suddenly released.

An earthquake may last only seconds, but the forces that produced it could have been building for many years.

Where Does an Earthquake Begin?

The underground point where the rock first begins to rupture is called the focus, or hypocentre.

From there, energy travels outward through Earth as vibrations called seismic waves.

The point on Earth's surface directly above the focus is called the epicentre.

People sometimes describe the epicentre as where an earthquake "started", but the actual rupture begins underground at the focus.

Seismic waves spread away from the earthquake, carrying energy through the ground.

These waves are what make buildings shake and the ground move.

Are There Different Kinds of Seismic Waves?

Yes.

Scientists study several types of seismic waves.

P-waves, or primary waves, are the fastest. They squeeze and stretch material as they travel and are usually the first waves detected after an earthquake.

S-waves, or secondary waves, arrive later and move the ground differently. Unlike P-waves, they cannot travel through liquids.

Other waves travel along Earth's surface. These surface waves can create strong rolling or side-to-side movements and may cause considerable damage near populated areas.

By studying when different seismic waves arrive, scientists can learn where an earthquake occurred and even gather clues about the structure deep inside Earth.

Why Do Some Earthquakes Cause More Damage?

Earthquakes vary enormously.

Some are so small that people never notice them. Sensitive instruments called seismometers may be the only things that detect them.

Others release huge amounts of energy.

The damage caused by an earthquake depends on more than its magnitude. It can also depend on how deep the earthquake is, how far away people are, the type of ground beneath buildings and how those buildings were constructed.

Soft sediments can sometimes amplify shaking, while well-designed earthquake-resistant buildings can greatly reduce the danger.

An earthquake beneath the ocean can also sometimes move the seafloor suddenly enough to create a tsunami, sending powerful waves across the ocean.

What Are Aftershocks?

A large earthquake changes the stresses in the rocks around a fault.

After the main earthquake, nearby rocks continue adjusting to their new positions. This can produce additional earthquakes called aftershocks.

Some are tiny. Others can be strong enough to cause further damage.

Aftershocks may continue for days, weeks, months or even longer after a major earthquake, generally becoming less frequent as time passes.

Can Scientists Predict Earthquakes?

Scientists know a great deal about where earthquakes are likely to happen.

They can map faults, measure plate movements, examine past earthquakes and calculate the probability of future earthquakes in particular regions.

What they cannot currently do is reliably predict the exact day, location and magnitude of a future earthquake.

Instead, earthquake science helps communities understand their risk and prepare for future shaking.

Engineers can design stronger buildings. Emergency services can plan responses. In some regions, earthquake early-warning systems can detect an earthquake after it begins and send alerts before the strongest shaking reaches places farther away.

That isn't prediction, but even a few seconds of warning can sometimes be useful.

Earth Is Always Changing

Earthquakes can be frightening, but they are also evidence that our planet is geologically active.

The same movement of tectonic plates that produces many earthquakes also helps create mountains, ocean trenches, volcanoes and new sections of Earth's crust.

The continents beneath our feet aren't fixed forever. They are travelling extremely slowly across the planet.

Most of that movement happens far too gradually for us to notice.

But when rocks become locked along a fault, stress can build until they suddenly slip and release energy as seismic waves.

So what causes an earthquake?

An earthquake happens when stressed rock suddenly moves along a fault, releasing stored energy that travels through Earth and shakes the ground.

The ground beneath us may feel permanent and motionless, but on a geological timescale, we're living on the moving pieces of an extraordinary planet.

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