What Causes Earthquakes?

Imagine sitting quietly at home when suddenly the floor begins to tremble. A glass rattles on the table, the windows shake, and the ground beneath your feet seems to move.

For a moment, it feels as though the whole world is wobbling.

But what could possibly make something as enormous and solid as Earth shake?

The answer is hidden deep beneath our feet.

Earth's surface is constantly moving, even when we can't feel it. Sometimes, enormous forces build up underground until rocks suddenly shift, releasing energy that makes the ground shake.

That's an earthquake!

Earth Isn't as Still as It Looks

When you stand outside, the ground feels solid and permanent. Mountains, roads and buildings all seem firmly fixed in place.

But Earth's outer layer is actually broken into enormous pieces called tectonic plates.

These plates are part of Earth's rigid outer shell, which includes the crust and the uppermost part of the mantle.

Imagine the cracked shell of a boiled egg. The cracks divide the shell into different pieces, even though they all fit together around the egg.

Earth's tectonic plates fit together around our planet in a somewhat similar way.

But unlike an eggshell, Earth's plates are moving.

Most travel only a few centimetres each year, roughly as fast as your fingernails grow.

That might sound incredibly slow, but the plates are enormous, and they've been moving for millions of years.

Their movement can build mountains, open oceans and trigger earthquakes.

What Makes Tectonic Plates Move?

Beneath Earth's rigid plates lies the hot, rocky mantle.

Although the mantle is mostly solid, some of its rock can slowly flow over extremely long periods.

Heat from Earth's interior helps drive this movement.

Gravity also plays an important role. Cold, dense sections of oceanic plates can sink into the mantle, pulling the rest of the plate behind them. This is called slab pull.

Together, these processes help move Earth's tectonic plates.

The plates don't all travel in the same direction.

Some move apart, others push towards each other, and some slide sideways past their neighbours.

These movements create enormous forces in the rocks.

What Happens When Rocks Get Stuck?

Here's where earthquakes begin.

Earth's crust contains fractures called faults. Along some faults, huge blocks of rock move past one another.

But rock surfaces aren't perfectly smooth.

They have bumps, cracks and rough edges that can catch against each other.

Imagine trying to slide two rough bricks past one another. They might get stuck, even while you continue pushing.

Something similar happens underground.

The tectonic plates continue moving, but sections of rock along a fault remain locked together because of friction.

As the forces build, the surrounding rocks slowly bend or deform.

Eventually, the stress becomes too great.

The rocks suddenly slip along the fault, releasing stored energy.

That energy travels through the surrounding rock, causing the ground to shake.

An earthquake may last only a few seconds, even though the forces responsible for it have been building for decades or centuries.

Imagine Bending a Ruler

Here's a simple way to understand how earthquakes release energy.

Imagine holding a flexible plastic ruler and gently bending it.

As you bend the ruler, you're storing energy inside it.

If you carefully release one end, the ruler suddenly springs back.

The energy that built up slowly is released very quickly.

Earthquakes work in a somewhat similar way.

Rocks can slowly deform as stress builds. When they suddenly slip along a fault, some of that stored energy is released as vibrations.

Scientists call this process elastic rebound.

Of course, Earth's rocks are far stronger and the forces involved are enormously greater than anything happening inside a plastic ruler!

How Does the Shaking Travel?

When an earthquake begins, energy spreads outward through Earth as vibrations called seismic waves.

There are several kinds of seismic waves.

P-waves (Primary Waves) are the fastest. They squeeze and stretch the rock as they travel, rather like pushing and pulling a spring.

S-waves (Secondary Waves) arrive later. They shake material sideways or up and down, at right angles to the direction the waves travel.

Surface waves move along Earth's surface and can produce powerful rolling or side-to-side shaking.

These waves are why an earthquake can be felt far away from the place where it began.

Scientists use sensitive instruments called seismometers to detect and record the movement.

By studying seismic waves, they can work out where earthquakes happened and learn about the hidden structure inside our planet.

Where Does an Earthquake Start?

Earthquakes usually begin underground.

The point where the rupture first starts is called the focus, or hypocentre.

The location on Earth's surface directly above it is called the epicentre.

Imagine drawing a straight line from the focus upwards to the surface. The place where that line reaches the ground is the epicentre.

Some earthquakes begin only a few kilometres underground, while others start hundreds of kilometres beneath the surface.

The depth matters because earthquakes that begin closer to the surface can sometimes cause stronger shaking nearby.

Why Are Some Earthquakes Bigger Than Others?

Earthquakes come in many sizes.

Some are so tiny that nobody notices them. Others are powerful enough to damage buildings, roads and bridges.

Scientists describe an earthquake's size using its magnitude.

A magnitude 3 earthquake is usually much smaller than a magnitude 6 earthquake.

But the numbers don't increase in a simple way.

For every whole-number increase in magnitude, an earthquake releases approximately 32 times more energy.

That means a magnitude 7 earthquake releases about 32 times as much energy as a magnitude 6 earthquake!

However, the amount of damage depends on more than magnitude.

The earthquake's depth, distance from populated areas, local ground conditions and the strength of buildings all make a difference.

Can Earthquakes Happen Under the Ocean?

Absolutely!

Many earthquakes occur beneath the seafloor, where tectonic plates meet.

Sometimes a powerful underwater earthquake suddenly lifts or lowers a large area of the ocean floor.

This can displace enormous amounts of water and create a tsunami.

Unlike ordinary waves caused by wind, tsunami waves can travel across entire oceans.

As they reach shallow coastal waters, they can grow into dangerous surges of water.

Not every underwater earthquake causes a tsunami. The movement of the seafloor, earthquake size and depth all matter.

Do Earthquakes Happen in Australia?

Yes!

Australia is located mostly within the Australian tectonic plate, away from many of the world's major plate boundaries.

But that doesn't mean Australia is earthquake-free.

Forces acting on the plate can cause old faults within the continent to move.

One well-known example is the 1989 Newcastle earthquake in New South Wales, which had a magnitude of approximately 5.6 and caused serious damage.

Earthquakes also occur in places such as Western Australia, South Australia and Victoria.

Most Australian earthquakes are relatively small, but stronger ones are possible.

Can Scientists Predict Earthquakes?

Scientists can identify areas where earthquakes are more likely to occur.

They study faults, measure plate movement and examine records of previous earthquakes.

But they cannot reliably predict exactly when and where a particular earthquake will happen or how powerful it will be.

Some regions have earthquake early-warning systems that detect an earthquake after it has begun.

Because electronic signals travel faster than seismic waves, these systems can sometimes provide a few seconds of warning before strong shaking arrives.

That isn't predicting an earthquake. It's detecting one very quickly.

What Should You Do During an Earthquake?

If an earthquake happens while you're indoors, the general safety advice is:

DROP, COVER AND HOLD ON.

• DROP onto your hands and knees so you're less likely to be knocked over.

• COVER your head and neck, preferably beneath a sturdy table or desk.

• HOLD ON to your shelter until the shaking stops.

Stay away from windows and objects that might fall. Don't run outside during strong shaking.

If you're outdoors, move away from buildings, trees and powerlines if you can do so safely.

After a strong earthquake near the coast, follow local tsunami evacuation advice.

Five Surprising Earthquake Facts

1. Earthquakes happen every day. Thousands are detected around the world, although most are too small to feel.

2. The largest earthquake ever recorded by instruments occurred in Chile in 1960. It had a magnitude of approximately 9.5.

3. Earthquakes can happen on the Moon! Scientists call them moonquakes.

4. Earthquakes can move the ground permanently. Large earthquakes sometimes shift sections of Earth's surface by several metres.

5. Earthquakes can have aftershocks. These smaller earthquakes happen as the surrounding rocks adjust after the main event.

Our Planet Is Always Changing

Earthquakes remind us that our planet is not a motionless ball of rock.

Deep underground, tectonic plates are moving, mountains are slowly forming, and enormous forces are constantly reshaping Earth's surface.

Most of these changes happen so slowly that we never notice them.

But sometimes, rocks that have been locked together for years suddenly slip, releasing energy that travels through the ground.

So what causes earthquakes?

Earthquakes happen when stress builds in rocks until they suddenly move along a fault, releasing energy as seismic waves that shake the ground.

The next time you look at a mountain, a coastline or the ground beneath your feet, remember that you're standing on a planet that's still changing, one tiny movement at a time.

Privacy Policy Terms & Conditions Refund & Digital Products Policy