When the ground
shakes.
who asks the hard questions.
Earthquakes happen at plate boundaries.
You already know the great secret of the Earth: its surface is cracked into enormous slabs of rock — the tectonic plates — and they are slowly, endlessly moving. An earthquake is what happens where two of these plates meet, at a plate boundary. Almost every earthquake on the planet happens along these lines, which is why a world map of earthquakes traces the edges of the plates exactly.
The plates do not slide past each other smoothly. Their rough edges lock together and stick, even as the rest of the plate keeps trying to move. The rock bends and strains, storing up energy like a bent ruler held under your thumb — for years, even centuries. Then, in an instant, the rock can take no more. The edges snap free, lurch past each other, and all that stored energy is released at once, shaking the ground in every direction. That sudden release is the earthquake.
This builds straight on the plate tectonics lesson. The plates meet in three ways: pulling apart, pushing together, or sliding past one another. All three can cause earthquakes, but the sliding kind — like California's San Andreas Fault — is famous for them, because the plates grind past each other and snag.
There are around half a million detectable earthquakes on Earth every year — more than one a minute. The vast majority are far too small to feel, picked up only by sensitive instruments. Only a hundred or so each year do real damage.
The focus, the epicentre, and the waves.
An earthquake starts underground. The exact point beneath the surface where the rock snaps and the energy is released is called the focus. The point on the surface directly above the focus is the epicentre — and because it is closest to where the energy came out, the epicentre is usually where the shaking is strongest and the damage worst.
From the focus, the energy travels outwards in all directions as seismic waves — the same kind of ripples you'd get from dropping a stone in a pond, but moving through solid rock. The waves spread out, lose strength with distance, and shake everything they pass through. The closer you are to the epicentre, the more violent the shaking.
Focus and epicentre — where an earthquake begins
The focus is underground — the place where it all starts. The epicentre is on the surface, straight above. Think of a firework: the focus is where it bursts, the epicentre is the spot on the ground right below the bang.
How we measure an earthquake.
We catch seismic waves with an instrument called a seismometer, which draws a wobbling line — a seismograph — that gets wilder the stronger the shaking. From that record, we work out the earthquake's magnitude: a single number for how much energy it released. You may have heard of the Richter scale; today scientists usually use a more accurate version called the moment magnitude scale, but the idea is the same — a bigger number means a bigger quake.
A seismograph — the line a quake draws
The magnitude scale has a sting in the tail: it is logarithmic. Each step up the scale is not a little bit bigger — it is about thirty times more energy released. A magnitude 7 quake is not slightly worse than a magnitude 6; it unleashes roughly thirty times the energy. That is why the difference between a 6 and an 8 is the difference between a serious quake and a catastrophe.
The most powerful earthquake ever recorded was the magnitude 9.5 quake in Chile in 1960. It released more energy than a thousand atomic bombs, set off a tsunami that crossed the whole Pacific Ocean, and was so vast it made the entire planet ring like a bell for days afterwards.
Primary effects, and secondary effects.
Geographers split the effects of an earthquake into two kinds, and the difference matters. Primary effects happen the instant the ground shakes — they are caused directly by the shaking itself. Secondary effects come afterwards — they are knock-on consequences, set off by the primary effects. Often it is the secondary effects that cause the most harm of all.
Primary effects
Caused directly by the shaking, in the moment:
- Buildings, bridges and roads collapse
- People are killed or injured by falling rubble
- Gas, water and power lines are torn apart
- The ground itself cracks and splits open
Secondary effects
Knock-on consequences, in the hours and days after:
- Fires, from broken gas pipes and fallen cables
- Tsunamis, if the quake is under the sea
- Landslides on shaken, unstable slopes
- Disease, as clean water and sewers fail
- Homelessness, hunger, and a halted economy
When a large earthquake strikes the sea floor, it can shove a whole column of water upwards and send out a tsunami — a wave that can cross an ocean and rise into a wall of water at the coast. The 2004 Indian Ocean tsunami, set off by an undersea quake, killed more people than the shaking ever could have on its own.
Why a quake is far deadlier in a poorer country.
Here is one of the most important ideas in all of geography. Two earthquakes of the exactly the same magnitude can have wildly different outcomes — one might kill a handful of people, another tens of thousands. The Earth shakes the same. What differs is the country it shakes, and above all, how rich or poor that country is.
A wealthier country can afford to prepare. Its buildings are designed to flex and survive. It has the money for warning systems, trained rescue teams, hospitals with supplies, and a swift, organised response. A poorer country often cannot. Buildings are weaker and packed close together. There is little spare money for defences. And when the quake hits, hospitals are overwhelmed, roads are blocked, and help is slow to arrive — so the secondary effects, the disease and the hunger, take a terrible toll.
In 2010, a magnitude 7.0 earthquake struck Haiti, one of the poorest countries in the world. Around 220,000 people died. The very same year, a much larger magnitude 8.8 quake — releasing hundreds of times more energy — struck Chile, a wealthier country with strict building rules. Around 500 people died. The bigger quake killed far fewer people, because the country was ready for it.
Japan, which sits on the meeting point of four plates, has some of the most earthquake-ready buildings on Earth. Many of its skyscrapers rest on giant shock absorbers and rubber bearings that let the building sway gently instead of cracking — the same idea as a tree bending in a storm rather than snapping.
How people prepare and respond.
You cannot stop an earthquake, and you cannot reliably predict the day one will strike. But people who live on plate boundaries are far from helpless. The work falls into two parts: preparing before a quake ever comes, and responding well in the hours after one does.
Build to survive
Design buildings that flex and sway — deep foundations, cross- bracing, and shock absorbers — so they ride out the shaking instead of collapsing.
Plan and practise
Hold earthquake drills so people know to “drop, cover and hold on”. Keep emergency kits with water, food and torches ready at home.
Warn and monitor
Networks of seismometers can give cities a precious few seconds of warning before the strongest waves arrive — enough to stop trains and shut off gas.
Respond fast
Trained search-and-rescue teams, field hospitals, clean water and shelter, brought in quickly, save lives in the days after — when the secondary effects bite.
Seismic waves travel fast, but not instantly. A warning system can detect the first, gentler waves and send an alert that outruns the slower, more destructive ones. Even ten seconds is enough to stop a high-speed train, halt a surgeon's hand, and let a child get under a desk. It is not much — but it is the difference between being caught and being ready.
Where do almost all earthquakes happen?
Focus or epicentre?
How big is a step up the scale?
The waves from a very large earthquake can travel right through the centre of the Earth and be detected on the far side of the planet. It is by reading these waves that scientists first worked out that the Earth has a liquid outer core — the waves bend as they pass through it.
Primary or secondary?
Why so different?
Name the point.
Fresh one. The instrument that catches seismic waves and draws a wobbling line that gets wilder the stronger the shaking — what is it called?
Name the parts of a quake.
Four parts of this earthquake are numbered. Pick a chip from the pool, then tap the number it belongs to. Two of the chips don't belong on this diagram at all — read each one before you place it.
Match each label to its pin
Explain why.
This is the kind of question that rewards you generously when you write in full sentences and develop your points — you say something, then you say why, then you say so what. Aim for 100–150 words. No bullet points.
Explain why an earthquake of the same magnitude can be far more deadly in a poorer country than in a wealthier one.
Use the words buildings, prepare and response. Give a reason for each — and a real example earns marks.
A few thoughts on your answer, Florence
strong You went straight to the buildings, which is the heart of this question — earthquakes don't kill people, falling buildings do, and you said why a stronger building survives. That cause-and-effect is exactly the move a marker is looking for.
try this You mentioned the response, but it stops a little short. Carry it on: in a poorer country, the hospitals are overwhelmed and the roads are blocked, so help is slow — and that is when the secondary effects, the disease and hunger, take their toll. The second half of the sentence is where the mark lives.
to add One real pair of examples would lift this answer. Haiti and Chile in 2010 — the smaller quake killing far more — is the perfect proof of your whole argument. Name them, and the marker sees you can apply the idea, not just state it.
Worth watching.
Two short films to watch alongside today's lesson — each shows you something the words and pictures can't.
The words from today.
- Plate boundary
- The line where two of the Earth's tectonic plates meet — where almost all earthquakes happen.
- Focus
- The point beneath the surface where the rock snaps and the earthquake's energy is first released.
- Epicentre
- The point on the surface directly above the focus, usually where the shaking is strongest.
- Magnitude
- A single number for the energy an earthquake releases; each step up the scale is about thirty times more energy.
- Primary effect
- Damage caused directly by the shaking — collapsed buildings, broken pipes, cracked ground.
- Secondary effect
- A knock-on consequence after the shaking — fires, tsunamis, landslides, disease and hunger.
You understand the shaking.
You know now why the ground moves — plates straining and snapping at their boundaries — and you can name the focus, the epicentre, and the waves that race out from them. You can read how an earthquake is measured, tell a primary effect from a secondary one, and explain the hardest truth of all: that the same quake is far deadlier where people are poorer and less prepared. That last idea, Florence, is geography at its most human.
Big earthquakes can very slightly change the length of a day. The 2011 Japan quake shifted so much of the planet's mass that it sped up the Earth's spin by a tiny fraction — shortening every day since by about 1.8 millionths of a second.
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