Earthquakes and Tsunamis

When the ground itself moves

We usually think of the ground beneath us as the one thing that never moves. So an earthquake — a sudden shaking of the Earth's surface — can be frightening and surprising. Most earthquakes are small and barely noticed, but the largest ones can topple buildings, crack roads and, when they happen under the sea, set off giant waves called tsunamis.

In this lesson we will discover what causes earthquakes deep inside the Earth, how scientists measure them, how undersea quakes create tsunamis, and what people can do to stay safe.

The broken shell of the Earth

To understand earthquakes, we first have to remember what the Earth is made of. Its hard outer shell, the crust, is not one solid piece. It is cracked into about a dozen giant slabs called tectonic plates, which fit together like a cracked eggshell.

These plates are not still. They float on the hot, slowly flowing rock of the mantle beneath them and drift a few centimetres each year — about as fast as your fingernails grow. The places where two plates meet are called plate boundaries, and this is where most earthquakes happen.

How an earthquake happens

The edges of tectonic plates are rough and jagged, so as the plates try to move past each other, they often get stuck. But the forces pushing the plates do not stop. So pressure, or stress, slowly builds up in the rock, year after year, like the tension in a bent ruler or a stretched elastic band.

Eventually the stress becomes too great. The rock suddenly snaps and slips along a crack called a fault. All the energy that had been building up for years is released in just a few seconds. That sudden release is the earthquake.

Two important words describe where it happens:

• The focus (also called the hypocentre) is the exact point underground where the rock first slips.
• The epicentre is the point on the surface directly above the focus. The shaking is usually strongest here.

The deep Earth forces behind earthquakes are the same ones that build mountains and feed volcanoes, which you can explore further in volcanoes and earthquakes.

Seismic waves and how we measure them

When the rock slips, the released energy travels outward in all directions as seismic waves — vibrations that ripple through the ground, a bit like the ripples that spread when you drop a stone into a pond. These waves are what we feel as shaking.

Scientists who study earthquakes are called seismologists, and they use an instrument called a seismometer (or seismograph) to detect and record seismic waves. The wiggly line it draws shows how strong the shaking is and helps locate where the earthquake came from.

The strength, or magnitude, of an earthquake is given as a number. You may have heard of the old Richter scale; today scientists usually use the moment magnitude scale, but the idea is similar. Each step up the scale means much more energy:

• A magnitude 3 quake is barely felt.
• A magnitude 5 can rattle windows and crack plaster.
• A magnitude 7 or higher can cause serious destruction.

Importantly, each whole number up the scale releases roughly 32 times more energy than the one below, so a magnitude 8 is vastly more powerful than a magnitude 6.

How a tsunami forms

When a large earthquake happens under the sea, it can do something even more dangerous: it can create a tsunami (a Japanese word meaning "harbour wave").

Here is how. If an undersea fault suddenly lifts or drops the seafloor, it shoves a huge amount of water up or down with it. That displaced water has to go somewhere, so it spreads out across the ocean as a series of powerful waves carrying enormous energy.

A tsunami behaves very differently from an ordinary wind-made wave:

1. In the deep ocean, a tsunami may be only half a metre tall but stretches for hundreds of kilometres and races along at up to 800 kilometres per hour — as fast as a jet plane. Ships out at sea may not even notice it passing beneath them.
2. As it nears the coast, the shallow seabed slows the front of the wave while water behind keeps rushing forward. The wave's energy gets squeezed into a smaller space, so it piles up, sometimes into a towering wall of water many metres high.
3. At the shore, the tsunami floods far inland, carrying tremendous force. It is the sheer volume and weight of moving water, not just its height, that makes a tsunami so destructive.

The terrible Indian Ocean tsunami of 2004 and the Japanese tsunami of 2011 were both triggered by giant undersea earthquakes and caused enormous loss of life. They are reminders of how the ocean, usually so familiar, can become powerful in moments — you can learn more about the sea in oceans and sea life.

Warning signs and staying safe

We cannot yet predict exactly when an earthquake will strike, but science and preparation save many lives.

For tsunamis, there are warning signs to know:

• A strong earthquake felt near the coast.
• The sea suddenly pulling far back, exposing the seabed, before the wave arrives.
• A loud roaring sound from the ocean.

If any of these happen, the rule is simple: move to high ground immediately and stay there. Tsunamis arrive as several waves, and a later wave may be bigger than the first.

For earthquakes themselves, people in quake-prone places like Japan, Chile and California prepare in several ways:

• Drop, Cover and Hold On — drop down, take cover under a sturdy table, and hold on until the shaking stops.
• Building strong, flexible structures designed to sway without collapsing.
• Practising regular earthquake drills at school and work.
• Keeping an emergency kit with water, food, a torch and a first-aid kit.

Many countries also run warning systems that use seismometers to detect a quake's first, faster waves and send out alerts seconds before the strong shaking or a tsunami arrives — precious time that can save lives.

Try it yourself: model seismic waves and a shake test

You can explore the science of earthquakes with two simple activities.

1. Wave model. Lay a length of rope or a slinky spring along the floor and hold one end. Give it a sharp shake. Watch the wave of energy travel along the rope, just as seismic waves travel through the ground away from an earthquake's focus.

2. Shake test. Build two towers out of building blocks on a tray: one tall and thin, and one short and wide with a broad base. Gently shake the tray to mimic an earthquake. Which tower falls first? You will usually find the tall, narrow one topples sooner. This is exactly why engineers design earthquake-safe buildings with strong, wide foundations and flexible frames.

By modelling these forces, you are thinking like a real seismologist and engineer, working to understand — and survive — one of nature's most powerful events.