An earthquake is the ground shaking because rocks deep underground suddenly slip past each other. Around 500,000 earthquakes are detected worldwide every year. About 100,000 of those are big enough for someone to feel, and around 100 do damage. Most happen along the edges of the Earth's tectonic plates, and one country — Japan — feels roughly 1,500 of them a year.
The Ground Is Not Solid
The Earth's outer shell is broken into about fifteen large slabs called tectonic plates, plus a lot of smaller ones. They sit on hotter, softer rock underneath and drift a few centimetres a year.
Where two plates meet, they can push together, pull apart, or slide past each other sideways. None of it is smooth. The rock catches, sticks, and strain builds up like a bent ruler.
Eventually the friction gives way. The rock snaps back into a new position, and all the stored energy is released at once as waves travelling through the ground. That is an earthquake.
Faults
A fault is a crack in the Earth's crust where the rock on one side can move relative to the other. Most earthquakes happen on existing faults, because the rock there is already broken and weaker than the rock around it.
The San Andreas Fault in California is around 1,200 kilometres long and marks where the Pacific Plate slides north-west past the North American Plate at about 5 centimetres a year. Los Angeles is moving slowly towards San Francisco.
Fences, roads and streams that cross a fault get bent and offset over decades. After the 1906 San Francisco earthquake, fences were found to have jumped sideways by up to six metres in a matter of seconds.
Focus and Epicentre
Two words that get muddled. The focus, sometimes called the hypocentre, is the point underground where the rock actually breaks. The epicentre is the point on the surface directly above it.
Shallow earthquakes — under 70 kilometres deep — do the most damage, because the energy has less rock to travel through before it reaches buildings. Deep ones, down to about 700 kilometres, are felt over a wider area but more weakly.
Nothing deeper than about 700 kilometres has ever been recorded, because below that the rock is too hot and soft to snap. It bends instead.
Three Kinds of Wave
Earthquake energy travels as seismic waves, and they arrive in a specific order.
P-waves, or primary waves, come first. They push and pull the rock in the direction they travel, like sound, and they move at around 5 to 8 kilometres a second. They can travel through solids and liquids.
S-waves arrive next and move the ground up and down or side to side. They are slower but stronger, and they cannot travel through liquid. Then come the surface waves, which are slowest of all and cause most of the damage, rolling along the ground like waves on the sea.
How We Know What Is Inside the Earth
Nobody has ever drilled more than about 12 kilometres down. Everything we know about the deep interior comes from earthquake waves.
Because S-waves cannot pass through liquid, and there is a shadow zone on the far side of the planet where they never arrive, scientists worked out that part of the Earth's core must be molten. Because P-waves bend in particular ways, they mapped the layers.
It is a bit like tapping a wall to find out where the studs are, except the wall is a planet and the tapping is done by earthquakes. Every large quake gives seismologists another scan of the inside of the Earth.
Measuring the Shaking
You have probably heard of the Richter scale. Seismologists mostly do not use it any more.
They use the moment magnitude scale, which measures the total energy released and works better for very large earthquakes. It is logarithmic: each whole number is about 32 times more energy than the one below. A magnitude 7 releases roughly a thousand times more energy than a magnitude 5.
There is a second kind of scale entirely, the Modified Mercalli Intensity scale, which measures what the shaking actually did in a particular place — from "not felt" up to "total destruction". One earthquake has a single magnitude but many different intensities, depending on how far away you are and what the ground under you is made of.
Build a Shake Table
Put a tray on top of two round pencils so it can slide back and forth. That is your shake table.
Build two towers on it from wooden blocks or Lego: one tall and thin, one short and wide. Slide the tray sharply from side to side and see which falls first.
Now try again with a tower built on a base of jelly or wet sand, and one on a hard board. The soft base will amplify the shaking dramatically — which is exactly why buildings on soft soil and reclaimed land suffer far more damage than buildings on solid rock, even in the same earthquake.
The Biggest Ever Recorded
The largest earthquake ever measured was magnitude 9.5, near Valdivia in Chile on 22 May 1960. It raised and lowered stretches of coastline by several metres and sent a tsunami across the entire Pacific.
The 2004 Indian Ocean earthquake off Sumatra was magnitude 9.1 and generated a tsunami that killed around 230,000 people in fourteen countries. The 2011 Tohoku earthquake off Japan, magnitude 9.0, moved the main island of Honshu about 2.4 metres east and shifted the Earth's axis by several centimetres.
Both of those released so much energy that they very slightly shortened the length of the day — by a few millionths of a second, but measurably.
Tsunamis
A tsunami is not a normal wave. Ordinary waves are made by wind pushing on the surface, and only the top layer of water moves. A tsunami is made when the seafloor itself lurches upwards or downwards, and the entire depth of the ocean is displaced.
In deep water a tsunami may be less than a metre high and hundreds of kilometres long, travelling at around 800 kilometres an hour — as fast as a jet aircraft. Ships out at sea barely notice it pass.
When it reaches shallow water it slows down and the energy piles up into a wall of water. One warning sign is the sea suddenly draining away from the beach — that is the trough arriving before the crest. In 2004 a ten-year-old British girl on a beach in Thailand recognised it from a geography lesson and helped clear the beach, saving around a hundred people.
Can We Predict Them?
Not yet, and possibly not ever. Scientists can say which regions are likely to have earthquakes and roughly how often, but not that one will happen on a particular day.
People have looked for warning signs for centuries: unusual animal behaviour, changes in well water, strange lights in the sky. Some have been observed, none has proved reliable enough to base an evacuation on.
What does work is early warning. Because P-waves travel faster than the damaging surface waves, a network of sensors near a fault can detect the first arrival and send an alert electronically, which travels at the speed of light. That buys seconds to tens of seconds — enough to stop trains, close gas valves, halt surgery and get under a desk. Japan, Mexico and California all run systems like this.
What to Do if the Ground Shakes
The advice in most countries is drop, cover and hold on. Get down on the floor before the shaking knocks you down, get under a sturdy table if there is one, and hold on to it so it stays over you.
Stay away from windows and from anything heavy that could fall. Do not run outside during the shaking — most injuries happen from falling glass and debris around the outside of buildings. If you are already outdoors, move away from buildings, trees and power lines.
Aftershocks follow almost every large earthquake, sometimes for months, and they can be nearly as strong as the first one. That is why buildings damaged in a main shock are dangerous to re-enter even after the shaking has stopped.
Buildings That Survive
Earthquakes rarely kill people directly. Collapsing buildings do.
Engineers have several tricks. Base isolation puts a building on rubber and steel bearings so the ground can move underneath while the building stays relatively still. Tuned mass dampers are heavy weights near the top that swing to counteract the sway. Cross-bracing and shear walls stop a frame folding sideways.
Old techniques worked too. Japanese pagodas have a heavy central pillar that swings freely, and almost none has ever collapsed in an earthquake in 1,400 years. Countries with strict building codes lose far fewer people: two earthquakes of the same size can kill dozens in one country and tens of thousands in another. To keep exploring, find out about how volcanoes work and read about the five oceans where tsunamis travel.
