Earthquake Facts

The earthquake and tsunami that devastated north-east Japan in March 2011 demonstrate the terrifying power of these natural phenomena. Almost 16,000 people died and more than a million buildings wholly or partly collapsed.

A year after the event, 330,000 people were still living in hotels or in other temporary accommodation, unable to return home. A further 3,000 people were still listed as missing. The gigantic tsunami waves spawned by the earthquake inundated the power supply and cooling of three reactors at the Fukushima Daiichi power station. The subsequent nuclear accident – the worst since Chernobyl – caused worldwide panic.

Earthquakes are unstoppable and strike with little or no warning, but we know a growing amount about how they work. Scientists have developed networks of sensors for monitoring ground movements, changes in groundwater and magnetic fields, which may indicate an impending quake. Engineers, meanwhile, have created new forms of architecture to resist earthquakes when they do strike. So without further ado, let’s learn some earth-shattering facts…

Top 25 Earthquake facts

1 .What’s the deepest epicentre on record?

750km

2. Do more earthquakes occur in hot weather?

No

3. What is Earth’s crust made of?

The crust consists of rock broken into moving slabs, called plates. These plates float on the denser rocks of the mantle, a sticky layer lying between the planet’s core and the crust. Granite is the commonest rock in the crust that makes up Earth’s continents. This continental crust is an average 35 kilometres (22 miles) thick, deepest beneath mountain ranges. Ocean floor crust is thinner – on average six kilometres (four miles) – and mainly made of denser volcanic rocks, such as basalt. Granite is 75 per cent oxygen and silicon. Basalt is denser as the silicon is contaminated with heavier elements like iron.

4. Did the 2011 quake in Japan shorten the days on Earth?

Yes, but you’re unlikely to notice. Every day is now 1.8 microseconds shorter, according to NASA. The Japan earthquake made Earth spin slightly faster by changing its rotation around an imaginary line called the figure axis. The Earth’s mass is balanced around the figure axis, and it wobbles as it spins. That wobble naturally changes one metre (3.3 feet) a year due to moving glaciers and ocean currents. The 2011 Tohoku earthquake moved the ocean bed near Japan as much as 16 metres (53 feet) vertically and 50 metres (164 feet) horizontally – that’s the equivalent horizontal distance to an Olympic swimming pool! The shifting ocean bed increased Earth’s wobble around the figure axis by 17 centimetres (6.7 inches). As the wobble grew, Earth sped up its rotation. It’s the same principle as when a figure skater pulls their arms closer to their body in order to spin faster.

5. What is the shadow zone of an earthquake?

A shadow zone is the location on the Earth’s surface at an angle of 104-140 degrees from a quake’s origin that doesn’t receive any S-waves or direct P-waves. S and P-waves are seismic waves that can travel through the ground. Seismic waves are shockwaves created when a fault suddenly moves. The shadow zone occurs as S-waves can’t pass through the Earth’s liquid outer core, while P-waves are refracted by the liquid core.

6. Where is the quake capital?

Around 90 per cent of earthquakes occur on the so-called Ring of Fire, a belt of seismic activity surrounding the Pacific Plate. The Ring of Fire is a massive subduction zone where the Pacific Plate collides with and slides beneath several other crustal plates. Most earthquakes are measured in Japan, which lies on the Ring of Fire at the junction of the Pacific, Philippine, Eurasian and Okhotsk Plates. Japan has a dense earthquake-monitoring network, which means scientists can detect even small quakes. The volcanic island chain of Indonesia probably experiences the most earthquakes based on landmass, however it has fewer instruments for measuring them.

7. Are earthquakes more likely to occur in the morning?

No.

8. What are tremors?

A tremor is simply another word for an earthquake. It’s also another word for the vibrations you experience when a quake hits. The earth trembles because movement energy is released in an earthquake, causing the ground to vibrate.

9. How can scientists tell how far away an earthquake occurred?

Scientists use a seismometer to record earthquake waves called P and S-waves. P-waves travel faster than S-waves and can pass through liquids. By measuring the delay between the P and S-waves arriving, they can calculate the distance the waves travelled.

10. What’s the earliest recorded major earthquake in history?

The first earthquake described was in China in 1177 BCE. By the 17th century, descriptions of the effects of earthquakes were published worldwide, although of course these accounts were often exaggerated and less detailed than data recorded today.

11. What do the lines on a seismometer reading represent?

The wiggly lines on a seismogram represent the waves recorded. The first big wiggles are P-waves. The second set of wiggles are S-waves. If the latter are absent, the quake happened on the other side of the planet.

12. Why do quakes at sea lead to tsunamis?

Earthquakes trigger tsunamis by generating ripples, similar to the effect of sloshing water in a glass. Tsunamis are giant waves, which can cross oceans at speeds similar to jet aircraft, up to 700 kilometres (435 miles) per hour, and reach heights of 20 metres (66 feet) as they hit the coast. They sweep inland faster than running speed, carrying away people and buildings alike. For example, the 2004 Indian Ocean tsunami claimed 300,000 lives and made nearly 2 million more homeless.

13. Are there different types of earthquake?

Strike-slip fault

Roads can be sheared apart along strike-slip faults. They’re straight cracks in the crust where two plates are sliding horizontally past each other. Every time a section of the fault moves, an earthquake occurs.

Normal fault

Earth’s brittle crust becomes fractured along fault lines. Quakes occur along a normal fault when the two sides move apart. Rock slabs sitting above the fault slide down in the direction the plates are moving, like at the Mid-Atlantic Ridge.

Thrust fault

The 2011 Tohoku quake ruptured a thrust fault in a subduction zone. These zones are associated with Earth’s most violent quakes as oceanic crust grinds beneath continental crust, creating great friction. Huge stresses can build here and release the same energy as a thousand hydrogen bombs!

14. How do P and S-waves move?

Primary (compressional) waves

P-waves are the fastest waves created by an earthquake. They travel through the Earth’s interior and can pass through both solid and molten rock. They shake the ground back and forth – like a Slinky – in their travel direction, but do little damage as they only move buildings up and down.

Secondary (shear) waves

S-waves lag behind P-waves as they travel 1.7 times slower and can only pass through solid rock. However they do more damage because they’re bigger and shake the ground vertically and horizontally.

15. How thick is the Earth’s crust?

5-70km

16. How many quakes occur each year?

500,000

17. Do earthquakes happen off Earth?

There’s evidence of ‘marsquakes’ on Mars as well as quakes on Venus. Several moons of Jupiter and Titan – a moon of Saturn – also show signs of quakes. Seismometers on the Moon detected tidal ‘moonquakes’ caused by the pull of the Earth’s gravity, vibrations from meteorite impacts and tremors caused by the Moon’s cold crust warming after the two-week lunar night.

18. Why is the San Andreas Fault prone to large quakes?

Longer faults have larger earthquakes, which explains why the strike-slip San Andreas Fault has had several quakes over magnitude 7. The San Andreas Fault extends 1,300 kilometres (800 miles) along the coast of California. When a fault ruptures, it ‘unzips’ along its length. Each section of the fault releases energy – the longer the fault, the more energy released and so the bigger the quake. Scientists believe the San Andreas Fault is overdue for a potential magnitude 8.1 earthquake over a 547-kilometre (340-mile) length. The southern segment has stayed static for more than a century, allowing enormous stresses to build.

19. Could Africa ever be split from Europe by an earthquake?

The Eurasian and African Plates are not splitting apart; they’re actually moving towards each other at about one centimetre (0.4 inches) each year. In the future, it’s possible that the Eurasian Plate may begin to slide beneath the African Plate. Even if the plates were moving apart, you’d need a mega-quake to yank Africa away from Europe in one go. There is no known fault long enough to create a mega-quake above magnitude 10. The most powerful earthquake in history was magnitude 9.5.

20. How many people jumping would it take to re-create the same reading as the Tohoku earthquake?

You’d need a million times Earth’s population, all jumping at once, to generate the energy released by the March 2011 Tohoku quake. How do you calculate that? You assume Earth’s population is 10 billion and each person generates 200 joules of energy by jumping 0.3 metres (0.98 feet).

21. How did the Japan Trench form?

A 390-kilometre (242-mile) stretch of the Japan Trench is associated with Japan’s 2011 Tohoku earthquake. The trench is a vast chasm in Earth’s crust at the junction between the Pacific Plate and tiny Okhotsk Plate beneath Japan. The Pacific Plate is moving westwards and diving beneath the Okhotsk. Friction between the two plates causes them to lock together and pressure to build. Sudden slippages release the tension in a violent burst of energy.

22. How long do quakes last?

10-30 secs

23. Can animals predict quakes?

There’s little evidence for whether animals can predict earthquakes, but many stories exist of odd behaviour. These include hibernating snakes fleeing their burrows in China in 1975, a month before the Haicheng quake.

24. Where is the safest place to be during an earthquake?

The safest place inside is underneath a sturdy table, away from light fittings and windows. The safest place outside is out in the open away from any buildings and electricity cables.

25. If I were stood on a beach during an earthquake would I sink?

Perhaps, but it’s unlikely you would drown. During an earthquake, wet sand or soil can behave like quicksand – a process called liquefaction. A quake vibrates the sand, separating the grains so that they flow like a liquid. It’s extremely unusual and even then people will rarely sink below their chests during liquefaction as they will float.