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The year of the quake

Earthquakes and tsunamis are the most destructive natural phenomenon we are ever likely to encounter. Luckily, the more destructive things such as a meteor impact and super-volcano eruptions only happen only every few thousand years.


The difference is that there's an earthquake somewhere in world everyday, and a destructive one every year.

Annual earthquakes:

7.0 magnitude earthquakes

The Haiti earthquake had a magnitude of 7.0. We would expect to see around 18 earthquakes of this size or larger every year.

Haiti Earthquake 1

Haiti Earthquake 2

Great category earthquakes

The Chilean earthquake comes into the 'Great' category. We would only expect to see about one of these a year, thankfully!

Chile Epicenter

Magnitude or Richter scale?

When news broadcasters talk about earthquakes they always mention the Richter Scale. This scale actually went out of use in the 1970s! What they use now is the Modified Magnitude Scale (MMS).

Shaking buildings - the difference in just one magnitude

The tricky bit to understand is that both scales are logarithmic. A difference of just 1 magnitude, say from 5 to 6, or 6 to 7 is much bigger than most people think. In fact, 1 magnitude is equal to an increase in amplitude of 10 times. This is why buildings in a magnitude 7 earthquake experience 10 times the shaking compared to a magnitude 6 quake.

Energising the destruction

When we measure energy, the difference of an increase of just 1 magnitude is dramatic. This represents an increase in energy released of 31.6 times. This is why such a small increase in magnitude can result in a huge increase in destruction. It's the energy that really does the damage.

Chile Earthquake 1

Chile Earthquake 2

10,000 Hiroshima-sized bombs

Atomic Bomb

A large earthquake releases about as much energy as about 10,000 Hiroshima-sized bombs! When the US dropped the atomic bomb called 'Little Boy' on Hiroshima in 1945, official figures confirm that 89,833 people died.

It is estimated that another 50,000 were killed (but were not identified and cannot be included in the official figures) and between 350,000 and 360,000 were identified as 'atomic bomb' victims suffering from many related injuries.

This is the result of just one bomb; now compare that destruction to 10,000 bombs being dropped -that's your average large earthquake!

Where does the energy come from?

The surface of the earth is broken up into a number of tectonic plates. As the plates bump and grind into each other the kinetic energy of the movement is transferred into elastic potential energy within the rocks. The rocks eventually snap and release this stored energy. This energy may have been stored up over hundreds of years, but is released in just a few seconds. The fault that caused the Haiti earthquake had not moved in about 250 years. This released a huge amount of stored energy.

How can earthquakes of similar magnitude cause such different damage?

The Haiti and Chilean earthquakes

The Haiti earthquake was only magnitude 7.0 and yet it killed over 220,000 people and made over a million people homeless. The Chilean quake was a monster of magnitude 8.8 and yet killed only around a thousand people. How can this be?

It's all in the location

Most earthquakes occur in areas with relatively low population densities. One of the most powerful earthquakes ever recorded occurred in Alaska on Good Friday in 1962. Only nine people were killed. This is because the region it struck was mostly unpopulated. So, the destruction, in human terms, was very low.

How does the energy get to the surface?

It's all in the waves! Waves transfer energy from place to place. Earthquakes produce two kinds of waves: P and S waves.

P and S Waves

P waves - punching the ground

P waves are longitudinal waves just like sound waves. They punch the ground and cause it move in an up and down direction. They also move fast, much faster than S waves.

S waves - shaking the earth

These are transverse waves and will shake the ground from side to side. These are often the more destructive type. They are slower and always arrive after the P waves. People who live in earthquake zones learn quickly that if they feel a vertical jolt hit them, they only have a few seconds to get to a place of safety.

Time-delayed S waves

The time difference depends on the distance to the earthquakes, the epicentre (or the centre). The greater the distance - the greater the delay. During the Haiti earthquake the distance to the epicentre was so short that both sets of waves arrived practically at the same time. Sadly, there was no warning.

Is it possible to detect the waves?

Scientists use seismographs to detect these waves. These are basically microphones buried in the ground. Data from a number of seismographs allow scientists to calculate the location of earthquakes.

The Haitian earthquake proved to be so devastating for a number of reasons:

  • It started only about 10Km below the surface

Many earthquakes occur deep in the earth's crust. This gives the energy time to spread out and dissipate before reaching the surface. This causes less destruction, but not on Haiti.

  • It occurred slap bang in a densely populated area

A recipe for disaster

These two facts combined with very poor building standards makes for a recipe for destruction. Also, Haiti is the poorest nation in the western hemisphere. All these things add up to disaster on the scale we have just witnessed.

The big questions:

What does magnitude 8.2 on the Richter Scale actually mean?

How much energy is released during an earthquake?

Where does the energy come from?

Why are some earthquakes so much more destructive than others?

What shakes the buildings?