What causes earthquakes and how is magnitude measured? | The Sun

EARTHQUAKES are known to be one of the world's most terrifying and destructive natural disasters.

But how do earthquakes occur, and when have been the strongest ones recorded?

What causes an earthquake?

The earth's crust is made up of different pieces known as tectonic plates.

These plates fit together like a jigsaw and continuously move at a rate of a few centimetres a year, in different directions and at different speeds.

It's common for plates to slides past each other, bump into each other and move away from each other.

As plates carry on moving in different directions over long periods of time, friction causes energy to build up.

Eventually it becomes so great that the energy is released, which creates a shock wave – an earthquake.

If the earthquake is beneath the ocean it can create a series of huge waves, called a tsunami.

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Earthquakes occur around the world everyday with some so small, they can only be detected using specialist equipment.

Others can be powerful enough to damage and destroy towns and cities.

The size of such earthquakes would be measured on the Richter magnitude scale.

There are earthquakes in the UK, but they are rare and so small that most people do not feel them.

How is magnitude measured?

Magnitude is the most common measurement for the strength of an earthquake.

Typically magnitude will be recorded by measuring the amplitude of seismic waves that are recorded on a seismometer.

Seismometers are instruments that respond to noises and shaking in the ground.

A magnitude readout is typically represented by a small number.

What's interesting about magnitude is that it climbs on a logarithmic base-10 scale.

So each time the number increases by one, the amplitude is actually 10 times greater.

That means that a magnitude 4 earthquake isn't twice as strong as a magnitude 2 quake, but 100 times.

The strongest quake on record peaked at around 9.4 to 9.6 magnitude.

What have been the strongest ever earthquakes?

Also known as the Great Chilean earthquake, the Valdivia Earthquake was the most powerful earthquake ever recorded measuring 9.5 on the Richter scale.

The earthquake hit on May 22, at 19:11 GMT (15:11 local time), approximately 100 miles off the coast of Chile – parallel to the city of Valdivia. 

It lasted approximately 10 minutes and triggered a massive tsunami with waves up to 25m.

The total number of fatalities from the earthquake and subsequent tsunamis have been estimated between 1,000 – 6,000, with about 3,000 people injured.

Earthquakes by magnitude:

  • May 22, 1960 – Valdivia, Chile (9.4 to 9.6)
  • June 11, 1585 – Aleutian Islands (9.25)
  • July 8, 1730 – Valparaiso, Chile (9.1 to 9.3)
  • Marcy 27, 1964 – Prince William Sound (9.2)
  • December 26, 2004 – Indian Ocean, Sumatra, Indonesia (9.1 to 9.3)
  • October 17, 1737 – Kamchatka Peninsula, Russia (9.0 to 9.3)
  • November 17, 1837 – Valdivia, Chile (8.8 to 9.5)
  • March 11, 2011 – Pacific Ocean, Tōhoku region, Japan (9.1)
  • December 16, 1575 – Valdivia, Chile (9.0)
  • November 24, 1604 – Arica, Chile (9.0)

What to do during an earthquake

The CDC has lots of advice for what to do in an earthquake.

The official advice is to Drop. Cover. Hold On:

  • DROP down onto your hands and knees before the earthquake knocks you down. This position protects you from falling but allows you to still move if necessary.
  • COVER your head and neck (and your entire body if possible) underneath a sturdy table or desk. If there is no shelter nearby, get down near an interior wall or next to low-lying furniture that won’t fall on you, and cover your head and neck with your arms and hands.
  • HOLD ON to your shelter (or to your head and neck) until the shaking stops. Be prepared to move with your shelter if the shaking shifts it around.

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