The Richter Scale


Map of recent New Zealand Earthquakes

Map of recent New Zealand Earthquakes

Dramatic news from New Zealand about the recent Earthquake. I have never experienced an Earthquake and it sounds like one of the most baffling and terrifying experiences a human being could ever live through: How could the Earth itself move?! Now every ‘Earthquake story’ is accompanied by a statement of the strength of the Earthquake on the Richter Scale and for once the BBC has done us proud with a relatively comprehensible article on the subject. Thank you BBC.

The recent Earthquake registered 6.3 on the Richter scale and has killed 75 people so far, whereas the magnitude 7.1 Earthquake last September 2010 killed no one. The Haiti earthquake in January 2010 was a magnitude 7.0 shake and killed tens of thousands. Is this Richter scale really a measurement that reflects the deadliness of an Earthquake? No. The Richter scale measures the amount of energy released in an event. But the scale is very strange:

  • Every 2 points along the scale corresponds to a change in energy released by a factor 1000. So a magnitude 8.0 earthquake is 1000 times more energetic than a magnitude 6.0 earthquake and 1,000,000 times more energetic than a magnitude 4.0 earthquake.
  • To find the ratio of the amounts of energy released in two different Earthquakes one needs to do a sum. First one subtracts one Richter number from the other, and then raises 10 to the power of the difference. So for the two recent New Zealand earthquakes one subtracts 6.3 from 7.5 to give 1.2. One then raises 10 to the power of 1.2 using a  calculator to yield 10^1.2 = 15.84 or approximately 16.

So the recent earthquake was 16 times less powerful than the September earthquake. One can calculate the amount of energy associated with these events. Wikipedia has a tabulation but for reference, a magnitude 7.0 earthquake corresponds to the release of a phenomenal amount of energy:

  • 2 Peta joules of energy i.e.
  • 2000 million million joules of energy i.e.
  • 2 000 000 000 000 000 joules of energy, which is equivalent to an explosion caused by
  • 474 thousand tons of TNT or around 22 bombs similar to that which was dropped on Nagasaki.

And just like a bomb, the amount on damage inflicted depends on several factors

  • How far away one is from the bomb site – the epicentre of the Earthquake:
    • The further away one is, the less the effect
    • The deeper the Earthquake the less the effect at the surface.
  • How well protected one is:
    • In a bomb shelter one can gain a certain amount of protection from a bomb.
    • Similarly, buildings designed to resist the shaking of Earthquakes can afford protection.

The magnitude 6.3 quake was equivalent to the explosion of ‘only’ 4 atomic bombs, 10 km from Christchurch at a depth of 5 km. To my mind that sounds like a truly terrifying prospect. I take my hat off to the people who developed building regulations to withstand that. And I express my sympathy to all the people in the locale for their loss.

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