Unfortunately, the past few weeks have seen 2 devastating earthquakes hit different parts of the world. The first was of course the one that ravaged Haiti on January 12th. It recorded a magnitude of 7.0 on the Richter scale. The second hit Chile yesterday morning, with a magnitude of 8.8 on the Richter scale.
Richter was an extremely annoying man who set up the most misleading kind of scale. What’s the difference between a 5 and a 7-magnitude? How come an earthquake of 7.0 killed 217,000 in Haiti, while a 8.8 one appears to have killed only 700 so far? (even one person’s loss is a devastating one, but the human death toll in Haiti was more than 300-times that in Chile so far, so you really can’t ignore the numbers)
Let’s start off by explaining the how the Richter scale works.
“The Richter Scale is the best known scale for measuring the magnitude of earthquakes. The magnitude value is proportional to the logarithm of the amplitude of the strongest wave during an earthquake” < I didn’t understand anything from this, for those of you who did, good for you. For the rest of us, here is sone more:
The energy release of an earthquake, which closely correlates to its destructive power, scales with the 3⁄2 power of the shaking amplitude. Thus, a difference in magnitude of 1.0 is equivalent to a factor of 31.6 ( = (101.0)(3 / 2)) in the energy released; a difference in magnitude of 2.0 is equivalent to a factor of 1000 ( = (102.0)(3 / 2) ) in the energy released.
Basically, each degree is 10 times as powerful as the degree below it (earth motion-wise). So, a 7 magnitude is 10 times as bad as a 6 magnitude and a 100 times worse than a 5 magnitude. The 10-fold increase applies to the relative motion of the Earth in each case. A 31.6-fold increase in energy though is seen between each degree. So, the energy involved in a 5 magnitude earthquake releases 31.6 times more energy than a 4 magnitude one, explaining the massive difference in damage and casualties between seemingly close-range earthquake magnitudes.
Next is the frequency of earthquakes. We’ve heard about 2 this year so far but in fact, some 1,000,000 earthquakes happen each year, that’s some 2740 earthquakes a day (2732 earthquakes a day on a leap year =P)
The Richter scale however isn’t the preferred scale for scientists and seismologist. Experts in the field take many other factors into consideration besides wave amplitude. Most notably, the direction of the waves (up and down, or side-to-side), the population density in the affected area, and the strength of buildings and infrastructure.
So, that’s the reason we all get so mixed up when we hear about earthquakes:
1- The Ricther scales isn’t as simple as it looks
2- Scientists prefer to use extra criteria to classify magnitudes
So, the next time you hear that the Chilean earthquake was 800 times worse than the Haitian one, while its just 1.8 degrees difference and wonder why, or why if it was so much worse, was the death-toll was 1/300th that of the earthquake with 1/800th the size? It’s because of the misleading scale, which does not take into consideration a nation’s readiness, population density and proximity of an earthquake’s epicenter.
UPDATE on 11 March 2011
The earthquake that struck Japan today was an 8.9 magnitude, which as you can see is very severe and fairly rare when it comes to earthquakes. Japan, arguably the most prepared and experienced country in terms of earthquake and tsunami response, has suffered hundreds of casualties so far, demonstrating how powerful and unpredictable plate tectonics are.
The quake was felt over 400km away from the epicenter, 10-meter-high tsunamis were reported and the whole pacific basin issued a tsunami warning.