Recent discussions on Thaivisa regarding the recent tsunami threat has made it clear that a surprising number of people do not understand the Richter scale. The problem seems to be that they do not understand a logarithmic scale.
So with apologies to those who do, here is, I hope, a simple explanation.
Many things in nature have a very wide range, size is a good example, from a tiny atom to a mega sun, when we measure the sensitivity of the human eye to light, or the ear to sound, we find these also have a very wide range, from a candle a mile away, to the sun at full noon, or the whine of a mosquito to a pneumatic drill.
Earthquakes also have a broad range, from tremors so slight they can only be detected by sensitive instruments, to movements so vast they can be felt thousands of miles away.
To classify such a vast range of energies it is convenient to use a logarithmic scale, this mean a scale where size is shown as the logarithm of the actual number.
log 10 =1
log 1000 =3
Put simplistically the log of a number is the number of trailing zeros and an increase of 1 in the log number is an increase of 10 in the actual size.
Looking now at the Richter scale this means a magnitude 9.0 earthquake is 10 times more powerful than a magnitude 8.0 and 100 times more powerful than a magnitude 7.0
In reality Richter values less than about 3.0 are not noticed by humans, only by some animals and sensitive instruments.
Values around 4.0 are felt but rarely cause damage.
We have to get above the value 5.0 before they start to really impact.
Now the 2004 quake was a 9.2 roughly equivalent to a 950 megaton bomb. This week's quake was an 8.6 roughly 140 megatons, so about nine times less. Note the non linearity in this.
Just for the record, the largest quake recorded was a 9.5 in Chile in 1960.
The meteor impact of 65 million years ago is estimated as being a Richter 12.55 event.
The damage caused by an earthquake will depend on location, densely populated or uninhabited, also the strength of building.
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