There's a whole lot of shakin goini on ....

I was in a few earthquakes during my sojourn in the Philippines. Most were around 6 on the Richter scale. But one in Cabanatuan was 7.8. ( and that folks is a biggy!)

The photo to the left is the aftermath of the San Francisco earthquake of 1906. Magnitudes between 7.7 and 8.25 have been suggested for that disaster.

Which brings me to the point of this here posting today - the Richter Scale.

The news media have been doing a good job covering the Christchurch earthquake, but one thing they fail to explain is the Richter scale. I have a feeling many people including those in the press don't understand it.

For example some people think a magnitude 6 is twice as big as a magnitude 3.  Not so.

In fact a magnitude 6 is 1000 times bigger than a magnitude 3!!

Why? I hear some ask. Surely not! Others cry.


The Richter scale is related to the base ten logarithmic scale ( ooops  ... some eyes are beginning to glaze over!).

The gap between each whole number (1, 2, 3, 4, 5, 6, 7, 8 ....) represents a 10 fold increase in the amplitude of the earth quake.

This simple graph shows what I mean. As does the table below.

However, it is not quite as simple as that (darn ... it never is in mathematics ... the people cry).

But in terms of the energy of the earthquake, each whole jump represent around 31 times the energy.

The recent Christchurch earthquake was scaled down from a 7.4 to a 7.1. Not much many may think. But in actual fact a 7.4 magnitude is 2.5 times the magnitude in terms of amplitude of a 7.1.

The Cabanatuan earthquake of 7.8 was 5 times the amplitude of Christchurch's 7.1. Fortunately, I was some 400 km from the epicentre. But even 400 km away we were shakin!

A final thought. The Christchurch earthquake represents just over 40 megatons of TNT in terms of energy released.

That is more than the energy of the largest thermonuclear weapon ever tested.

Some would argue we should ditch the richter scale and speak of earthquakes in terms of their TNT equivalent. They suggest this would be more meaningful. Thus Christchurch's earthquake would have been 40 megatons and San Francisco would have been at least 800 megaton.

What do you all think?


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