Unravelling the Richter scale of earthquakes

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

22 July 2007

When George Onyango was awakened by yet another Earth tremor this week, the first thought that crossed his mind was this: “Earthquakes occur below the ground and the weather is in the atmosphere above; why then is it that whenever the ground shakes it is the weathermen who appear on TV instead of geologists?”

Good question George and even I don’t know the answer. Nonetheless, there is another interesting difference between two phenomena – their uncertainty. Geologists can accurately pinpoint where an earthquake will happen (to the nearest kilometre) but it is impossible tell when it will occur.

Meteorologists, on the other hand, can predict with good accuracy (to the nearest hour) when it will rain, for example, but it is very difficult to pinpoint where it will fall. This is why we hear of phrases like “highlands east of the Rift Valley…” Now this is a region that covers several thousand square kilometres. Some parts of it will rain and others will remain dry and the meteorologist remains correct!

Onyango also wondered about the so-called Richter scale: “what does it mean?” This method of measuring the magnitude of an earthquake was introduced in 1935 by Charles Richter. It is a way of representing the relative size of the ground vibrations during a quake.

It is not an absolute scale but a relative one. That is, it measures the ratio between the vibrations using an arbitrarily selected value as its base.

But since the magnitudes of the vibrations vary widely, Richter devised a convenient scheme which records the number of factors of ten. Thus a magnitude of one means that the vibration is ten times bigger than the base. A magnitude of two means two factors of ten or 100 times bigger (since 10 x 10 = 100). And so on

Clearly then, the vibration corresponding to magnitude 2 are NOT double the size of magnitude one: they are 10 times bigger! This is the peculiar property of this scale. An increment by one represents a factor of ten in the size of vibrations.

At the time writing this article (Wednesday), the largest tremor recorded was magnitude 6.1 and the smallest was 4.4. The difference is 1.7 magnitudes; which represents a factor of about 50. That is, the largest vibrations were 50 times bigger than the smallest.

But what is the base of the scale, that is, what vibration corresponds to magnitude zero? This is defined at a reference point at 100km from the centre of the earthquake. At that distance, a vibration of one micrometer is equivalent magnitude zero.

Now, a millimetre is equal to 1,000 micrometres, (3 factors of ten or 10 x 10 x 10), therefore, this definition is equivalent to stating that 1mm vibrations recorded at 100km away represent a magnitude 3 earthquake.

That sounds complicated, but today is Sunday: take some time and think about it and you see that it is not that difficult!