Basically, it measures only the amount of energy released by a quake and takes no account of other factors such as depth below the earth’s surface or direction of movement, which can have a significant bearing on the amount of destruction caused.
The Richter scale is logarithmic, so a Richter 2 magnitude quake is not twice as strong as a Richter 1 magnitude; it’s roughly 30 times as strong.
The 9.2 quake that caused the 2004 Asian Tsunami was roughly 5.5 times as powerful as the 8.7 quake of April 11 this year.
The largest man-made release of energy was the Tsar Bomba, the world’s most powerful nuclear device, detonated in 1961 in a remote part of the Soviet Union.
The amount of energy released by the bomb, as recorded by seismographs, rated an
8.35 on the Richter scale.
Another method for measuring earthquakes is the Modified Mercalli Intensity scale, based on a 19th Century system of measurement, and revised in 1931. The Mercalli measures the effects on the ground, rather than the energy released by a quake.
Numbered in Roman numerals, it runs from I, for a quake that is “generally not felt at all”.
A Mercalli VI quake (Strong), is described as “Felt by all; many frightened and run outdoors, walk unsteadily. Windows, dishes, glassware broken; books fall off shelves; some heavy furniture moved or overturned; a few instances of fallen plaster. Damage slight.”
The most extreme quake on the Mercalli scale is a XII (Cataclysmic), described thus: “Total destruction – Everything is destroyed. Lines of sight
and level distorted. Objects thrown into the air. The ground moves in waves or ripples.”
The Richter scale, which can be extrapolated from seismograph measurements halfway round the world, is much easier to apply than the Mercalli, which requires first-hand observation on the ground. This is why the Richter scale is much more widely used.
Why there was no tsunami this time:
At 8.7 on the Richter scale, with two aftershocks of 6.5 and 8.1, the April 11 quake to the south of Sumatra, Indonesia, was a big one – large enough to set off tsunami alerts all around the Bay of Bengal and even as far away as Kenya.
But in the end, seismologists have explained, the likelihood of it causing a catastrophic tsunami was almost non-existent.
This is because it was a different kind of quake from the quake of 2004 that set off the deadly Asian Tsunami.
In 2004, the quake was caused by subduction along what is known as the Sunda Megathrust. This is where two “protoplates” of the earth’s crust – the Indian and Australian plates – are gradually sliding under two smaller ones, the Sunda and Burma plates.
To explain in very simplistic terms, imagine you are holding an eraser with its end against a sheet of glass. Keep the pressure on as you push it across the glass, and the eraser will bend. Eventually it reaches a point where it snaps back into its original shape.
Pressure builds up between the plates until it reaches unsustainable levels – as with the eraser – and a huge amount of energy is released in a very short time.
Usually this results in the surface – in the case of the 2004 quake – snapping upwards or downwards. If it is under the ocean this will cause tsunami waves.
On April 11, however, the quake – although in the same area close to Sumatra– was a result of a pressure build-up in what is known as a strike-slip fault, where two plates are sliding past each other, rather than one under the other.
Such quakes can cause significant damage on the surface but in this case all the action was more than 30 kilometres below the seabed, and had no effect on the seabed itself.
This is not, of course, to say that we are safe now. Scientists believe that some parts of the Sunda Megathrust have been the cause of two or more quakes in history, so 2004 could be repeated.
The big question that no-one can answer is “When?”
The world’s most powerful quakes:
1 In 1960 the world’s most powerful quake, near Temuco in Chile, registered 9.5
on the Richter scale and killed more than 1,600 people, injured another 3,000, and
left two million homeless. It also caused tsunami waves that killed 231 people in Japan,
Hawaii and the Philippines.
2 The 1964 quake in Prince William Sound, Alaska, measured at 9.2, killed just 15 people, though the tsunami it generated killed another 113. It caused devastation in Anchorage, the nearest city, flattening 30 blocks.
3 Much more deadly was the 9.1 quake off Sumatra, Indonesia on December 26, 2004. This was the one that released the Asian Tsunami, killing a record 228,000 people in 14 countries around the rim of the Indian Ocean.
4 Last year’s Tohoku 9.0 earthquake, off the coast of Honshu Island, Japan, and the subsequent tsunami, killed at least 15,703 people, and left 130,927 homeless. At Miyako the waves reached a height of almost 38 metres – equivalent to a 13-storey building.
5 The Kamchatka Quake of 1952 registered 9.0 on the Richter scale. Thanks to the sparseness of the population in Russia’s Far East, no deaths were recorded, but the resulting tsunami caused damage in Hawaii, Alaska, Chile and New Zealand.
6 In 1868 a 9.0 earthquake centred on the port of Arica, Chile, caused havoc along the coastline and was felt as far away as 1,400 kilometres away. At least 330 people were killed by the three tsunami waves that followed.
7 The Cascadia earthquake of 1700 – believed to have been around magnitude 9.0 – rattled northern California, Oregon, Washington, and southern British Columbia, and set off waves that damaged coastal villages in Japan. The death toll was not recorded.
8 The 2010 Chile quake killed 523 people, injured 12,000, destroyed 370,000 homes and left 800,000 homeless. More than 300 aftershocks of 5.0 or greater have been recorded since.
9 In 1906, off the coast of Ecuador, an 8.8-magnitude quake generated tsunami waves that killed as many as 1,500 people in that country and neighbouring Colombia.
10Like the Kamchatka quake (No. 5) the 8.8 quake in 1965 among the Rat Islands, off Alaska, caused no casualties, mainly because almost no one lives in the area.
The power of a quake does not necessarily correlate with the damage to society. In terms of death tolls, even the Sumatra quake of 2004, with 228,000 dead, was by no means the most deadly. The top five are:
1 The Shaanxi Quake of 1556 in China killed an estimated 830,000 people. It is thought to have been around 8.0 Richter.
2 The 7.0 quake of 2010 in Haiti killed 316,000 people and injured some 300,000.
3 The Tangshan Earthquake of 1976, in China, officially killed 242,769 people, though some estimates put the toll as high as 655,000.
4 So long ago it’s almost in prehistory, the Aleppo, Syria, quake of 1138 is estimated
to have killed 230,000 people.
5 The 9.1 Sumatra Quake of 2004 (see No. 3 in the list above) is infamous not because of the number of people the quake itself killed, but for the record number of people the subsequent tsunami waves killed – 228,000.
Source: US Geological Survey