THE destructive power of earthquakes is measured in different ways, but the one most people have heard of is the Richter scale.
Here’s how the force is calculated – and what it means if you are caught up in a disaster like the Lombok quake that killed at least 142.
What is the Richter scale?
Seismologists have tried to measure the force of earthquakes for more than a century.
One measure is intensity – how violently the ground and buildings shake.
Intensity is greater close to the epicentre than further out, and is also greater if the quake strikes close to the surface. Generally this corresponds with how much damage is caused.
Another measure is magnitude – the total energy released over the duration of the quake.
The Richter magnitude scale was devised in the 1930s by American scientist Charles Richter as a way to measure tremors in California.
A modified version became standard across the world but failed to accurately measure a number of super-energetic “great” earthquakes that split the earth along a 600-mile rupture.
In the late 1970s it was replaced by the moment magnitude scale, which handles the biggest quakes better and is still used today.
The two systems work in similar ways and are roughly equivalent numerically, so some laymen still informally refer to the Richter scale when talking of magnitude.
How is earthquake magnitude measured?
Sensitive probes called seismographs detect shaking in the ground, which could be generated by a quake nearby or one on the other side of the world.
The amplitude (maximum motion) caused by the displacement waves are entered into a mathematical formula to determine the magnitude.
The scale is logarithmic, with each step equivalent to multiplying by 10 to the power of 1.5.
This means a 6.0 magnitude earthquake has approximately 32 times more energy than a 5.0 quake, and a 7.0 quake has 1,000 times more energy.
Seismologists say the likely effects of quakes with increasing magnitudes are as follows:
less than 2.0 = Micro earthquake. Not felt.
2.0-2.9 = Minor earthquake. Rarely felt. No damage to buildings.
3.0-3.9 = Minor earthquake. Often felt. May rattle items indoors but rarely causes damage.
4.0-4.9 = Light earthquake. Felt by most people. Objects may fall off shelves and light sleepers may be woken. Significant damage unlikely.
5.0-5.9 = Moderate earthquake. Felt by everyone. Can cause major damage to poorly constructed buildings over a small area.
6.0-6.9 = Strong earthquake. Felt hundreds of miles away. Violent shaking in central area causes moderate damage to well-built structures. Can be destructive in areas up to 100 miles across.
7.0-7.9 = Major earthquake. Damage to most buildings, some to the point of collapse, with destruction over large areas.
8.0-8.9 = Great earthquake. Severe damage likely to even the strongest buildings, with many collapsed. Can cause serious damage in areas 1,000 miles across.
9.0 and greater = Great earthquake. At or near total destruction with severe damage or collapse to all buildings. Heavy damage and shaking to distant locations. Causes permanent changes in the landscape.
Does the scale go up to ten?
No, it is not a number out of ten. There is no mathematical maximum.
There has never been an earthquake measured of magnitude 10.0 or above, although some scientists think it is possible.
Researchers estimate a disaster on that scale – which could hit once every 10,000 years – would shake the earth for an hour and trigger tsunamis for several days.
Every year there are up to 20 major earthquakes (magnitude 7.0 or above) and an average of one great earthquake of 8.0 or above.
The largest ever recorded was the Great Chilean Earthquake on May 22, 1960, which measured 9.5 and killed up to 7,000 people. Violent shaking lasted for ten minutes and caused a 35ft tsunami that struck as far away as Japan and Australia.
The deadliest ever was a 9.1 quake that struck off the coast of Banda Aceh, Indonesia, on Boxing Day 2004. It caused a tsunami that killed more than 240,000 in coastal areas around the Indian Ocean.
Another of 9.0 in Sendai, Japan, left 1,000 dead in March 2011.
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On August 5, 2018, Gary Barlow and Chrissy Teigen were among thousands of holidaymakers caught up in a magnitude 6.9 quake that hit Lombok and Bali.
Nearly 100 people died and 20,000 were evacuated.
In September 2017, at least 370 died after a 7.1 quake brought down buildings in Mexico City, one of the world’s earthquake hot spots.
In July 2018, a 3.1 magnitude tremor that hit Surrey and Sussex was the seventh in the Crawley area in three months.
Britain’s biggest ever was the 6.1 quake centred on Dogger Bank in 1931, which damaged buildings 60 miles away on the East Coast.
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