The Richter scale was developed by seismologist Dr Charles Richter in 1935 to provide a single objective number to describe the strength of an earthquake.
Before that, earthquakes were just described as 'violent' or 'strong', which made it hard to compare earthquakes around the world.
The Richter scale is based on the maximum amplitude of the trace on a seismograph – those devices with a pen that twitches across a roll of paper, the ones you see in disaster movies.
It’s also logarithmic, so moving from a magnitude 4 earthquake (where books fall off shelves) to a magnitude 5 (involving some damage to poorly constructed buildings) means the seismograph needle’s twitches are 10 times bigger and the energy released is around 32 times greater.
The problem is, the Richter scale was designed around a very specific kind of seismograph, that isn’t used anymore and doesn’t measure very large or very distant earthquakes very well.
This means that earthquakes larger than magnitude 7 on the Richter scale don’t result in readings that grow proportionally with the actual energy released (and therefore with the likely damage to buildings and infrastructure).
In 1979, Prof Hiroo Kanamori and Dr Thomas C. Hanks proposed an alternative measure called the ‘moment magnitude scale’. This measures the strength of the much slower vibrations with periods of 30 seconds to a couple of minutes.
Although these frequencies don’t carry most of the earthquake’s energy, they penetrate better through the crust and scale more evenly with the size of the earthquake.
So, they provide a more reliable way to predict the energy of the earthquake using instruments that might be far from the source.
Since the early 2000s, all major seismology agencies have switched to using the moment magnitude scale.
Although, ironically, this scale doesn’t scale well for very small earthquakes, so seismologists will still sometimes use the Richter scale for earthquakes below magnitude 1.
This article is an answer to the question (asked by Britt Gill, Peterborough) 'What's the best way to measure an earthquake?'
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