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The Richter Scale is a way of measuring earthquakes and talking about the magnitude of earthquakes without having to use enormously big numbers. I'm going to explain what the Richter Scale numbers mean and how you can understand earthquake news reports as easily as weather reports giving the temperature in so-many degrees Celsius or degrees Fahrenheit.
First, though, I've got to explain the basis for "logarithmic scale" values, because unlike weather report temperatures, the power of earthquakes varies by vast amounts, some being a million times bigger than others. An elephant might be described as "big", but then if you wanted to talk about a mountain, and how "big" that is in relation to an elephant, putting simple numbers on it would be tricky. This is where logarithmic scales come in handy.
To illustrate this, let's use the imaginary "Rich Scale"! This is a notional way of talking about how rich someone is without using big-sounding words like "million" or "thousand". To express the magnitude of richness of a person on a scale with simple numbers, you could use the concept involved in terms like "six figure amount", so if someone is a millionaire, they could be said to be "magnitude 6 on the rich scale", whereas if someone's fortune was a hundred million, they would be "magnitude 8 on the rich scale". Even less well-off people could be described on this notional "rich scale", for example someone who'd got 1000 in their bank account would still be "magnitude 3 on the rich scale".
Although this "rich scale" is something I made up, and you don't generally hear money being talked about like that, it illustrates how the Richter Scale really works, because earthquakes really are measured like that! Here's how it works:
A magnitude zero earthquake is rated as one in which the earth's surface quakes/moves one micro-metre at a distance of 100 kilometres from the epicentre of the earthquake (epicentre = "ground zero" for earthquakes) (one metre = 1 million micro-metres). As with the "rich scale", magnitude 1 is ten times as big, so it would cause the ground to move about 0.01mm at 100km from the centre. A magnitude 2 earthquake would cause movement of 0.1mm, and a magnitude 3 would cause movement of one millimetre at that distance. By the time you get to magnitude 4 earthquakes, they are starting to be noticed, because 10mm (1centimetre) of movement is noticeable. A magnitude 5 earthquake causes 10 centimetres of ground shake at 100km away, and that is usually enough to be on the news. Magnitude 6, with its associated one metre of quaking (at 100km distant) is likely to cause buildings to collapse. In a magnitude 7 earthquake, it's best not to be within 100km of it, and seismographs of the type used for calibrating the Richter scale don't work above magnitude 6.3 , so some estimation has to be done. With the larger magnitude earthquakes the survival rate for people and seismometers near the event is poor, so estimates then have to be based on how much displacement is measured a few thousand miles away, or how big a tsunami is caused, etc.
To sum this up so far, the numbers on the Richter scale measure the magnitude of earthquakes by the order of magnitude of the amount of earth-movement caused at a distance from the epicentre of the event.
This is different from the amount of energy released, so if you want to compare earthquakes with nuclear explosions measured in megatons, you need to consider that energy doesn't translate directly into distance, because it spreads out. In a practical sense, it takes 1000 times as much energy to cause 100 times as much movement. Or to put it another way, for each extra Richter magnitude number multiply the distance moved by 10 and the energy by 32. Even so, a magnitude 4 earthquake is equivalent to 1 kiloton of bomb explosion, and a magnitude 7 is equivalent to 1 megaton. Note: the bombs cause more damage per energy as they go off on the surface or up in the air, and there is blast and fallout, whereas the earthquakes are so-many miles underground and the damage is caused by the shaking-about of the ground surface, and any other problems such as fire from burst gaspipes etc.
A one megaton bomb is approximately equivalent to a magnitude 7 on the Richter Scale. Be careful when doing calculations based on this, as a 10 megaton bomb would be equivalent to magnitude 8. This is also assuming the bombs were exploded underground, which they seldom are!
Displacement = 10[Richter Number] micro-metres (at 100km away from the epicentre)
Energy = 10((1.5 x [Richter Number]) - 9) megatons.
At the time of the Market Rasen earth tremor 2008, I was about 100km from Market Rasen, and on the news they said it was a Richter scale magnitude of 5.2 , so I should have felt an earth movement of 105.2 micro-metres = 15.8 centimetres (says the scientific calculator). I thought at the time it was probably about 3-4in, so that's near enough to say the method is good. Energy = 63 kilotons equivalent underground nuclear test... I'm sure the people of Market Rasen would be happier with it being an earthquake rather than a bomb!
* The Richter Scale was invented by Charles Richter who was a geologist in California some time ago. The name is pronounced with a Germanic splitch in the middle so the "ch" sounds like it does in Scottish "loch".
* The Richter Scale is nothing to do with the Red Baron or with the stuff that forms in kettles, those being Von Richthofen and lime scale respectively.
* There is no gadget that directly gives readings "on the Richter scale". Seismometers give readings of displacement at that point, but to assess where on the Richter scale an earthquake is requires calculation from readings from several seismometers in different places. It's a bit like the way barometers don't tell you the exact size of a storm! Also, thermohygrographs measure temperature and Relative humidity, not anything Absolute!
* There's no such thing as "OFF the Richter scale". It's not limited to between 0 to 10, any more than temperature is limited to 0-100 degrees C. Earthquakes of Richter scale numbers higher than 10 can happen, and if they do happen, then anyone who is still alive can speculate about the numbers while they try to survive in the post-cataclysmic world.
* Small tremors such as magnitude 1 or magnitude 0 can occur, being minor earth-moving events, and, for even smaller movements, negative numbers on the Richter scale can also appear, and represent events of various levels of softness. Zero on the Richter scale is equivalent to 6 ounces of explosives, -1 (being a tenth of that) would be more like hitting the concrete with a sledgehammer very hard, -2 ... dropping half a brick, and -3 a cat landing softly from off a table. If you like you can figure out what negative magnitude would be represented by a pin drop.
* It's worth asking your insurance company if your home insurance policy covers earthquakes. It probably does. It's a good bet from an insurance perspective, as earthquakes are rare, at least those big enough to cause much damage, in most places.
* Unlike the weather, earthquakes aren't predictable, although the risk profile can be assessed from past form. So, enjoy San Francisco and Los Angeles while you still can!