Q: Why are earthquake magnitudes always measured on the Richter scale and not in Richters? We never say 鈥�240 on the Volta scale鈥� or 鈥�200 degrees on Baron Kelvin鈥檚 scale鈥�.
A: The Richter scale for earthquakes is an arbitrary one based on the logarithmic scale of measurements of seismic waves recorded by seismographs.
Other examples of scales are the Beaufort scale of wind force based on the observation of the wind-blown movement of objects on land or the nature of waves at sea, and the Mohs scale of hardness based on an arbitrary set of substances from talc to diamond.
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The volt and the kelvin are absolute units and can be expressed in a simple relationship to other units. For example, the kelvin is the rise in temperature when 1 calorie is absorbed by 1 gram of water. Because they represent real things, like mass or energy, they are expressed as simple abbreviations. Degrees Celsius are an intermediate, for each degree celsius is 1 kelvin, but the starting point at 273.15 kelvin is arbitrary. The scales could be replaced by absolute measures, as the Beaufort has been by kilometres per hour, but the intervals between each point on the absolute scale would be very irregular.
A: Earthquake magnitudes are not always measured on the Richter scale. Although widely used since 1935, Charles Richter鈥檚 system has now been superseded by the moment-magnitude scale, which is considered more accurate.
The Richter scale measures the quake鈥檚 magnitude by using a seismograph to measure seismic waves, the type of energy released in an earthquake. The moment-magnitude scale measures the total energy released in the quake and is calculated in part by multiplying the area of the fault rupture surface by the distance the Earth moves along the fault.
A: It would be easier and save seismologists a lot of trouble if earthquake sizes were quoted simply in Richters. Many scales (such as the Beaufort scale) comprise a set number of integral values or points, so we are often asked if an earthquake magnitude is on a 鈥渟cale of 10鈥�.
In fact, earthquake intensities, which describe earthquake effects at particular places, are given on such scales, usually from 0 (not felt) to 12 (total destruction). Magnitude, however, is a measurement related to energy release and can be given to any appropriate precision. It is not restricted to integral values and has no upper limit imposed by the scale. It is thus sometimes referred to as 鈥渙pen-ended鈥�, but as the scale is logarithmic, any quake of magnitude much more than 10 destroy the whole Earth.