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Every day, the Earth shakes thousands of times. The vast majority of these earthquakes are far too small for anyone to feel, and they are detected only by sensitive instruments called seismometers. But when a large earthquake strikes, the first question people ask is: how big was it?
For much of the twentieth century, the answer was given on the Richter scale, developed in 1935 by the American seismologist Charles Richter. The scale is based on the size of the waves recorded by a seismometer, and it is logarithmic. This means that each step up the scale represents a tenfold increase in the size of the ground motion. In terms of the energy released, the difference is even greater: each whole number corresponds to roughly thirty-two times more energy. A magnitude 7 earthquake therefore releases about a thousand times more energy than a magnitude 5.
The Richter scale worked well for small and medium earthquakes in California, where it was designed, but it was less reliable for very large earthquakes and for those far away. Since the 1970s, seismologists have therefore preferred the moment magnitude scale, which is based on the physical size of the break in the Earth's crust and how far the rock moved. For most earthquakes, the numbers are similar to Richter's, which is why news reports often still mention his name.
Magnitude, however, tells us only how much energy an earthquake released. It says nothing directly about the damage. That depends on many other factors, including how deep the earthquake was, how close it was to towns and cities, the type of ground, and the quality of buildings. To describe the effects in a particular place, scientists use intensity scales, which are based on what people felt and how much damage was caused.
This explains why two earthquakes of similar magnitude can have very different consequences. A powerful earthquake deep beneath an empty region may cause little harm, while a smaller one directly under a crowded city with poorly built houses can be devastating.
The largest earthquake ever recorded struck Chile in 1960, with a magnitude of 9.5. It caused huge waves, called tsunamis, that crossed the Pacific Ocean and caused deaths as far away as Japan. Events like this have taught engineers and governments that preparation, such as strict building codes and warning systems, is the most effective way to reduce the human cost of earthquakes.