Measuring Earthquakes

Earthquakes are not all the same size, and to make sense of them we need a way to measure them. The book gives us two distinct ways of doing this, and the key point is that they measure completely different things. One measures the energy released at the source of the quake; the other measures the damage it actually causes at the surface. Confusing the two is a common error, so keep that distinction clear from the start.

The first method is the magnitude scale, better known as the Richter scale. Magnitude is a measure of the energy released during the earthquake itself. It is expressed as a number on a scale that runs from 0 to 10. Because it measures the physical energy of the quake, a single earthquake has one magnitude, no matter where you are when you feel it. The number tells you how powerful the shaking was at its origin.

The second method is the intensity scale, named after Mercalli, an Italian seismologist. Unlike magnitude, intensity does not measure energy. Instead, it takes into account the visible damage caused by the event — how buildings, roads, and the ground itself respond. The intensity scale runs from 1 to 12. A quake's intensity is not a single fixed number; it varies from place to place, depending on how far you are from the epicentre and how solid the local ground is. The same earthquake could be intensity 12 near the epicentre and intensity 3 far away.