Magnitude and intensity are not the same number

When an earthquake happens you will see a single number attached to it - M6.2, say. That number is the magnitude, and there is exactly one of it per earthquake. What you felt where you were standing is a different measurement called intensity, and it has a different value in every town.

Magnitude: one number for the whole earthquake

Magnitude describes the size of the earthquake itself - the event at the fault, not the experience of it. The USGS puts it plainly: an earthquake has a single magnitude.

The scale is logarithmic, which is why the numbers feel misleadingly close together. According to the USGS, each whole number step is a tenfold increase in the amplitude measured on a seismogram, and roughly 32 times more energy released.

Intensity: a different number in every place

Intensity describes the shaking at a particular location. The USGS notes that it varies from place to place, depending mostly on distance from the fault rupture area - which is why one earthquake produces a whole map of intensities rather than a single figure.

In the United States this is reported on the Modified Mercalli Intensity scale, written in Roman numerals. The USGS describes it as traditionally a subjective measure, derived from human observations and reports of felt shaking and damage, though instrumental data can also be used to estimate it.

Why the Richter scale is not the answer any more

The Richter scale is the one most people have heard of, but the USGS says it is not commonly used any more except for small earthquakes recorded locally. For everything else it uses moment magnitude, which it describes as a more accurate measure of earthquake size.

The reason is a real limitation rather than a preference. For very large earthquakes the USGS notes that some amplitude-based magnitudes overestimate the true size and some underestimate it, so the agency now works from measurements of the physical effects of the earthquake instead.

Why the number sometimes changes afterwards

A magnitude published in the first minutes is a preliminary estimate from whichever stations had reported by then. As more data arrives the solution is refined, and the published figure can move - usually by a tenth or two, occasionally more for a large or distant event.

Different agencies can also publish different magnitudes for the same event at the same moment, because they are using different station networks and sometimes different magnitude types. That is why a quake can appear twice with slightly different numbers depending on where you look.