After every earthquake you hear two words: magnitude and intensity. People often use them as if they meant the same thing, but they answer different questions. Magnitude asks how big the earthquake was. Intensity asks how strongly it was felt in a particular place.

Magnitude: one number for the earthquake

Magnitude describes the size of the earthquake at its source. It is calculated from instrument records, called seismograms, and it does not depend on who is standing where. A single earthquake has one true magnitude, whether you sit next to the fault or on the other side of the country.

In practice, each agency computes its own estimate from the stations it has, so you may see slightly different values for the same event. That is normal and is explained in a separate article about where earthquake data comes from.…