Earthquake Magnitude
The magnitude of an earthquake describes its size. There is only one magnitude for each earthquake. Scientists also talk about the intensity of shaking from an earthquake, and this varies depending on where you are during the earthquake.
The magnitude of an earthquake depends on the size of the fault and how far the plates move. This is not something scientists can measure with a ruler or measuring tape, since faults are deep beneath the earth’s surface. So how do they measure an earthquake? They use recordings (known as seismograms) made on the surface of the earth to measure motion and determine how large the earthquake was.
The recordings look like a series of wiggly lines that appear during ground vibration (see image below). A series of short wiggly lines means a small earthquake, and a series with long wiggly lines means a large earthquake. The length of the wiggle depends on the size of the fault, and the size of the wiggle depends on the amount of slip.
The magnitude of an earthquake describes how much energy was released based on the maximum motion recorded. There are different scales used to convey this information, but the moment magnitude scale, abbreviated MW, is preferred because it works over a wider range of earthquake sizes and is applicable globally.
Magnitudes are based on a logarithmic scale (base 10). What this means is that for each whole number you go up on the magnitude scale, the amplitude of the ground motion recorded by a seismograph goes up ten times. Using this scale, a magnitude 5 earthquake would result in ten times the level of ground shaking as a magnitude 4 earthquake (and 32 times as much energy would be released).