What is the magnitude of an earthquake and how is it measured?

The magnitude of an earthquake is a measure of the energy it releases. This means that it doesn't matter that the earthquake might not "feel" as strong farther away from its source; the magnitude just depends on the earthquake's total energy.

The more energy released by an earthquake, the higher the magnitude. The magnitude scale is logarithmic — moving up a level of magnitude means that the strength of the earthquake multiplies, in this case, by a factor of about 31. The energy of a magnitude 4 earthquake is 31 times the energy of a magnitude 3 earthquake. The energy of a magnitude 5 earthquake is 31 times the energy of a magnitude 4 earthquake.

Earthquake magnitudes are determined from seismic waves, the ground-bending waves generated by the earthquake fault. The energy in these waves lessens with distance. As you move farther from the earthquake fault, the intensity of the shaking decreases. At a fixed distance from the fault, the larger the earthquake magnitude, the greater the shaking.

In 1935, Charles Richter was the first western scientist to publish the earthquake magnitude scale, later called the Richter scale. Richter was inspired to develop the Richter scale by the magnitude scale for brightness of stars used in astronomy. In collaboration with Beno Gutenberg, he proposed the Gutenberg-Richter magnitude-frequency statistical law for earthquakes. This law states that there are fewer stronger earthquakes than weaker earthquakes — about 90% fewer per level of magnitude. For instance, the law states that the number of magnitude 5 earthquakes is about 10% of the number of magnitude 4 earthquakes, the number of magnitude 6 earthquakes is about 10% of the number of magnitude 5 earthquakes, and so on.

Richter realized that by analyzing the seismic waves recorded on many seismograms at various distances, he could determine what he called the magnitude of the earthquake. For this purpose he used a specific type of seismograph called the "Wood-Anderson Torsional" seismograph. This early type of seismograph has some important limitations, in that it only allows certain frequencies of earthquake waves to be recorded.

Today, we determine earthquake magnitudes in a different way than Richter's method. This newer magnitude scale is called the "Moment Magnitude" scale, and was developed by Thomas Hanks and Hiroo Kanamori in 1979. We use broad band digital seismometers that allow all wave frequencies to be measured. We also use models of the earthquake process to match the waves generated by the earthquakes.

With this method, we can be sure that the earthquake magnitude we compute is as accurate as possible.