How are earthquakes recorded? How are earthquakes measured? How is the magnitude of an earthquake determined?

Earthquakes are recorded by a seismographic network. Each seismic station in the network measures the movement of the ground at that site. The slip of one block of rock over another in an earthquake releases energy that makes the ground vibrate. That vibration pushes the adjoining piece of ground and causes it to vibrate, and thus the energy travels out from the earthquake hypocenter in a wave.

There are many different ways to measure different aspects of an earthquake:

• Magnitude is the most common measure of an earthquake's size. It is a measure of the size of the earthquake source and is the same number no matter where you are or what the shaking feels like. The Richter scale is an outdated method for measuring magnitude that is no longer used by the USGS for large, teleseismic earthquakes. The Richter scale measures the largest wiggle (amplitude) on the recording, but other magnitude scales measure different parts of the earthquake. The USGS currently reports earthquake magnitudes using the Moment Magnitude scale, though many other magnitudes are calculated for research and comparison purposes.
• Intensity is a measure of the shaking and damage caused by the earthquake; this value changes from location to location.

Learn more:

• Earthquake Magnitude, Energy Release, and Shaking Intensity
• UC Berkeley Seismo Lab Earthquake FAQ
• How Big Was That Earthquake?