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 the 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 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 The Richter scale is an outdated method that is no longer used that measured the largest wiggle on the recording, but other magnitude scales measure different parts of the earthquake.
Intensity is a measure of the shaking and damage caused by the earthquake, and this value changes from location to location.
UC Berkeley Seismo Lab FAQ on How do seismologists measure earthquakes?
UC Berkeley Seismo Lab FAQ on Why are there so many magnitudes?