Earthquakes are measured using seismographs, which monitor the seismic waves that travel through the Earth after an earthquake strikes.
Scientists used the Richter Scale for many years but now largely follow the Modified Mercalli Intensity Scale, which the U.S. Geological Survey says is a more accurate measure of size.
The Richter scale measures magnitude, whereas the MMI scale measures intensity.
Here are the magnitude classes, according to Michigan Tech:
2.5 or less
Typically not felt, but can be recorded by seismograph.
2.5 to 5.4
Often felt, but only causes minor damage.
5.5 to 6.0
Slight damage to buildings and other structures.
6.1 to 6.9
May cause significant damage in very populated areas.
7.0 to 7.9
Major earthquake. Widespread damage.
8.0 or greater
Great earthquake. Can cause total destruction near the epicenter.
And this is the MMI scale, according to the US Geological Survey:
I Felt by very few people, if at all.
II Weak: Felt by a few people.
III Weak: Felt by several people, typically indoors and on upper floors.
IV Light: Felt by many indoors, few outdoors.
V Moderate: Felt by almost everyone.
VI Strong: Felt by everyone. Some slight damage.
VII Very strong: Considerable damage to poorly built structures.
VIII Severe: Considerable damage to standard buildings, severe damage to poorly built structures.
IX Violent: Buildings shifted off foundations. Noticeable cracks in the ground, widespread destruction.
X Extreme: Most masonry structures destroyed.