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 the California Earthquake Authority:
3.0 - 3.9
Minor, may be felt.
4.0 - 4.9
Light, likely felt.
5.0 - 5.9
Moderate, some minor damage may occur.
6.0 - 6.0
Strong, damage likely.
7.0 - 7.9
Major, widespread damage expected.
8.0 or higher
Great, significant damage.
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.