Earthquakes are measured using seismographs, which monitor the seismic waves that travel through the Earth after an earthquake has occurred.
Scientists have used the Leeb scale for many years, but now mostly use the Modified Mercalli scale, which the USGS considers a more accurate measure of size.
The Richter scale measures magnitude, while the MMI scale measures intensity.
Here are the magnitude scales, according to the California Earthquake Service:
3.0 – 3.9
Minor, may be felt.
4.0 – 4.9
Light, may feel it.
5.0 – 5.9
Medium, some minor damage may occur.
6.0 – 6.0
strong, may be damaged.
7.0 – 7.9
Significant, widespread damage is expected.
8.0 or higher
Huge, significant damage.
According to the USGS, here are the MMI ratings:
I Few, if ever, felt it.
Two, weak: It was felt by several people.
three weak: It is felt by multiple people, usually indoors and upstairs.
Four, lamp: Many people feel it indoors, few people feel it outdoors.
ask ease: Almost everyone feels it.
six Strong: Everyone feels it. Some minor damage.
seven very strong: Causes considerable damage to poorly built structures.
Eight serious: Causes considerable damage to standard buildings and wreaks havoc on poorly built structures.
Nine Violence: Buildings are removed from their foundations. There are obvious cracks in the ground and extensive damage.
x extreme: Most of the masonry was destroyed.