I am going to explain what happens in an earthquake and how it is measured. When plates in the earth's crust grind against each other, often these plates fracture, or break. If this happens, an earthquake will most certainly occur. To help us measure the magnitude(size) of the quake, seismologists use the Richter Scale to give it a measurement between 3.5 and 8(check out the chart below). A 3.4 on the Richter Scale wouldn't be felt by most people, but an 8 would most definitely be felt! If you enjoyed hearing about the Richter Scale, you should check out the Mercalli Scale and investigate the difference.
Less than 3.5 Generally not felt, but recorded.
3.5-5.4 Often felt, but rarely causes damage.
Under 6.0 At most slight damage to well-designed buildings.
Can cause major damage to poorly constructed buildings
over small regions.
6.1-6.9 Can be destructive in areas up to about 100 kilometers
across where people live.
7.0-7.9 Major earthquake. Can cause serious damage over larger areas.
8 or greater Great earthquake. Can cause serious damage in areas several
hundred kilometers across.