Earthquakes happen not too frequently, and sometimes they cause serious natural disasters. Haiti just experienced a earthquake of 7.0 on the Richter Scale. Just to give you a sense of how powerful that is it is a major quake.
I'll go lay out the Richter scale and it's intensity.
Earthquakes less than 2.0 are not felt at all.
Earthquakes with the range between 2.0 -2.9 are recorded but you don't feel much.
Earthquakes in the range of 3.0-3.9 are considered minor, indoor shaking may occur.
Earthquakes in the 4.0-4.9 range, indoor noises are louder. Light damage occurs.
A quake in the 5.0-5.9 range is moderate and major damage to some buildings may occur. Strong buildings may sustain little damage.
A 6.0-6.9 Earthquake is strong and causes major damage to buildings.
A 7.0-7.9 Earthquake causes substantial building collapses, (ouch!)
A 8.0-8.9 Earthquake causes great damage, which stretches for hundreds of miles.
Quakes in the 9.0-9.9 range can cause destruction even thousands of miles away...
Just to give you an idea of how strong these earthquakes can be, an earthquake of 4.0 is 32 times stronger than that of a quake with a strength of 3.0
Today, you hear these ranges because the Richter Scale is being used. It was invented by Dr. Charles Richter to measure the magnitude of the earthquake by recording the seismic waves onto a scale.
Another question that may be raised from earthquakes is where the earthquake stared, this is called the epicenter. The epicenter may be determined by using seismographs. At least three seismographs are used to determine where the earthquake started. The distance between the epicenter and the seismograph is detected and a radar circles around the seismograph. The area where the three radar seismograph circles meet is the area of the epicenter. Here's one image:
Earthquakes don't usually occur that frequently, but when they do, they cause damage from minor to major, and it usually cost millions of dollars to fix damages. Earthquakes in the ocean can cause tsunamis, just like the one in the Indian Ocean in 2004.