You may have heard of the Richter scale used to study earthquakes. In 1935 Charles Richter developed a system to measure the magnitude --or amount of energy released--of an earthquake. Each whole number on the Richter scale indicates a tenfold increase in amplitude (greatness in size). Thus, a 7.5 earthquake on the Richter scale actually has ten times the amplitude of a 6.5 earthquake. There is no upper limit on the Richter scale, meaning that it could be used to measure earthquakes of a ten or more magnitude if one ever occurred. The most devastating earthquakes we know of are 8 or 9 on the Richter scale.
Scientists also measure seismic waves, or movements in the earth's crust. Special machines called seismographs record movement in the earth, including earthquakes that are so low in magnitude that people cannot feel them. You can make a simple seismograph to demonstrate how this machine works.
Fill a 2-liter soda bottle with water and use wire to suspend it about 1' above the surface of a table, using a sturdy stick or ruler set across stacks of heavy books. (The bottle should hang freely between the books.)
Tape a sheet of paper with aluminum foil underneath it to the table, beneath the bottle.
Roll a felt-tip pen in a piece of paper, and tape it loosely enough for the pen to slide up and down.
Tape this roll to the side of the bottle so that the pen's tip touches the paper.
Shake the table back and forth, gently at first and then a little harder. (Don't move the table's legs; shaking is enough.)
When you're done, examine the paper. What kind of record is there of the 'quake'?
There's another measurement used for earthquakes: the Modified Mercalli Scale is used to measure intensity, or how strong the effects of the quake are. The intensity varies based on position relative to the epicenter of the earthquake, so one earthquake does not have a set number from the scale assigned to it as with the Richter scale. Intensity is measured in Roman numerals I-XII. For a list of effects at each level, visit www.geo.mtu.edu/UPSeis/Mercalli.html
If it isn't confusing enough with so many things to measure, there's one more method for determining magnitude. The Moment Magnitude scale is more accurate than the Richter scale for measuring large earthquakes. In the world's worst recorded earthquake--the 1960 one in Chile--the magnitude on the Richter scale was 8.5, but on the Moment Magnitude (Mw) scale it was 9.5.