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Measuring the Strength of Earthquakes

When earthquakes happen, the vibrations in the Earth are recorded on very sensitive instruments called seismographs. The seismograph records the movement with zig zag lines. The strength of an earthquake is measured by looking at the zig zag lines on the seismograph and measuring their size. A special mathematical formula is used to convert the size of the lines into a number. This method was developed by Charles Richter and is known as the Richter magnitude scale. This scale represents the strength of an earthquake using a whole number and decimal fraction (a magnitude 6.3 for example). Each whole number increase in magnitude represents an increase of 10 times in the size of the lines recorded by the seismograph. You would think then that a magnitude 7.3 earthquake would release 10 times more energy than a magnitude 6.3 earthquake. Actually, because of the nature of the scale, a 7.3 earthquake releases about 31 times more energy!

Below is a general description of the relative strength of earthquakes ranging from a magnitude 1 (not felt) to a magnitude 9 (EXTREMELY strong). There is not upper limit to the scale (which means there could be magnitude 10 or more earthquake) but none have been measured larger than the magnitude 9.5 in Southern Chile in 1960.

Richter Magnitude Scale Definitions

 M=1 to 3 Usually not felt but recorded. M=3 to 4 No damage, but often felt. M=5 Some damage, felt a lot. M=6 A lot of damage, old buildings destroyed. M=7 Big earthquake causes serious damage, usually happens in Taiwan, Turkey, Japan and California. M=8 Great earthquake destroys lots of buildings. Earthquakes felt over an area of about 1000 km. M=9 Dangerous earthquake, big damage, destroys large region, earthquakes measures larger than 1000 km.