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The Power of Earthquakes

February 27, 2010

Last night a powerful earthquake struck the country of Chili.  It registered 8.8 on the Moment Magnitude Scale (MMS), making it much more powerful than the recent quake in Haiti that registered 7.0MMS.

News coverage shows the devastating power a 7.0 earthquake possesses, a large portion of Haiti is in ruins.  Still, understanding the scale is not as straight forward as other scales we use, like the ones on thermometers.  Growing up, I remember earthquakes measured in points on the Richter scale.  Now, the Moment Magnitude Scale, for major quakes, replaces that system.  Only a scientist is really interested in the difference between the two, for the general public, its best to simply understand the new scale provides a higher degree of accuracy.

What does a one digit increase, or the case of Chili vs. Haiti, a 1.8 digit increase, mean?  The numbers really don’t hold value on their own.  Unlike the thermometer, starting at a number like zero and increasing incrementally to something like 250, the measurement for earthquakes is measure with a logarithmic scale.  A logarithmic scale uses an index based on a formula rather than simply increasing in whole numbers.  In the case of the Moment Magnitude Scale, an increase of one number means the power increased 31.6 times.  That is difficult to conceptualize, think about it applied to measuring gallons of water.  If 1MMS equals 1 gallon of water, 2MMS equals 31.6 gallons or 31.6 times as much water.  3MMS does not double to equal 63.2 gallons but increases 31.6 times again to about 1,000 gallons.

If MMS Measured Water

MMS        Gallons

1              1.00
2              31.60
3              998.56
4              31,554.50
5              997,122.07
6              31,509,057.53
7              995,686,217.81
8              31,463,684,482.92
9              994,252,429,660.36
10             31,418,376,777,267.50
11             992,820,706,161,653.00
12             31,373,134,314,708,200.00

Another way to look at it, the average home swimming pool in the United States has about 22,000 gallons of water. In the gallon example above, that equals 3.7MMS.  It is easy to see that a one point increase in scale equates to a huge increase in volume.  To increase from one to over twenty-thousand gallons takes less than a two-point move.

Of course, the scale does not deal with gallons of water but the power of earthquakes.  Perhaps the best thing to remember from all this is a small increase up the scale means a huge increase in power.  Across the planet, there are a dozen or so earthquakes a day people can really feel, in the 4.0 to 6.0 range, they barely make the news.  A 7.0 quake destroyed much of Haiti.  The damage to Chili by this 8.8 one, though not known, is expected to be major.  Therefore, when the news reports an earthquake, remember there is more to the number than you might think.  It is not the same as increasing the temperature in your home by a degree or two.  A one point increases in the Moment Magnitude Scale can have disastrous results.

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