Magnitude

jill's picture

Measuring the size of an earthquake is a complicated process, which is the main reason most of us have trouble understanding just how much power is released by an earthquake of a certain “magnitude”. I searched around for the simplest explanation possible. If you understand a little more about the potential power that can be unleashed during an earthquake, then you might also appreciate how important it is to be prepared.

The most common way scientists use to communicate the size of an earthquake is to convert the total energy released into a range of “magnitude” numbers. In general, if you can feel the ground move during an earthquake, the magnitude assigned to it will be greater than 2.5 or 3, which is considered “minor”. Minor earthquakes are measured by sensitive instruments (called seismometers), but generally not “felt”. Almost a million minor earthquakes a year are recorded by seismometers.

"Light" to "moderate" earthquakes may cause slight damage to buildings, dams, reservoirs, and other manmade structures. These would be earthquakes that are between magnitude 4 to 5.9, and about 30,000 of these events occur each year worldwide. A “strong” earthquake of between magnitude 6 to 6.9 happens about 100 times a year somewhere in the world, and these can cause a lot of damage in urban areas. “Major” earthquakes, usually measured between magnitude 7 to 7.9, will cause serious damage in populous regions. About 20 of these occur worldwide per year. A “great” earthquake can completely destroy whole communities. Great earthquakes are anything measured above magnitude 8, and only occur once every few years.

One way to understand how much energy is released by earthquakes is to compare them with other ways we produce energy. For example, a magnitude 1 seismic wave releases as much energy as blowing up 6 ounces of TNT. A magnitude 7 earthquake releases as much energy as 8 million gallons of gasoline or enough energy to heat 20,000 single family homes per year in the U.S. A magnitude 8 earthquake releases as much energy as detonating over 8 million tons of TNT. For comparison, the two atom bombs dropped on Japan to end World War II had TNT equivalents of 13,000 tons on Hiroshima and 21,000 tons on Nagasaki.

If you feel an earthquake, or if you hear about one that happens anywhere in the world, you can go online and either report what you felt, or you can see what others have reported in an area that recently shook. A website called “Did You Feel It?”  is managed by the U.S. Geological Service (USGS), and features lots of other great information on earthquakes as well.

Now you know a little more about why it’s important to figure out what the earthquake potential is in your own community. If you live in an area that could experience anything above a magnitude 5, you should make sure you have taken steps to prepare in order to protect your home and your loved ones. Use OpenHazards tools such as the "Hazard Viewer" and "Home Response" to determine your risk today!

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