January 14 – Did you feel it?

Today’s Factismal: There were nearly 4,000 earthquakes today.

But don’t worry. There were that many earthquakes yesterday, too. And there will be that many tomorrow as well. The fact is that every year, there are nearly one and a half million earthquakes across the globe.

One year of earthquakes across the world. (Image courtesy the USGS)

One year of earthquakes across the world. (Image courtesy the USGS)

These earthquakes happen because the Earth is very slowly cooling down. Radioactive decay in the mantle (the thick solid section between the liquid outer core and the crust) and solidification of the outer core create heat inside the Earth. That heat, plus a little “fossil heat” from the Earth’s formation, creates convection in the mantle. And the motion of the mantle drives motion of the Earth’s crust, breaking it into large rigid sections called plates. As the plates collide to form mountain ranges or scrape alongside in transform zones, they release energy as earthquakes.

The different layers of the Earth. Only the outer core is molten; everything else s solid!

The different layers of the Earth. Only the outer core is molten; everything else s solid!

And what a lot of energy they release! A magnitude 2.5 earthquake will give off enough energy to power a home for 14 hours, and there are nearly 1,300,000 earthquakes that large every year. Even better, the energy goes up much faster than the magnitude. A magnitude 4 earthquake gives off enough energy to power a home for 1.6 years. Fortunately, the number of earthquakes also decreases faster than the magnitude; there are only about 13,000 magnitude 4 earthquakes every year.

There are a lot more small earthquakes, but the big ones release a lot more energy!

There are a lot more small earthquakes, but the big ones release a lot more energy!

And that relationship between energy and magnitude is why we can’t prevent a large earthquake by triggering a lot of small ones. It takes about 33 magnitude 7 earthquakes to release the same energy as one magnitude 8. So let’s suppose that you live in a place where you get a magnitude 8 about once every hundred years. You’d need to have a magnitude 7 every three years to release the strain. Or you could do it with a magnitude 6 every month. Or a magnitude 5 every day. Or a magnitude 4 every 45 minutes. Or a magnitude 3 every minute. Obviously, this is not a good idea.

What is a good idea is keeping up with the most recent earthquakes using either the Rapid Earthquake Viewer or the USGS Earthquake Monitor. And please contribute to science by telling the USGS if you felt the earth move!
http://earthquake.usgs.gov/earthquakes/dyfi/

2 thoughts on “January 14 – Did you feel it?

  1. I am uneasy at seeing a round core called a ‘layer’. I am almost as uneasy at seeing concentric shells that were not ‘laid’ one atop the other in a sedimentary sense called ‘layers’. Asthenosphere should appear with lithosphere, not between crust and mantle. Otherwise, an excellent site, I am posting the link for my students. I like the rapid earthquake viewer.

    • There is a long history of calling the core and the other regions of the Earth “layers”; for example, one of the most important papers in seismology is “The density variation of the earth’s central core” (K. E. Bullen, Bulletin of the Seismological Society of America, January 1942, vol. 32, no. 1, 19-29) which defined the D’’ (“D double prime”) layer that we now usually refer to as the core-mantle boundary. Though sedimentary layers are the ones that most people think of, geologists and geophysicists use the term somewhat more broadly.

      As for the aesthenosphere, there is a confusion of terms in geophysics. And it didn’t help that I didn’t note that the diagram showed the layers that are defined by their chemical properties (crust, aesthenosphere, mantle {which can be subdivided further}, core-mantle boundary, outer core, and inner core) as opposed to the ones that are defined by their mechanical properties (lithosphere, aesthenosphere, mantle, outer core, inner core). You may notice that some of those layers appear in both lists; that’s because the chemical differences (such as the slight amount of water in the aesthenosphere) lead to mechanical differences (the weakness of the aesthenosphere).

      Thank you for letting folks know about the site! If there are any topics that you’d like to see covered, please drop me a line!

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