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The Way We Measure Earthquakes is Stupid

posted by janrinok on Friday August 14 2015, @06:15PM   Printer-friendly
from the shake,-rattle-and-roll dept.

AnonTechie writes:

THIS WEEKEND, A 3.3-magnitude earthquake rattled San Francisco ever so slightly. The small quake, like so many before it, passed, and San Franciscans went back to conveniently ignoring their seismic reality. Magnitude 3.3 earthquakes are clearly no big deal, and the city survived a 6.9-magnitude earthquake in 1989 mostly fine—how how much bigger will the Big One, at 8.0, be than 1989?

Ten times! As smarty-pants among you who understand logarithms may be thinking. But...that's wrong. On the current logarithmic earthquake scale, a whole number increase, like from 7.0 to 8.0, actually means a 32-fold increase in earthquake energy. Even if you can mentally do that math—and feel smug doing it—the logarithmic scale for earthquakes is terrible for intuitively communicating risk. "It's arbitrary," says Lucy Jones, a seismologist with the US Geological Survey. "I've never particularly liked it."

[Suggested New Earthquake Scale]: Seismological Review Letters

Maybe SN could suggest a better way to measure earthquakes ...

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  • (Score: 5, Interesting) by takyon on Friday August 14 2015, @06:45PM

    by takyon (881) <takyonNO@SPAMsoylentnews.org> on Friday August 14 2015, @06:45PM (#222947) Journal

    Maybe SN could suggest a better way to measure earthquakes ...

    3.3 magnitude = 5.6 gigajoules = 5.6 x 109 J = 1.34 tons of TNT
    6.9 magnitude = 1.4 petajoules = 1.4 x 1015 J = 340 kilotons of TNT
    7.0 magnitude = 2 petajoules = 2.0 x 1015 J = 480 kilotons of TNT
    8.0 magnitude = 63 petajoules = 6.3 x 1016 J = 15 megatons of TNT

    https://en.wikipedia.org/wiki/Richter_magnitude_scale [wikipedia.org]
    http://www.wolframalpha.com/ [wolframalpha.com]

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  • (Score: 0, Disagree) by Anonymous Coward on Saturday August 15 2015, @01:57AM

    by Anonymous Coward on Saturday August 15 2015, @01:57AM (#223108)

    Yes, but from the article:

    Many years ago, Hugo Benioff proposed describing earthquake size with energy. It never took hold in part because we could never agree just how much energy is released.

    But wait, you say, she's wrong because it is right there in the Wiki! I read it on the Internet, so it must be true.

    • (Score: 2) by takyon on Saturday August 15 2015, @04:43AM

      by takyon (881) <takyonNO@SPAMsoylentnews.org> on Saturday August 15 2015, @04:43AM (#223143) Journal

      If you had continued reading the paragraph you quoted, you would have found these bits:

      I have hoped that seismic moment could deliver us from The Scale. It has everything a scientist could love, a physical reality that connects field geology to geodesy to seismology. Just tell them the moment and we won't have to explain that this earthquake is really much larger than that other earthquake, even though M 7 does not seem that much more than M 5. But deliverance has not come. We seismologists all cling to The Scale even though calculating seismic moments is now routine. The first thing we do with a seismic moment is use it to calculate a moment magnitude, and we are back where we started, without units, saying, “Well, actually the energy goes up as the square root of 1,000....” What is this power that The Scale holds over us?

      Back to evil Wikipedia [wikipedia.org]:

      The moment magnitude scale (abbreviated as MMS; denoted as MW or M) is used by seismologists to measure the size of earthquakes in terms of the energy released. The magnitude is based on the seismic moment of the earthquake, which is equal to the rigidity of the Earth multiplied by the average amount of slip on the fault and the size of the area that slipped. The scale was developed in the 1970s to succeed the 1930s-era Richter magnitude scale (ML). Even though the formulae are different, the new scale retains the familiar continuum of magnitude values defined by the older one. The MMS is now the scale used to estimate magnitudes for all modern large earthquakes by the United States Geological Survey.

      The author wants to use a unit called Aki, with 1 Aki equal to 1021 dynes-cm or 1014 N-m. Hmm... N-m... aka a newton-meter... what could that be a measurement [wikipedia.org] of?

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      • (Score: 2) by maxwell demon on Saturday August 15 2015, @09:55PM

        by maxwell demon (1608) on Saturday August 15 2015, @09:55PM (#223366) Journal

        So 1 Aki = 100 Terajoule? Then why not simply say the latter?

        --
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        • (Score: 2) by takyon on Saturday August 15 2015, @10:02PM

          by takyon (881) <takyonNO@SPAMsoylentnews.org> on Saturday August 15 2015, @10:02PM (#223368) Journal

          I would define the Aki to be equal to 1021 dynes-cm (or 1014 N-m for the SI police). The smallest earthquakes routinely recorded by most networks (M 2.0), would be about 0.01 Akis, a barely damaging earthquake (M 5) would be 400 Akis, Northridge would be 120,000 Akis, and the great Chilean earthquake would be two hundred billion Akis. Essentially every earthquake we would ever talk about would involve a range of numbers similar to our monetary system. From a penny for our thoughts to a 1980's United States federal deficit, these are numbers that may not fit on our hands but are ones we can live with.

          Lucy Jones seems to want to normalize the imperceptible tremors to a fraction of 1 unit, small earthquakes from 1 to low thousands of units, and then hundreds of thousands all the way to billions for the actual and biggest earthquakes. But it's all joules in the end.

          Jones wants this unit so that the public and journalists can understand earthquake strength more intuitively than "7.0 mag is 32 times stronger than 6.0 mag".

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          • (Score: 2) by maxwell demon on Sunday August 16 2015, @10:34AM

            by maxwell demon (1608) on Sunday August 16 2015, @10:34AM (#223489) Journal

            Let's rewrite his statement to Terajoule:

            The smallest earthquakes routinely recorded by most networks (M 2.0), would be about 1 Terajoule, a barely damaging earthquake (M 5) would be 40,000 Terajoule, Northridge would be 12 million Terajoule, and the great Chilean earthquake would be twenty trillion Exajoule.

            So how are his Akis better, again? Heck, even the "reporting threshold" is perfect with Terajoule

            --
            The Tao of math: The numbers you can count are not the real numbers.