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posted by chromas on Tuesday November 20 2018, @05:36PM   Printer-friendly

Phys.org:

World War II-era oil pumping under Los Angeles likely triggered a rash of mid-sized earthquakes in the 1930s and 1940s, potentially leading seismologists to overestimate the earthquake potential in the region, according to new research published in AGU's Journal of Geophysical Research: Solid Earth.

Six independent earthquakes and two aftershocks of magnitude 4.4 to 5.1 shook L.A. between 1935 and 1944, a rate of about one every two years. The area also experienced a higher rate of low-intensity earthquakes during that time frame. After 1945, the rate dropped to one moderate earthquake every seven years.

The new study re-examined historical information about the earthquakes from archived newspaper reports of earthquake damage, postcard questionnaires collected by the Coast and Geodetic Survey, and other sources pulled from old files and the dusty corners of cyberspace.

The authors used this information to refine the earthquake locations identified by early earth-motion sensors. The first seismometers in the United States began monitoring earthquake activity in the L.A. region in 1932. It is known that these early instruments could err in locating earthquake epicenters by tens of miles because of the limitations of their clocks, and because the low-sensitivity instruments were so few, according to the study's authors. Extrapolation of earthquake location from seismometer recordings relies on accurate time measurements.

The oil companies active at the time associated their activities with increased earthquakes, because the quakes would often sever pipes and shutdown production for months.


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  • (Score: 2) by deimtee on Wednesday November 21 2018, @04:56AM (9 children)

    by deimtee (3272) on Wednesday November 21 2018, @04:56AM (#764591) Journal

    If you have an ideal spring that stretching 1cm takes 1 joule, then stretching 1 cm and releasing it 10 times will take 10 joules of energy. If you stretch that same spring 10 cm in one go and release it you will use 100 joules.

    I'm afraid I don't understand how that is relevant. Ground under stress doesn't act like an ideal spring. There is considerable deformation. It can even flow like a fluid with enough stress and pressure.

    I was pointing out that releasing the same amount of deformation tension can result in orders of magnitude difference in the energy involved depending on how you do it.

    If you have ten little earthquakes with a displacement of 10 cm each, it is going to cause far less damage than one big one with displacement of a metre, but either will release the same amount of tension in the rocks. The energy stored in the stressed rocks comes from continental plate drift, it is a tiny fraction of the energies involved, and the drifting plate does not care at all if you store up a huge amount and release it all at once, or if you keep releasing the tension and don't store much at all.

    Claiming that you can't release the tension with small earthquakes is akin to claiming there is a Law of Conservation Of Earthquake Damage.

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  • (Score: 1) by khallow on Wednesday November 21 2018, @01:42PM

    by khallow (3766) Subscriber Badge on Wednesday November 21 2018, @01:42PM (#764698) Journal

    I was pointing out that releasing the same amount of deformation tension can result in orders of magnitude difference in the energy involved depending on how you do it.

    Even using the ideal spring law, you have ten times the tension at the point of release for a 10 cm displacement. And said tension is not linear over time precisely because of the potential energy build up required.

  • (Score: 1) by khallow on Wednesday November 21 2018, @01:47PM (7 children)

    by khallow (3766) Subscriber Badge on Wednesday November 21 2018, @01:47PM (#764701) Journal

    Claiming that you can't release the tension with small earthquakes is akin to claiming there is a Law of Conservation Of Earthquake Damage.

    Didn't read that at first. Sure, you're right, but in the real world, the bigger earthquakes happen anyway. There appears to be two sorts of faults. The first is slippery and has earthquakes inversely proportion to the energy of the earthquake. The second is stuck and tends to have earthquakes inversely proportional to the moment of the earthquake (which is crudely measured by moment magnitude). The catch is that the energy of earthquake is roughly proportional to the power of 1.5 of the moment. So high magnitude earthquakes are much more likely on stuck faults.

    • (Score: 2) by deimtee on Wednesday November 21 2018, @02:16PM (6 children)

      by deimtee (3272) on Wednesday November 21 2018, @02:16PM (#764713) Journal

      The point I should have made is that the amount of slippage is the value that is 'conserved' not the energy involved. If the plate drifts 5 cm a year, you are going to average 5 metres of slippage per century. That might be one massive earthquake or one hundred tiny ones. The amount of energy involved is not fixed, the amount of slippage is.

      That website referred to in one of the gg...p posts is completely wrong when they use the energy of quakes to claim that you can't relieve the tension with small quakes.
      It might be difficult to relieve tension that already exists, because that energy is already stored, but you could certainly prevent future buildups by triggering small quakes.

      --
      If you cough while drinking cheap red wine it really cleans out your sinuses.
      • (Score: 1) by khallow on Thursday November 22 2018, @02:44AM (5 children)

        by khallow (3766) Subscriber Badge on Thursday November 22 2018, @02:44AM (#765048) Journal

        The point I should have made is that the amount of slippage is the value that is 'conserved' not the energy involved. If the plate drifts 5 cm a year, you are going to average 5 metres of slippage per century. That might be one massive earthquake or one hundred tiny ones. The amount of energy involved is not fixed, the amount of slippage is.

        The thing is, the energy involved is fixed. You're not going to release more energy than the system can build up as potential energy. And that tends to be a fairly constant rate for the given fault system.

        That website referred to in one of the gg...p posts is completely wrong when they use the energy of quakes to claim that you can't relieve the tension with small quakes.

        I agree. The "slippery" systems, for example, release a lot more energy in small earthquakes than large. But "stuck" faults are a different story. They could relieve the tension of the fault with small earthquakes, but they don't.

        • (Score: 1) by deimtee on Thursday November 22 2018, @03:35AM (4 children)

          by deimtee (3272) on Thursday November 22 2018, @03:35AM (#765059) Journal

          That's why I brought up the spring thing. The amount of energy is not fixed. Deform twice as far before slipping and you have four times the energy. Deform ten times as far and have one hundred times the energy. Okay, it is probably not as straightforward as that, but it is certainly not a straight line graph of so many MJ/cm.

          Energy conservation doesn't come into it because there is a massive torrent of it going past in the form of plate movement. How much or how little the fault stores matters not at all to the drifting plate, it will continue at the same pace.

          --
          If you cough while drinking cheap red wine it really cleans out your sinuses.
          • (Score: 1) by khallow on Thursday November 22 2018, @02:36PM (3 children)

            by khallow (3766) Subscriber Badge on Thursday November 22 2018, @02:36PM (#765198) Journal

            The amount of energy is not fixed.

            Conservation of energy says otherwise.

            • (Score: 1) by deimtee on Thursday November 22 2018, @02:58PM

              by deimtee (3272) on Thursday November 22 2018, @02:58PM (#765208) Journal

              Conservation of energy says the total energy of the system doesn't change. The tiny fraction of it stored as potential energy in the strain of deforming the rocks absolutely can change.

              --
              If you cough while drinking cheap red wine it really cleans out your sinuses.
            • (Score: 1) by deimtee on Thursday November 22 2018, @03:06PM (1 child)

              by deimtee (3272) on Thursday November 22 2018, @03:06PM (#765214) Journal

              Ok, I just re-read this thread. I think you are talking about faults that are already under strain. I agree it would be difficult to release those without triggering a big quake, stored strain energy would have to go somewhere, and it wouldn't be easy to get rid of it without damage.
              I was talking more about preventing the buildup by triggering small quakes. This would be relatively easy, and I guess the best time to start would be soon after a big quake.

              --
              If you cough while drinking cheap red wine it really cleans out your sinuses.
              • (Score: 1) by khallow on Thursday November 22 2018, @03:30PM

                by khallow (3766) Subscriber Badge on Thursday November 22 2018, @03:30PM (#765228) Journal

                I was talking more about preventing the buildup by triggering small quakes. This would be relatively easy, and I guess the best time to start would be soon after a big quake.

                That's a reasonable thing to consider. One apparent solution is to pump water into earthquake faults that have recently ruptured. Since they've ruptured, they're already low friction so you aren't changing things instantaneously, but rather over the course of decades or centuries. I think currently liability law in a good portion of the world would prevent this from being a good solution for a while, but it could be attempted in some low population areas to see how well it works.