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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.
(Score: 2) by deimtee on Wednesday November 21 2018, @02:16PM (6 children)
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)
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.
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)
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)
Conservation of energy says otherwise.
(Score: 1) by deimtee on Thursday November 22 2018, @02:58PM
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)
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
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.