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Physics of Human Caused Earthquakes?

  1. Feb 20, 2017 #1
    I'm not sure if this is the right area of the forum (it may be more Earth physics) or even the right site for this (it might be more of a geology question), but I'm wondering if someone more knowledgeable in physics could explain the physics of human caused earthquakes to me. Recently the United States Geological Survey updated its seismic activity maps to account for human activity and determined that Oklahoma has an earthquake risk profile similar to Alaska and California. Here's the map:

    yellow-map-chance-of-earthquake-oklahoma.jpg

    However, Oklahoma doesn't have much of an earthquake history before 2011, when the area seems to have awakened, and fluid and steam injection has been done for decades. Oklahoma is a major center of hydraulic fracturing, which is a relatively new technology, but there doesn't seem to be similar levels of induced seismic activity elsewhere as a result of hydraulic fracturing.

    Could someone clarify the physics of how this process might work, and why other activities involving large amounts of mass (such as reservoirs) and energy (such as nuclear tests) don't seem to have created induced seismic activity, or at least nothing as dramatic as what's occurring in Oklahoma?
     
  2. jcsd
  3. Feb 20, 2017 #2

    Evo

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  4. Feb 20, 2017 #3
    So the faults in the area had reached equilibrium, but human activity in the area upset that?

    It seems strange that dams and nuclear testing has never resulted in something so dramatic. There are reservoirs in areas known to have seismic activity, sometimes even near fault lines, and nuclear tests (even atmospheric tests/detonations) are powerful enough to create earthquakes. Why have relatively stable faults in Oklahoma burst back into activity when active faults in areas such as the Columbia Basin and California weren't as dramatically disturbed by water resource development? The area around Las Vegas, Nevada, even has the Hoover Dam and Nevada Test Site nearby, and despite being a seismically active region it seemingly wasn't disturbed by those activities either.
     
  5. Feb 20, 2017 #4

    Evo

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    They were saying that's it's when the faults are pushed in the same direction.

    From the article
    Definitely more studies need to be done.
     
  6. Feb 20, 2017 #5

    Vanadium 50

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    Fracking actually goes back to the 40's, but the USGS says it's not the main cause of human-induced earthquakes. I also would not take absence of evidence as evidence of absence, especially from Wikipedia. Especially squared when the cited source says "list incomplete".
     
  7. Feb 21, 2017 #6
    Current wisdom has it that induced seismicity is related to static friction at a fault. Injecting liquids into the subsurface in large volumes, such as in secondary oil recovery and injection of aqueous wastes, raises the pore pressure within the receiving formations. Prior to injection, the rock strata were in a historical state of stress established over geological time scales. Increasing the pore pressure disturbs the state of effective stress in the rocks, both normal to fault faces and tangent to the faces. If the ratio of the tangential stress to the normal stress exceeds the coefficient of static friction, the fault is expected to be activated. Most of these injections occur at depths > 3000 ft.
     
  8. Feb 21, 2017 #7

    FactChecker

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    Good info. The statistical evidence of a connection between fracking activity and earthquake frequency is extremely strong. It's not just a general correlation between fracking waste injection sites and earthquake frequency. It's the correlation in many specific locations, the timing of sudden extreme increases, and the correspondence between injection depth and earthquake depth, that makes it so convincing.
     
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