wastewater well
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Wastewater well above-ground injection process.
Between 1978 and 2008, Oklahoma had just two earthquakes with a magnitude over 3.0. In 2014, thus far, there have been around 200 such earthquakes there, more even than the highly unstable state of California. (They've had 140.) Experts believe the unusual increase in earthquakes is linked to the number of wastewater wells connected to oil and gas drilling.

diagram, wastewater effect
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Wastewater's effect on earthquake faults.
Wastewater wells occur when oil and gas companies inject wastewater deep underground. Scientists believe that the wastewater acts as a lubricant in existing fault lines, causing more movement. Hydraulic fracturing, commonly known as fracking, has also been linked to earthquakes, though the majority of Oklahoma's tremors were caused by wastewater wells.

Comment: The principal seismic hazard from injection-induced earthquakes comes from the disposal of wastewater into deep strata, i.e: basement formations. When the balance of applied shear stress is less than the strength of contact, the fault remains locked. Injection wells can promote a slip by increasing pore pressure, compounded by a change of load above a fault (such as the removal of oil). As faults slip, earthquakes release stored energy. The intensity of the earthquake is proportional to the stored energy and the triggers for release.

map, earthquakes due to tracking
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Awakened regional earthquakes from remote triggers due to wastewater injection.
The USGS and Oklahoma officials are adding monitor stations to best determine which wastewater wells are causing the earthquake issue. There are currently 15 permanent stations and 17 temporary stations.

Thus far, none of the earthquakes in Oklahoma have caused major damage. However, USGS geophysicist Rob Williams believes it is only a matter of time, "Given the rate of earthquakes over the last six months, it's concerning enough to be worried about a larger, damaging earthquake happening, let alone what might happen in the future."