Explanation of the figure: In N and NE Oklahoma horizontal wells are drilled into unconventional reservoirs, typically the Mississippian Lime. These wells produce vast quantities of formation (salt) water mixed with a small percentage of oil. The oil is separated and the water needs to be disposed. For this purpose a deeper nearby vertical salt water disposal well pumps the water down into the 500 million year old Arbuckle formation that sits on granite basement rocks. The basement and Arbuckle often have ancient inactive faults from continental collisions that formed the North American continent. Salt water injection that is too rapid, at too high a pressure or from wells too close together can reactive the faults and be felt as earthquakes. The earthquake waves radiate out from the fault and, sometimes, can be felt 100 miles away in Fayetteville, AR where I live. The hilly terrain of NW Arkansas scatters and focusses the earthquake waves so that the felt effect varies widely from one neighborhood to the next.
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| Sketch of NE OK earthquake as felt in Fayetteville, AR |