Man-made earthquakesFracking-induced earthquakes in Oklahoma
Oklahomans are no strangers to Mother Nature’s whims. From tornadoes and floods to wildfires and winter storms, the state sees more than its share of natural hazards. But prior to 2009, “terra firma” in Oklahoma meant just that — earthquakes rarely shook the state. Then, after decades of seismic quiet where the state averaged less than two quakes of magnitude 3 or greater a year, Oklahoma suddenly saw a sharp uptick, to twenty such quakes in 2009. NASA says that the earthquakes were human-induced, resulting from wastewater injection, rather than a naturally caused quakes.
Oklahomans are no strangers to Mother Nature’s whims. From tornadoes and floods to wildfires and winter storms, the state sees more than its share of natural hazards. But prior to 2009, “terra firma” in Oklahoma meant just that — earthquakes rarely shook the state.
Then, after decades of seismic quiet where the state averaged less than two quakes of magnitude 3 or greater a year, Oklahoma suddenly saw a sharp uptick, to twenty such quakes in 2009. By 2013 there were 109 such quakes. Since then, the numbers have soared, reaching 903 in 2015 before dipping last year to 623. In the process, Oklahoma has surpassed California to become the most seismically active of the lower 48 U.S. states.
In 2011, a magnitude 5.7 quake and two related magnitude 5.0 quakes struck near the Oklahoma town of Prague, causing damage and injuries. Then last 3 September, a magnitude 5.8 quake struck a few miles northwest of the city of Pawnee, population 2,200. That quake, which occurred on a previously unmapped fault, was the strongest ever measured by instruments in Oklahoma. It shook a large area of north-central Oklahoma and was felt throughout the Midwest and as far away as Phoenix and Pittsburgh.
A Seismic detective story, with satellites
Even before NASA studied the Pawnee earthquake, studies published since late last year by the United States Geological Survey and other institutions suggested that the earthquake was human-induced due to increases in wastewater injection related to petroleum operations. Injection wells place fluids underground into porous geologic formations, which scientists believe can sometimes enter buried faults that are ready to slip.
NASA says that to shed additional light on the source of the Pawnee quake, a team led by geophysicist Eric Fielding of NASA’s Jet Propulsion Laboratory in Pasadena, California, used enhanced seismic data and satellite image analysis to more accurately estimate the location and extent of the fault responsible for the quake, its hypocenter (the point below Earth’s surface where the quake began) and its aftershocks, and to measure how the fault moved. Results of their study were published recently in Seismological Research Letters.
To help pinpoint which fault ruptured and where the main quake started, Fielding’s team updated the locations of earthquakes published in an Oklahoma Geological Survey catalog of aftershocks. The catalog included nearly 2,200 earthquakes of greater than magnitude 1.0 within about 31 miles (50 kilometers) of the 3 September main shock.
