Did You Feel It? Fracking Earthquakes Are Less Intense

Photographer: Ty Wright/Bloomberg
A rig drills for natural gas at a hydraulic fracturing site owned by EQT Corp. located atop the Marcellus shale rock formation in Washington Township, Pennsylvania, on Oct. 31, 2013.

Bloomberg BNA -- Earthquakes and tremors from hydraulic fracturing shake the ground less than naturally occurring earthquakes of the same magnitude, therefore causing less damage, according to new U.S. Geological Survey research.

USGS seismologist Susan Hough analyzed 11 induced earthquakes in the central and eastern United States from 2011-2013, evaluating the ground tremors these events generated.

Using a USGS database known as the “Did You Feel It?” system, Hough said the observations of those who experienced the quakes were “very straightforward—in every single case the intensities are low.”

Hough's study, published online Aug. 19 in the “Bulletin of the Seismological Society of America,” concludes that the hazards of these quakes are lower than what might be expected, chiefly because induced events are 16 times weaker than natural earthquakes with the same magnitude.

The earthquakes associated with fracking also tend to lose energy about six miles from their epicenter, Hough said, presumably because the fault is lubricated by the injected wastewater, making it easier to slip.

Earthquakes have become a concern for states experiencing hydraulic fracturing, or fracking, of their shale reserves, with some reporting a significant increase in seismic activity, possibly due to deep water injection associated with this type of drilling

Hough's study looked at fracking-related quakes in Arkansas, Colorado, Oklahoma, Ohio and Texas using data from the “Did You Feel It” questionnaires completed online by people who felt the earthquakes and went to the USGS site to report them.

She compared the induced quakes to 10 tectonic earthquakes from 2002 to 2011. The natural earthquakes had magnitudes between 4.0 and 5.8; the magnitude of the induced earthquakes was between 3.9 and 5.7.

System Provides Shaking Intensity Characterization

While instrumental recordings of injection-induced quakes are scant, the DYFI system provides an “excellent characterization of shaking intensities caused by induced earthquakes,” Hough said.

The way an induced quake felt was equivalent, on average, to a natural quake that was of a magnitude 0.8 or less, Hough said.

For the 11 events studied, estimated intensities were lower by 0.4-1.3 units than the event magnitudes, she said, with an average difference of 0.8 units.

The largest, a 5.7 magnitude quake in Prague, Okla., in November 2011, felt like a 5.1 magnitude natural quake, Hough said, noting that the effective intensity magnitude for each induced quake was less than natural tectonic events.

Based upon the USGS scale, a drop in 0.8 magnitude translates to about 16 times less strength or energy released, she said.

Force of Energy Called Shallow

Along with lower energy levels, Hough said the data suggest that the force of energy is shallow, perhaps due to the presence of fracking fluids, and the tremor's force tends to dissipate at around six miles from the quake's epicenter.

Hough's results suggest that damage from injection-induced earthquakes will be especially concentrated in the immediate epicentral region.

Induced earthquakes may have lower stress drops than natural ones because the fluids injected into the ground lubricate geological faults and allow them to slip more smoothly, Hough said.

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