Derechos | Equipo Nizkor
Study identifies 2012 U.S. quake as "man-made"
A new study has identified a 4.8-magnitude earthquake in East Texas in 2012, the largest recorded in the region, as man-made event triggered by the injection of large volumes of wastewater from oil and gas activities into rocks deep beneath the surface.
The findings, published in the journal Science, was the result of a satellite study using a remote sensing technique called Interferometric Synthetic Aperture Radar, or InSAR, to detect tiny, centimeter-scale changes in the shape of Earth' s surface, exposing ground deformations near the wastewater injection wells where the quake occurred.
While the East Texas temblor and several strong aftershocks were suspected to be "induced" by the nearby wells, "our research is the first to provide an answer to the questions of why some wastewater injection causes earthquakes, where it starts and why it stops," said study co-author William Ellsworth, a geophysics professor at Stanford University's School of Earth, Energy & Environmental Sciences.
In what the researchers claim to be the first observations of surface uplift associated with wastewater injection, Ellsworth and his co-authors focused on four high-volume wells used for disposing wastewater, located near the town of Timpson, Texas, where the 2012 quake was centered. According to a news release from Stanford, the four wells began operations between 2005 and 2007 and at their peak injected about 200 million gallons of wastewater per year underground.
Brackish water naturally coexists with oil and gas within the Earth. After extracting this slurry using hydraulic fracturing or other techniques, drilling companies separate the "produced water" from the oil and gas and then reinject it into Earth at disposal wells, as the wastewater is too salty and too contaminated with other chemicals to treat economically. Approximately 180,000 of these disposal wells are now in operation in the United States, primarily in Texas, California, Oklahoma and Kansas.
Injecting wastewater at a depth of over 1 mile, or 1.6 kilometer, two of the wastewater disposal wells the researchers examined lie directly above where the earthquake occurred; and the other two wells injected similar volumes of wastewater, but at shallower depths, just over a half mile, or 800 meters, below the surface.
The InSAR measurements revealed that wastewater injection at the shallow wells resulted in detectable ground uplift up to 5 miles, or 8 kilometers, away but only a modest rise in pore pressure, which is the pressure of fluids within the fractures and cavities of rocks, at the depth at which earthquakes happen 2 or more miles, or 3.2 kilometers, below the surface.
As increasing pore pressure within a geologic fault is known to be able to cause the two sides of the fault to slip and release seismic energy as an earthquake, the combination of stiffer rock and the impermeable "blocking formation" above at the deep-well injection sites allowed the rising pore pressure to migrate downward and build up until it triggered earthquakes in 2012 along an ancient fault line.
"The detection of uplift when combined with well-injection records provides a new way to study wastewater injection," Ellsworth said.
The researchers said their study highlights the importance of understanding the local geology for wastewater injection operations. "The recent upturn in seismicity in Oklahoma and Kansas commonly happens where injection occurs close to the crystalline basement, so we're getting lots of earthquakes in those places," Ellsworth said.
[Source: Xinhua, San Francisco, 22Sep16]
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