The
U.S. Geological Survey (USGS) has taken its first stab at quantifying
the hazard from earthquakes associated with oil and gas development. The
assessment, released in a preliminary report today, identifies 17 areas
in eight states with elevated seismic hazard. And geologists now say
that such induced earthquakes could potentially be large, up to
magnitude 7, which is big enough to cause buildings to collapse and
widespread damage.
The new bull’s-eyes on the map, regions such as central Oklahoma,
have short-term hazards that are comparable to the those in traditional
earthquake states, like California, says Mark Petersen, chief of the
USGS National Seismic Hazard Mapping Project in Golden, Colorado. “These
earthquakes are occurring at a higher rate than ever before and pose a
much greater threat to people living nearby,” he says. “This report
represents our first step in identifying and quantifying the ground
shaking from induced earthquakes.”
Geoscientists have known for decades that the injection of fluid can
increase pressures within the pores of deep rock formations, pushing
faults that are already critically stressed by forces in Earth’s crust
past the snapping point. But the phenomenon has been brought to the fore
by an extraordinary rise in small earthquakes across parts of the
central United States. That surge has coincided in time and place with
the boom in unconventional oil and gas extraction such as hydraulic
fracturing, or "fracking," in which high-pressure fluid is injected into
the ground to break up the underlying rock and release trapped gas or
oil. In most cases, the earthquakes are not due to fracking itself,
which is usually completed in hours or days. Rather, the culprit is
typically wastewater disposal, where high volumes of water extracted in
oil and gas operations is reinjected into deep basement rocks, where the
bigger and more dangerous faults lie.
So far, the largest induced earthquake in the United States has been
the 2011 magnitude-5.6 earthquake in Prague, Oklahoma, which damaged
dozens of buildings. But geoscientists now say there is no reason why
oil and gas operations couldn’t end up triggering something much larger.
“There are certainly faults large enough to produce a magnitude 7,”
says Justin Rubinstein, a geophysicist at USGS in Menlo Park,
California, and a co-author of the new report. “We can’t rule this out.”
In July 2014, USGS published its most recent update to its national seismic hazard map,
which is incorporated into engineering codes for buildings and bridges.
The hazard is expressed in terms of the probability of exceeding a
certain level of shaking in 50 years—not only because the hazard in
places like California is not expected to change much over that time
period, but also because 50 years—the typical life span of a building—is
a useful period of time for engineers.
But that map ignored the threat of induced earthquakes, precisely
because the frequency of these earthquakes was likely to change over the
course of 50 years. Economic or policy conditions could affect the
frequency of induced earthquakes, Petersen says. In short order,
wastewater wells could be drilled in new locations, the falling price of
oil could shut down certain operations, or regulators could change
their policies in certain regions. To account for that uncertainty, the
new map describes probabilities over an extremely short, 1-year
interval.
The report is a preliminary study that is going to be revised later
this year, Petersen says. But William Ellsworth, a geophysicist at USGS
in Menlo Park, suggests that it could still be useful—for instance, for
state transportation departments trying to prioritize which bridges
should be singled out for repairs or retrofits first.
Science agencies and regulators finally seem to be taking induced
earthquakes seriously. For a long time, the Oklahoma Geological Survey
was reluctant to link the earthquakes to oil and gas operations. But in a
statement released on 21 April, the agency now says that it is “very
unlikely” that the surge in earthquakes represents a natural process. At
a teleconference the following day, held in conjunction with unveiling
the new USGS report, Austin Holland, the state seismologist at the
Oklahoma Geological Survey in Norman, went even further, saying, “The
vast majority of these [earthquakes], we suspect, are from waste water
disposal.”
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