WASHINGTON (AP) — Thousands of times every day, drilling deep
underground causes the earth to tremble. But don't blame the surprise
flurry of earthquakes in Oklahoma on man's thirst for oil and gas,
The weekend quakes were far stronger than the puny
tremors from drilling — especially the controversial practice of
hydraulic fracturing. The weekend quakes didn't have the mark of man.
They were a force of nature.
Hydraulic fracturing, called
fracking, involves injecting millions of gallons of water, sand and
chemicals deep underground to break up rock. While that may sound like
it could cause an earthquake, experts say the process doesn't pack
nearly the punch of even a moderate earthquake.
quake that rocked Oklahoma three miles underground had the power of
3,800 tons of TNT, which is nearly 2,000 times stronger than the 1995
Oklahoma City bombing.
The typical energy released in tremors
triggered by fracking, "is the equivalent to a gallon of milk falling
off the kitchen counter," said Stanford University geophysicist Mark
In Oklahoma, home to 185,000 drilling wells and hundreds
of injection wells, the question of man-made seismic activity comes up
quickly. But so far, federal, state and academic experts say readings
show that the Oklahoma quakes were natural, following the lines of a
"There's a fault there," said U.S. Geological
Survey seismologist Paul Earle. "You can have an earthquake that size
anywhere east of the Rockies. You don't need a huge fault to produce an
earthquake that big. It's uncommon, but not unexpected."
But there's a reason people ask if the quakes are man-made rather than from the shifting of the Earth's crusts.
the past, earthquakes have been linked to energy exploration and
production, including from injections of enormous amounts of drilling
wastewater or injections of water for geothermal power, experts said.
They point to recent earthquakes in the magnitude 3 and 4 range — not
big enough to cause much damage, but big enough to be felt — in
Arkansas, Texas, California, England, Germany and Switzerland. And back
in the 1960s, two Denver quakes in the 5.0 range were traced to deep
injection of wastewater.
Still, scientists would like to know if
human activity can trigger a larger event. The National Academy of
Sciences is studying the seismic effects of energy drilling and mining
and will issue a report next spring.
"This is an area of active research," said Rowena Lohman, a Cornell University seismologist. "We're all concerned about this."
issue is that areas that are prone to earthquakes are also places where
oil and gas flow along fractures, experts said. In some studies,
scientists have taken earthquake data and, like detectives, tracked its
causes to deep injections of lots of liquid under high pressure, such as
ones that peaked at magnitude 3.3 at the Dallas-Fort Worth airport in
2008 and 2009, said USGS geophysicist William Ellsworth. The Switzerland
quake was in the city, Basel, so it did cause damage, he and others
"How big an earthquake might we trigger? That is an open
question at this point," Ellsworth said. "We do know we can trigger
magnitude 5 earthquakes."
When lots of liquid is injected into the
ground it changes the stress and pressure in a place that probably
already was a fault, said Oklahoma Geological Survey seismologist Austin
Holland. Putting the liquid in is similar to injecting water between
two adjacent bricks, it allows them to slide more easily and "the water
under pressure is helping push the bricks apart ever so slightly,"
Holland, who has documented some of the biggest
shaking associated with fracking, compared a man-made earthquake to a
mosquito bite. "It's really quite inconsequential," he said.
fracturing has been practiced for decades but it has grown rapidly in
recent years as drillers have learned to combine it with horizontal
drilling to tap enormous reserves of natural gas and oil in the United
About 5 million gallons of fluid is used to fracture a
typical well. That's typically not nearly enough weight and pressure to
cause more than a tiny tremor.
Earlier this year, Holland wrote a
report about a different flurry of Oklahoma quakes last January — the
strongest a 2.8 magnitude — that seemed to occur with hydraulic
fracturing. Holland said it was a 50-50 chance that the gas drilling
technique caused the tremors.
That is the largest tremor
associated with fracking in the scientific literature, experts say. And
the strongest of this weekend's natural quakes, magnitude-5.6, released
nearly 16,000 times the energy of the worst from that January flurry.
industry-funded study into a 2.3-magnitude tremor in June near a
fracking site in England linked the drilling activity to the quake, but
it was a "worst case scenario" of fluid injection into the exact wrong
place in a fault, said German geologist Stefan Baisch, lead author of
But wastewater from hundreds of wells is often
collected and disposed of deep underground through so-called injection
wells. In Lincoln County, Okla., where the recent earthquakes hit, there
are approximately 1,982 active oil and gas wells, according to Matt
Skinner, spokesman for the state agency that oversees oil and gas
production. There are 181 injection wells.
These wells pump
wastewater often much deeper underground, all day and all night, for
years. The weight and pressure from all of this fluid has been known to
cause relatively large earthquakes, including recently in Arkansas, home
to another large shale gas field.
After a swarm of small
earthquakes hit north-central Arkansas near a formation called the
Fayetteville Shale, the state issued a temporary moratorium last year on
new injection wells. The state found that three wells were operating
near an unknown fault and were likely contributing to earthquakes. The
state shut those wells and banned future ones near the fault.
and gas production can lead to tremors another way: When drillers suck
all the oil from underground and leave nothing to fill the gap for where
the oil was, the emptying reservoir can collapse. If this happens at
all, it usually happens slowly over decades. It triggered a series of
earthquakes in Los Angeles county in the 1930s, according to the
Lawrence Berkeley National Laboratory.
Now, water is injected into depleting wells to maintain pressure. The water also helps keep oil flowing.
Fahey reported from New York.