(John Kemp is a Reuters market analyst. The views expressed are
By John Kemp
LONDON May 9 Central Oklahoma faces a
significantly increased risk of a damaging earthquake, and the
threat is probably related to oil and gas extraction, the U.S.
Geological Survey warned on Monday.
"As a result of the increased number of small and moderate
shocks, the likelihood of future, damaging earthquakes has
increased for central and north-central Oklahoma," USGS said.
More than 180 earthquakes of magnitude 3.0 or greater were
recorded in Oklahoma between October 2013 and the middle of
April 2014, compared with a long-term average of just two per
year between 1978 and 2008.
The risk of a more damaging earthquake of magnitude 5.5 or
greater has increased significantly, USGS warned. (here)
"Building owners and government officials should have a
special concern for older, unreinforced brick structures, which
are vulnerable to serious damage during sufficient shaking."
Oklahoma has long been known as an earthquake zone, but the
recent increase does not appear to be due to typical random
fluctuations in natural seismicity, according to USGS.
Instead, USGS experts believe a contributing factor has been
the disposal of waste water from oil and gas production by
injecting it into deep underground formations.
Scientists have long known the injection or withdrawal of
large volumes of fluid into or from an underground rock
formation can trigger earthquakes by changing stress along
existing fault lines and lubricating them.
Small and medium-sized earthquakes have been triggered by
mining, oil and gas extraction, geothermal energy, enhanced oil
recovery, carbon capture and storage and hydroelectric dams.
In the 1960s, the underground disposal of waste fluids from
the U.S. Army's chemical weapons facility at Denver, Colorado
triggered more than 1,500 earthquakes, including three
registering more than magnitude 5.0.
Water injection at Geysers geothermal power plant in
northern California has caused a large number of small tremors,
and the power plant has a well established programme for
compensating local residents for minor damage to their property.
The magnitude of the earthquake is related to the surface
area that ruptures, which in turn is related to the volume of
fluid injected or withdrawn from the underground rock formation.
So in general, larger fluid injections are capable of causing
The amount of waste water injected in a typical disposal
well is not normally large enough to trigger a significant
earthquake. But USGS has begun to worry that a series of small
quakes could lead to a larger one.
In November 2011, a magnitude 5.0 earthquake occurred in
close proximity to active fluid injection wells near the town of
Prague in Oklahoma. Less than a day later, a magnitude 5.7
earthquake was reported, with at least one magnitude 5.0
USGS geologists said the initial magnitude 5.0 quake "may
have promoted failure of the main shock and thousands of
aftershocks along the Wilzetta fault".
Before the Prague quake, the strongest tremor on record in
Oklahoma reached magnitude 5.5 and occurred in 1952.
The increasing number of earthquakes related to oil and gas
extraction is bound to heighten concern about the safety of
fracking, which in turn will intensify scrutiny of the industry.
But there are several important points to bear in mind.
First, the earthquakes are not related to hydraulic
fracturing but to disposal of contaminated waste water, which is
linked to all oil and gas extraction activities, not just
All oil and gas extraction produces large amounts of waste
water, which is normally reinjected underground.
Second, fluid injection is not restricted to oil and gas
production. Geothermal, carbon capture and storage and a range
of other human activities all pose a heightened risk of
earthquakes. Carbon capture and storage would probably pose the
biggest potential danger because of the volume involved.
Third, the risk of induced seismicity seems to be
concentrated in heavily faulted areas that are already under
high levels of natural stress. Fluid injection in areas already
known to be prone to earthquakes poses the greatest risk.
For safety reasons, it is vital that would-be drillers
understand the local geology: injecting waste water near
California's San Andreas fault would clearly not be a good idea.
The increasing number of small earthquakes reported in
Oklahoma, Arkansas, Ohio, Texas and Colorado stems from the
general increase in oil and gas production in the United States
from conventional and shale deposits, all of which produce large
volumes of waste water.
As evidence mounts that the U.S. oil and gas boom is
contributing to an increase in the number of earthquakes, energy
producers must get ready for another round of tough questions
If USGS is correct, it is only a matter of time before an
earthquake occurs that is strong enough to inflict noticeable
damage and is traced back to oil and gas production.
The risks cannot be eliminated entirely, but they can be
managed. It is important for the industry to take steps ahead of
time, including paying greater attention to local fault risks
before drilling, so it can demonstrate all reasonable
precautions were taken.
(editing by Jane Baird)