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IN THE NEWS Time yagt.3..._
Date
Average annual number of earthquakes Average annual number of earthquakes
Earth 21 greater than or equal to magnitude 3.0 '100+ greater than or equal to magnitude 3.0
in central and eastern U.S.,1967-2000 in central and eastern U.S.,2010-2012
Huge quakes foretell smaller ones appeared t initiate small tremors near
some wastewater water disposal wells,which
Ominous activity triggered at wastewater injection sites in turn presaged larger quakes.
At each of three sites in Oklahoma,
places.From 1967 to 2000,the central Texas and Colorado, the team saw
By Erin Wayman and eastern United States experienced seismic activity spike in the days follow-
Giant, distant earthquakes may help an average of 21 earthquakes per year ing at least one massive earthquake:
scientists identify places where humans of magnitude 3.0 or greater.From 2010 a 2010 magnitude 8.8 quake in Chile,a
I are liable to set off smaller tremors to 2012,the region saw more than 300 2011 magnitude 9.1 event in Japan and
when they inject fluid deep into geo- such quakes,reports William Ellsworth, a magnitude 8.6 earthquake in Sumatra
logic deposits. a seismologist at the U.S. Geological in 2012.Six to 20 months after the initial
Scientists have known for decades that Survey in Menlo Park,Calif.,in the same tremors,each of the three sites experi-
injecting huge volumes of liquid under- issue of Science. enced quakes ranging from magnitude
ground—such as waste from hydraulic The rise in earthquakes in Arkansas, 4.3 to 5.7.
fracturing, or fracking—can set off Colorado, Ohio, Texas and elsewhere The seismic waves from the giant
quakes.But in most cases it doesn't,and coincides with an increase in extracting temblors probably perturbed fluids in
scientists can't predict when or where natural gas and oil from shale forma- the faults,increasing the pressure,van
such earthquakes will happen. tions in those states.Oil and gas don't der Elst says.
! In the July 12 Science,seismologists easily flow through impermeable shale, If operators can identify when a fault
report that massive earthquakes unleash so petroleum companies drill hori- is about to rupture,they can adjust how
seismic waves that can trigger tremors zontal wells and pump in pressurized much fluid they are injecting into a well
near wastewater disposal wells half a fluid to fracture a small section of rock or stop the injection altogether.However,
, world away.The tiny quakes may warn (SN:9/8/12,p. 20). This fracking pro- the usefulness of this foreshadowing is
that a fault is close to rupture. cess itself doesn't cause earthquakes, limited because big earthquakes that
"When we do see remote triggering, Ellsworth says. But the fracking fluid can remotely trigger tremors occur only
it seems to foreshadow larger induced comes to the surface along with the oil rarely,says Cliff Frohlich,a seismologist
earthquakes," says coauthor Nicholas or gas.Contaminated by a mix of chemi- at the University of Texas at Austin.They
van der Elst of Columbia University's cals,the fluid is disposed of by injecting happen roughly once a year.
Lamont-Doherty Earth Observatory in it deep underground,where it puts Ares- Another limitation is that not all
Palisades,N.Y. "It shows the faults are sure on faults. wastewater sites that produced earth-
reaching a tipping point." "Any time we change the fluid regime quakes during the study responded to
Concerns over human-caused quakes underground, we could bring a fault remote triggering.Van der Elst suggests
have grown in recent years as earth- closer to failure,"Ellsworth says. that when a well is very close to a fault,
quake activity has shot up in unexpected The United States is home to tens of just a few months—rather than years
thousands of wastewa- or decades—of fluid injection can build
Increasingly shaky Over the last few years,the number ter disposal wells."The pressure and cause a fault to slip. In
of earthquakes in the central and eastern United States has vast majority,"Ellsworth these cases,there's little chance of a big
increased.So has the number of wastewater disposal wells says, "don't appear to earthquake happening before the fault
associated with fracking.The map shows all of the magnitude
I' 3.0 or greater earthquakes that occurred in the United States induce earthquakes." reaches its tipping point.
in 2009-2012.Places with the highest natural earthquake Scientists don't under- Even with caveats, the work helps
hazards are shown in red and orange while areas with lower stand why only some scientists better understand the nature
risk are shown in yellow and green.itip pose a risk. of induced quakes. For the most part,
:' The discovery by van earthquakes related to wastewater
��ss der Elst and colleagues disposal have been small to moderate
. g • �* ;it''` may help identify spots and not caused much damage. When
4
OCI + = where quakes will pop such wells were limited to remote places
_ • i up.In reviewing seismic like West Texas, small rumbles in the
IF y activity in the Midwest ground didn't bother anyone,Frohlich
s al from 2003 to 2013, the says.Now that they are being built near
t researchers discovered densely populated areas like Dallas,
'.� that distant earthquakes people are rethinking the hazards. {)
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16 I SCIENCE NEWS I August 10,2013 www.sciencenews.org