Myths and Facts on Wastewater Injection,
Hydraulic Fracturing, Enhanced Oil
Recovery, and Induced Seismicity
by Justin L. Rubinstein and Alireza Babaie Mahani
INTRODUCTION
The central United States has undergone a dramatic increase in
seismicity over the past 6 years (Fig.
1
), rising from an average
of 24
M
≥
3
earthquakes per year in the years 1973
–
2008 to an
average of 193
M
≥
3
earthquakes in 2009
–
2014, with 688 oc-
curring in 2014 alone. Multiple damaging earthquakes have
occurred during this increase including the 2011
M
5.6 Prague,
Oklahoma, earthquake; the 2011
M
5.3 Trinidad, Colorado,
earthquake; and the 2011
M
4.7 Guy-Greenbrier, Arkansas,
earthquake. The increased seismicity is limited to a few areas
and the evidence is mounting that the seismicity in many of
these locations is induced by the deep injection of fluids from
nearby oil and gas operations. Earthquakes that are caused by
human activities are known as induced earthquakes. Most injec-
tion operations, though, do not appear to induce earthquakes.
Although the message that these earthquakes are induced by
fluid injection related to oil and gas production has been com-
municated clearly, there remains confusion in the popular press
beyond this basic level of understanding.
In this article, we attempt to dispel the confusion for a
nonspecialist audience. First, we highlight six common misun-
858 M
3 Earthquakes 1973–2008
1570 M
3 Earthquakes 2009–April 2015
Central and Eastern US
Earthquakes
1973–April 2015
0
500
1000
1500
2000
2500
Number of M
3 Earthquakes
1975
1980
1985
1990
1995
2000
2005
2010
2015
▴
Figure 1.
Count of
M
≥
3
earthquakes in the central and eastern United States from
1973 to April 2015. Two abrupt increases in the
earthquake rate occurred in 2009 and 2013. (Inset) Spatial distribution of earthquakes. Red dots represent earthquakes that occurred between
2009 and April 2015, and blue dots represent earthquakes that occurred between 1973 and 2008. Red color becomes brighter when there are
more earthquakes in the area. The earthquake rate and distribution of earthquakes changed in 2009. Prior to 2009, earthquakes were spread
across the United States. Beginning in 2009 the earthquakes are tightly
clustered in a few areas (central Oklahoma, southern Kansas, central
Arkansas, southeastern Colorado and northeastern New Mexico, and multiple parts of Texas).
doi: 10.1785/0220150067
Seismological Research Letters Volume 86, Number 4 July/August 2015 1
SRL Early Edition
derstandings and correct them. Subsequently, we describe the
three main types of fluid injection used by the oil industry:
(1) hydraulic fracturing (commonly referred to as fracking),
(2) wastewater disposal, and (3) enhanced oil recovery. We then
explain how each of these processes can induce earthquakes.
Next, we review the evidence that shows that wastewater injec-
tionistheprimarycauseofthelargechangeinseismicityob-
served in the United States and demonstrate that this meets our
expectations of how seismicity should behave. Finally, we discuss
the possibility of mitigating this hazard. This article focuses on
the recent seismicity induced by fluid injection; we are not aim-
ing to provide a broad review of induced seismicity. Many ar-
ticles in this vein have already been written (
Nicholson and
Wesson, 1990
,
1992
;
McGarr
et al.
,2002
;
Ellsworth, 2013
).
COMMON MISCONCEPTIONS ABOUT FLUID
INJECTION AND EARTHQUAKES
The media commonly report on induced earthquakes incor-
rectly, and consequently policy makers and the public have
an incorrect or incomplete understanding of how and why they
occur. Here, we list common misconceptions about induced
earthquakes and then correct them.
1.
Misconception:
Hydraulic fracturing is causing all of the
induced earthquakes.
Correction:
Hydraulic fracturing is directly causing a small
percentage of the felt-induced earthquakes observed in the
United States. In contrast, felt earthquakes induced dur-
ing hydraulic fracturing operations are more common in
western Canada. Most induced earthquakes in the United
States are a result of the deep disposal of fluids (waste-
water) related to oil and gas production.
2.
Misconception:
The wastewater injected in disposal wells is
spent hydraulic fracture fluid.
Correction:
The amount of spent hydraulic fracturing
fluid injected into wastewater disposal wells is highly var-
iable. The fluids disposed of near earthquake sequences in
Youngstown, Ohio, and Guy, Arkansas, are believed to
consist largely of spent hydraulic fracturing fluid (
Horton,
2012
;
Kim, 2013
). In contrast, in Oklahoma spent hy-
draulic fracturing fluid represents 10% or less of the
fluids disposed of in salt-water disposal wells in Oklahoma
(
Murray, 2013
). The vast majority of the fluid that is dis-
posed of in disposal wells in Oklahoma is produced water.
Produced water is the salty brine from ancient oceans that
was entrapped in the rocks when the sediments were de-
posited. This water is trapped in the same pore space as oil
and gas, and as oil and gas is extracted, the produced water
is extracted with it. Produced water often must be disposed
in injection wells because it is frequently laden with dis-
solved salts, minerals, and occasionally other materials that
make it unsuitable for other uses.
3.
Misconception:
There would be no need for wastewater
disposal if hydraulic fracturing was not used.
Correction:
Salt water is produced at virtually all oil wells,
whether the wells were hydraulically fractured or not. In
fact, hydraulic fracturing is not used in the Hunton
Dewatering Play in central Oklahoma, yet it is one of the
largest producers of salt water in the United States (
Walsh
and Zoback, 2015
).
4.
Misconception:
Induced seismicity only occurs close to the
injection well and at a similar depth as injection.
Correction:
Seismicity can be induced at distances of
10 km or more away from the injection point and at sig-
nificantly greater depths than injection. In the classic case
of injection-induced seismicity at the Rocky Mountain Ar-
senal, seismicity was induced at distances of at least 10 km
laterally from the well and at depths of at least 4 km greater
than the depth of injection (
Healy
et al.
, 1968
;
Herrmann
et al.
,1981
;
Hsieh and Bredehoeft, 1981
). More recent re-
ports have argued that seismicity may be induced at 20 km
or more from the injection point (
Keranen
et al.
, 2014
).
5.
Misconception:
All injection wells (hydraulic fracturing,
wastewater disposal, and enhanced oil recovery) induce
earthquakes.
Correction:
Most injection wells do not cause felt earth-
quakes. There are approximately 35,000 active wastewater
disposal wells, 80,000 active enhanced oil-recovery wells,
and tens of thousands of wells are hydraulically fractured
every year in the United States. Only a few dozen of these
wells are known to have induced felt earthquakes. A
combination of many factors is necessary for injection to
induce felt earthquakes. These include faults that are large
enough to produce felt earthquakes, stresses that are large
enough to produce earthquakes, the presence of fluid path-
ways from the injection point to faults, and fluid pressure
changes large enough to induce earthquakes. It is likely that
some of these criteria are not met in areas that have very few
or no earthquakes that may be induced by wastewater in-
jection, such as theWilliston Basin in North Dakota (
Froh-
lich
et al.
, 2015
), the Michigan Basin, and the Gulf Coast of
Texas and Louisiana (
Weingarten
et al.
,2015
).
More injection wells may be inducing earthquakes, but cur-
rent studies are limited to the largest earthquakes and those
with the best seismological and industrial data available.
Further study of other earthquake sequences may reveal that
additional felt earthquakes are induced. It is likely that
smaller induced earthquakes are occurring and are too small
to detect.
In some sense, all hydraulic fracturing induces earthquakes,
although typically microearthquakes. When production en-
gineers hydraulically fracture, they are intentionally cracking
the rock, causing microearthquakes that are typically smaller
than
M
0.
6.
Misconception:
Wells injecting at zero injection pressure at
the wellhead (i.e., wells where the formation readily ac-
cepts fluid without requiring the fluid to be pushed into
the formation) cannot induce earthquakes.
Correction:
Wells injecting under gravity feed (i.e., wells
where you can pour fluid down the well without added
pressure) increase the fluid pressure within the injection
formation and thus can induce earthquakes. For example,
2 Seismological Research Letters Volume 86, Number 4 July/August 2015
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the vast majority of wells in the Raton Basin are injecting
under gravity feed and have induced an earthquake se-
quence that is ongoing since 2001 and includes an
M
5.3
earthquake and an
M
5.0 earthquake (
Barnhart
et al.
,
2014
;
Rubinstein
et al.
, 2014
).
INDUCED VERSUS TRIGGERED
Earthquakes stimulated by human activities are commonly re-
ferred to as being either induced or triggered. As originally pro-
posed by
McGarr
et al.
(2002)
, induced should be used for
earthquakes where human-introduced stresses are similar in
amplitude to the ambient stress state, and triggered should be
used for earthquakes where human-introduced stresses are only
a
“
small fraction of the ambient level.
”
In the seismology com-
munity, triggered has an additional meaning: earthquakes
caused by earlier earthquakes are triggered by the previous
earthquake (
Freed, 2005
). This process applies to natural and
man-made earthquakes alike. To avoid any confusion between
the two kinds of triggered earthquakes, we suggest using in-
duced exclusively to describe all anthropogenic earthquakes,
and we have done so in this article. Accordingly, triggered refers
to the physical interaction between parent and daughter events,
whether natural or anthropogenic in origin.
HYDRAULIC FRACTURING
Invented in 1947 (
Hubbert and Willis, 1957
), hydraulic frac-
turing (often colloquially referred to as fracking), is a technique
that has been used for decades in the oil and gas industry. Ap-
proximately one million wells were hydraulically fractured in
the United States between 1947 and 2010 (
Gallegos and Var-
ela, 2014
). Hydraulic fracturing is a technique that aims to im-
prove the production of wells by increasing the number of and
extending the reach of fluid pathways (i.e., fractures) between
the formation and the well. Hydraulic fracturing achieves this
by injecting fluid, typically water, at high pressure into low-per-
meability rocks, such that the fluid pressure fractures the rocks
or stimulates slip across pre-existing faults or fractures (Fig.
2a
).
Increasing the fracture density and extent of the fracture net-
work enhances fluid flow and allows for more distant fluids to
be accessed by a well. In addition to fluid, a propping agent
(e.g., sand) is injected to keep the newly formed fractures
open. Following hydraulic fracturing, which takes a few hours
to a few days, there is a period where the hydraulic fracturing
fluid is allowed to flow back to the surface where it is collected
for disposal, treatment, or reuse. Subsequent to flow back, the
wells begin production (i.e., the extraction of oil or gas begins)
(Fig.
2b
).
At first, vertical oil wells were hydraulically fractured to
increase production. Then, in the 1990s, extended reach hori-
zontal drilling technology was developed. This allowed drillers
to steer wells more precisely such that they could remain within
narrow horizontal and subhorizontal oil and gas reservoirs over
great distances. This enabled production along the length of
the well within the production formation. This technology,
(a)
(b)
(c)
(d)
▴
Figure 2.
Simplified diagrams of oil-field operations. The geology
in these diagrams is simplified from natural situations in which there
are many more rock layers. Arrows show the directions of fluid
being injected or withdrawn. Arrow color indicates the contents
of the fluid: black (oil, gas, and wastewater), yellow (oil and
gas), and blue (wastewater). (a) For a period of hours to days, fluids
are injected at high pressure into a production well. The pressures
are high so that the rock surrounding the well fractures and the
permeability is increased. Increased permeability allows the extrac-
tion of oil or gas from a larger region. This technique of high-pres-
sure injection is known as hydraulic fracturing. Following the
hydraulic fracturing of a well, the well goes into production. (b) Pro-
duction wells extract oil and gas from the ground. Some, but not all,
production wells are hydraulically fractured. (c) Production wells
extract oil and gas, and as a byproduct, salt water. The salt water
is found in the same formation as the oil and gas and is commonly
termed
“
produced water.
”
The oil and gas are separated from the
produced water, and the produced water is injected into a deeper
formation with the disposal well. In practice, the wastewater from
many production wells is injected into a single injection well. (d) An
alternative to wastewater disposal is enhanced oil recovery. In en-
hanced oil recovery, produced water is injected into the formation
holding the oil and gas. The injection of produced water is intended
to sweep oil and gas that is close to the injector toward the pro-
duction wells to enhance oil recovery.
Seismological Research Letters Volume 86, Number 4 July/August 2015 3
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combined with hydraulic fracturing, unlocked gas and oil
resources in tight formations (e.g., shales) and is largely respon-
sible for the recent boom in gas and oil production in the
United States.
In some sense, hydraulic fracturing is intended to cause
earthquakes, albeit very small, in that the intent is to fracture
the rock. These intentionally produced earthquakes, often
termed microseismic events, are typically on the order of
−
3
:
0
≤
M
≤
0
(
Warpinski
et al.
, 2012
). In cases when hy-
draulic fracturing induces earthquakes of larger magnitudes,
the earthquakes are likely the result of the reactivation of
nearby pre-existing faults (
Maxwell
et al.
, 2010
). Despite being
invented in 1947, the first report of a felt hydraulic fracturing-
induced earthquake was in 1991 (
Kanamori and Hauksson,
1992
). Since 2011, a number of other earthquake sequences
with felt earthquakes induced by hydraulic fracturing have been
reported (
Green
et al.
, 2012
;
Holland, 2013
;
Friberg
et al.
,
2014
;
Skoumal
et al.
, 2015
). The largest hydraulic fracturing-
induced earthquakes to date are two
M
L
4.4 earthquakes in
central west Alberta and northeast British Columbia (
BC Oil
and Gas Commission, 2014
). In these cases, the total injected
volumes were remarkably high for hydraulic fracturing (e.g.,
630,000 barrels or
100
;
000 m
3
for both
M
L
4.4 earthquakes;
H. Kao, personal comm., 2015).
WASTEWATER DISPOSAL
Waste fluids are often a byproduct of many oil and gas extrac-
tion operations. At times these fluids can be cleaned and
reused or applied for other purposes. In other instances they
are unsuitable for other uses and must be disposed. When
waste fluids must be disposed, they are often injected deep
underground into high-permeability formations, usually deeper
than the production reservoirs, for permanent sequestration
and isolation from oil/gas reservoirs and drinking-water aqui-
fers (Fig.
2c
). The wells in which these fluids are disposed
are known as wastewater wells or salt-water disposal wells.
Underground disposal of wastewater has a lengthy history be-
cause it is typically considered an economic and safe option
(
Ferguson, 2015
).
The contents of wastewater can be highly variable. In
some places, it is primarily spent hydraulic-fracturing fluid
(e.g., Ohio and Arkansas), whereas in other locations waste-
water often consists mostly of formation brines that come
to the surface at the same time as the oil and gas that is ex-
tracted. For instance, in Oklahoma, only 10% of the fluid
injected into disposal wells is spent fluid that was initially
used in hydraulic fracturing and cannot be reused (
Murray,
2013
). These formation brines (also termed produced water
or wastewater) are typically salt water that is trapped in the
same pore space as the oil and gas and comes up with the oil
and gas. This salt water is often laden with dissolved salts,
minerals, and occasionally other materials that make it unsuit-
able for other uses.
Injection rates of disposal wells range widely, with some
wells injecting 100 barrels (
16 m
3
)/month and other wells with
rates exceeding one million barrels
160
;
000 m
3
=
month
.
The Denver earthquakes of the 1960s, caused by injection
of chemical waste at the Rocky Mountain Arsenal, were the
first earthquakes to be identified as related to deep, under-
ground injection (
Evans, 1966
;
Healy
et al.
, 1968
). The largest
earthquake in the sequence was
M
4.9 (
Herrmann
et al.
, 1981
).
Many more injection-induced earthquakes have been identified
since this sequence, the largest being the August 2011
M
5.3
Trinidad, Colorado, earthquake (
Rubinstein
et al.
, 2014
) and
the November 2011
M
5.6 Prague, Oklahoma, earthquake
(
Keranen
et al.
, 2013
).
ENHANCED OIL RECOVERY
Enhanced oil recovery is a suite of injection techniques used by
the oil and gas industry to allow or encourage more oil and gas
to be produced from a reservoir than would come out on its
own (
Murray, 2013
)(Fig.
2d
). The techniques typically involve
the injection of water, steam, or carbon dioxide into the
production formation. Water flooding (the injection large
amounts of water) improves production by sweeping the oil
and gas toward the production wells. The injection of steam
and carbon dioxide is undertaken to improve production by
reducing the viscosity of oil. Enhanced oil-recovery operations
typically aim to keep the fluid pressure in the reservoir at or
below its original level.
Many cases of enhanced oil recovery-induced earthquakes
have been identified (see
Nicholson and Wesson, 1992
and
references therein). In fact, one of the best documented cases
of injection-induced earthquakes was from water flooding near
Rangely, Colorado (
Gibbs
et al.
, 1973
;
Raleigh
et al.
, 1976
).
Because this field had an extensive history of induced earth-
quakes from enhanced oil recovery, it was selected for an ex-
periment in earthquake control by the U.S. Geological Survey.
Raleigh
et al.
(1976)
demonstrated that by varying the fluid
pressure at depth, they could control whether or not earth-
quakes were induced. The largest earthquake known to be
induced by enhanced oil recovery is an
M
4.6 earthquake
in the Cogdell field near Snyder, Texas (
Gan and Froh-
lich, 2013
).
MECHANISM OF INDUCED SEISMICITY
There are many ways in which human activities induce earth-
quakes (e.g., reservoir impoundment, fluid injection, fluid ex-
traction, and mining). Each of these actions fundamentally
causes earthquakes in the same way: they change the stress con-
ditions on faults, which can facilitate failure. Fluid injection
can induce earthquakes in four different ways: (1) the injection
of fluids raises pore-fluid pressure within a fault, (2) the injec-
tion of fluids fills and compresses fluids within pore spaces
causing deformation (poro-elastic effects), (3) the injection
of fluid that is colder than the rock into which it is being in-
jected into causes thermoelastic deformation, and (4) fluid
4 Seismological Research Letters Volume 86, Number 4 July/August 2015
SRL Early Edition
injected adds mass to the injection formation. Observations
and numerical modeling indicate that increased fluid pressure
within faults most strongly influences whether an injection
well will induce earthquakes (
Raleigh
et al.
, 1976
;
Shapiro
and Dinske, 2009
;
McClure and Horne, 2011
).
It is critical to note that the injected fluids need not travel
the entire distance from the injection well to a fault for the
injection to affect the fault
’
s behavior. Injection can affect a
fault
’
s behavior via the change in fluid pressure, which can
be transmitted greater distances than fluids themselves. Like
stepping on the brakes in a car, the increase in the fluid pressure
that is initiated at the well (or brake pedal) is transmitted to the
fault (brakes) without the fluid traveling the full distance be-
tween the well and fault.
As fluid is injected into a reservoir, the fluid pressure
within that reservoir rises. If this fluid pressure increase is trans-
mitted to a fault, the increase in pore pressure counteracts the
stresses holding the fault closed (the normal stress), resulting
in a lower effective stress. With lower effective normal stress
clamping a fault, the frictional resistance to slip is lower and the
fault is more prone to slip. The effect of fluid injection on fault
failure is illustrated with a Mohr-Coulomb diagram, which
shows a failure envelope for a typical compressive medium
(Fig.
3
). As pore pressure increases, the Mohr circles shift to
the left and closer to the failure envelope. If the faults are suit-
ably oriented with respect to the local stress field, they may slip
and cause earthquakes.
In addition to the fluid pressure in candidate faults, there
are many other factors that influence whether or not injection
will induce earthquakes. These include injection parameters
(e.g., cumulative injected volume, injection rate, injection pres-
sure, fluid temperature, and injection depth) and reservoir con-
ditions (e.g., pore pressure, temperature, rock strength, the
presence of pre-existing faults and their orientation relative
to the local stress field, and reservoir permeability;
Shapiro
and Dinske, 2009
;
Zoback, 2012
). Many of these parameters
are not easily constrained or are unknown, which makes it diffi-
cult to determine the wells that will induce earthquakes and those
that will not. Most often, faults reactivated by injection activities
were unmapped before the earthquakes illuminated them.
WHICH METHOD OF FLUID INJECTION HAS THE
HIGHEST LIKELIHOOD OF INDUCING DAMAGING
EARTHQUAKES?
Hydraulic fracturing, long-term wastewater injection, and en-
hanced oil recovery have all induced earthquakes in the United
States and Canada in the past few years (
Horton, 2012
;
Gan
and Frohlich, 2013
;
Holland, 2013
). As discussed above, waste-
water disposal is responsible for the vast majority of the in-
crease, including the largest and most-damaging induced
earthquakes (
Horton, 2012
;
Keranen
et al.
, 2013
;
Frohlich
et al.
, 2014
;
Rubinstein
et al.
, 2014
). Here, we explore why
wastewater disposal is responsible for this change.
It is probable that the duration of injection, the magni-
tude of the fluid pressure increase, and the size of the region
affected by injection will strongly influence whether earth-
quakes will be induced and how large they will be. Larger fluid
pressure changes are more likely to induce earthquakes than
smaller pressure changes, larger volumes of injected fluid in-
crease the probability of a large fault being affected by the
fluid pressure change, and a longer duration of injection gives
earthquakes a longer time window during which they can
nucleate.
A quick examination of the above factors suggests that
wastewater injection into previously undisturbed formations
is more likely to induce felt earthquakes than hydraulic frac-
turing. Although the higher injection pressures suggest that hy-
draulic fracturing would be more likely to induce earthquakes
than wastewater disposal, the duration of injection and the to-
tal volume of injection of hydraulic fracturing is much smaller
than wastewater disposal. Wastewater disposal wells typically
operate for years or decades, whereas hydraulic fracturing lasts
Shear
Stress
Normal
Stress
σ
1
σ
1
′
σ
3
Failure Envelope
σ
3
′
C
T
P
0
▴
Figure 3.
Mohr circle diagram showing the effect of increased
fluid pressure on a fault. Normal stress on the horizontal axis
(compression when positive and tension when negative) and
shear stress on the vertical axis. The maximum and minimum nor-
mal stresses acting in any given location are plotted as
σ
1
and
σ
3
,
and a Mohr circle (shown in red) is drawn to represent the range
of stresses acting on a plane at one location, showing both the
shear and normal stress at a given location. When fluid pressure
(
P
) is increased, normal stresses are reduced by
P
, resulting in
new normal stresses
σ
′
1
and
σ
′
3
, moving the Mohr circle to the left
by
P
(purple). This also means that the Mohr circle is closer to the
failure envelope and makes shear or tensile failure more likely.
The blue line represents the failure envelope with the slope being
equal to the frictional resistance at that point on the plane. When
the stress conditions exceed the shear strength of the fault, slip
on that fault may occur. The failure envelope is computed as the
sum of the cohesion
C
(an intrinsic property of an individual rock)
and frictional resistance (resistance to slip on a fault). When the
minimum principal normal stress
σ
3
is less than
T
, the tensile
strength of the rock, the rock will fail in tension, that is, cracks
will open. Figure after figure 7 in
Maxwell (2013)
.
Seismological Research Letters Volume 86, Number 4 July/August 2015 5
SRL Early Edition
for days. Wastewater disposal wells also inject far more fluid
than hydraulic fracturing, so they affect a much larger region.
The largest hydraulic fracturing treatments inject approxi-
mately 250,000 barrels (
Gallegos and Varela, 2014
) over the
course of a few days; in the United States there are
well over 1000 disposal wells that inject 100,000 barrels/month
or more (
Weingarten
et al.
, 2015
). Within a matter of months,
all of these wastewater disposal wells will greatly exceed the
volumes injected by even the largest hydraulic-fracturing oper-
ations, which implies that they are more likely to induce earth-
quakes. By virtue of its longer duration and larger injection
volumes, wastewater injection is more likely to induce earth-
quakes at a greater distance and over a longer time span than
hydraulic fracturing and thus, is a much more important source
of induced earthquakes than hydraulic fracturing.
Wastewater injection into undisturbed formations is also
more likely to induce earthquakes than injection for enhanced
oil recovery. The durations and volumes for both kinds of
wells are similar. The difference between these wells is that
enhanced oil recovery injects large volumes of fluid into de-
pleted reservoirs where oil and gas have already been extracted
and recycles produced water such that the pressure within the
injection reservoir rarely exceeds the preproduction level. In
contrast, wastewater injection is injected into virgin forma-
tions and thus raises the pore pressure from their initial levels.
Avoiding pore-pressure increases within reservoirs reduces the
likelihood of enhanced oil-recovery operations inducing earth-
quakes.
These considerations are in accord with observations.
Wastewater injection is associated with all of the largest injec-
tion-induced earthquakes (
Horton, 2012
;
Frohlich
et al.
,
2014
;
Keranen
et al.
,2014
;
Rubinstein
et al.
,2014
), and there
are many more reported cases of wastewater injection-induced
earthquakes in the past five years in the United States alone
(
Frohlich
et al.
,2011
,
2014
;
Frohlich, 2012
;
Justinic
et al.
,
2013
;
Kim, 2013
;
Block
et al.
, 2014
;
Keranen
et al.
, 2014
),
and yet there are only approximately 35,000 wastewater disposal
wells that are active in the United States. This is in contrast to
hydraulic fracturing, which is far more common (
∼
1
:
8
million
treatments over
∼
1
million wells, 1947
–
2010 in the United
States;
Gallegos and Varela, 2014
) than wastewater disposal
wells, and yet there are only three reported cases of hydraulic
fracturing-induced earthquakes in the United States (
Holland,
2013
;
Friberg
et al.
, 2014
;
Skoumal
et al.
,2015
) and only a few
more worldwide (
BC Oil and Gas Commission, 2012
,
2014
;
Green
et al.
, 2012
;
Farahbod
et al.
,2015
). Likewise, enhanced
oil recovery is quite common (
∼
80
;
000
active wells United
States;
Weingarten
et al.
,2015
), yet there are few recent earth-
quakes associated with it (
Gan and Frohlich, 2013
).
Results from modeling analyses also support the notion
that wastewater injection is more likely to induce large earth-
quakes. The injection pressure, pressure within the field, dura-
tion of injection, and volume of rock affected by injection all
influence the likelihood of inducing earthquakes (
Langenbruch
and Shapiro, 2010
;
Bachmann
et al.
, 2011
;
McClure and
Horne, 2011
).
WHY NOW?
The recent increase in injection-induced seismicity is caused by
a corresponding increase in wastewater disposal in the central
United States. The earthquake rate increase in Oklahoma, where
the vast majority of the increase has occurred (585 of 688
M
≥
3
earthquakes in the central United States in 2014), corresponds
to a doubling of the wastewater disposal rate in the state from
1999 to 2013 (
Walsh and Zoback, 2015
). Focusing on the areas
of increased seismicity within Oklahoma, we find that injection
increased by factors of 5
–
10 (
Walsh and Zoback, 2015
). Other
areas of increased rates of induced earthquakes also experienced
sudden increases in wastewater disposal (
Frohlich, 2012
;
Hor-
ton, 2012
;
Frohlich
et al.
,2014
;
Keranen
et al.
,2014
;
Rubinstein
et al.
,2014
).
SUMMARY AND OUTLOOK
Although enhanced oil recovery and hydraulic fracturing
have been implicated in some recent seismicity, studies indicate
that the majority of the increase in seismicity is induced by the
deep disposal of fluids produced by oil and gas production
(wastewater disposal). Hydraulic fracturing does not play a
key role in the increase in that (1) hydraulic fracturing does
not typically induce felt earthquakes; (2) in Oklahoma, the lo-
cation of the largest increase in seismicity, spent hydraulic
fracturing fluid does not represent a large percentage of the
fluids comprising disposed wastewater; and (3) oil produced
from many fields with large volumes of produced water did
not involve any hydraulic fracturing. Similarly, enhanced oil
recovery does not play a major role in the increase in seismicity,
likely because operators attempt to keep fluid pressures in the
reservoir balanced with the fluid pressure prior to production.
Accordingly, wastewater disposal is responsible for
inducing the majority of the earthquakes. Increased fluid pres-
sure is the probable driving mechanism to induce earthquakes,
and of the three aforementioned processes, wastewater disposal
wells can raise fluid pressures more, over longer periods of time
and over larger areas, than either of the other injection
methods.
Although seismicity associated with salt-water disposal has
caused damaging earthquakes, we have not yet seen a cata-
strophic event or fatalities. Preliminary results in a number
of areas of induced seismicity indicate that the earthquake haz-
ard in these areas is comparable to the hazard in areas more
traditionally known for earthquakes, such as California (
Pe-
tersen
et al.
, 2015
). The increase in hazard is undoubtedly
of concern and efforts to assess the hazard from induced earth-
quakes are ongoing. Fortunately, some authors have suggested
that there is hope for mitigating the likelihood of damaging
earthquakes through detailed seismic monitoring, careful selec-
tion of injection locations, variation of injection rates and
pressures in response to ongoing seismicity, and a clear man-
agement plan (
Zoback, 2012
;
McGarr
et al.
, 2015
;
Walters
et al.
, 2015
). Mitigation of hazard from future-induced events,
however, requires a detailed understanding of the physical
6 Seismological Research Letters Volume 86, Number 4 July/August 2015
SRL Early Edition
processes involved in inducing large magnitude events and a
detailed understanding of the geology and hydrology at the
site of the earthquakes. To reach this goal, three kinds of data
will be necessary: (1) seismic data: high-quality, real-time earth-
quake locations, which require dense seismic instrumentation;
(2) geologic data: hydrological parameters, orientation and
magnitude of the stress field, and the location and orientation
of known faults; and (3) industrial data: injection rates and
downhole pressures sampled and reported frequently. Manag-
ing the likelihood of induced earthquakes is an ambitious, but
possible task that will require collaboration between scientists,
industry, and regulators.
ACKNOWLEDGMENTS
The authors thank C. Frohlich, J. Kaven, W. Ellsworth, A.
McGarr, H. Collela, R. Gertson, M. Agard, J. Brandon, F.
Terra, and Editor Z. Peng for their thoughtful comments
on the manuscript. We thank A. Barbour, F. Terra, C. Potter,
and M. Weingarten for their help in developing Figure
1
.We
also thank M. Szczepanik, who created Figure
2
.
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Justin L. Rubinstein
U.S. Geological Survey
Menlo Park, California 94025 U.S.A.
jrubinstein@usgs.gov
Alireza Babaie Mahani
Pacific Geoscience Center
Geological Survey of Canada
Sidney, British Colombia
Canada V8L 5T5
Published Online 10 June 2
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