# White Paper: Induced Seismicity and Enhanced Geothermal Systems

E. L. Majer

Center for Computational Seismology

Ernest Orlando Lawrence Berkeley National Laboratory

Berkeley, California

## Purpose of Paper

The objective of this summary is to present an up-to-date review of the
various factors concerning the potential causes of induced seismicity
due to enhancing production from geothermal systems. Although there are
no reliable methods for predicting individual earthquakes this paper
will also discuss various factors that have affected seismicity in
geothermal areas and the potential to mitigate induced seismicity.
Although seismicity may have a negative connotation we will also examine
the positive impacts of seismicity such as a tool for reservoir
monitoring and management, evidence for fracture creation and mapping
existing/potential flow paths.

## Seismicity

It must be kept in mind that there are many different mechanisms that
have been proposed for inducing earthquakes. Induced seismicity has not
only been noted in geothermal reservoirs but in reservoir impoundment
(water behind dams), waste injections, and oil and gas operations.
Another type of induced seismicity is seismicity associated with
hydrofracturing. Hydrofracturing is distinct from many types of induced
seismicity because hrdrofracturing by definition is only created when
the "fract" gradient in fluid injection is exceeded and tensile failure
occurs creating a "driven" fracture. Shear failure has been observed
associated with hydrofracturing operations, in fact due to the very high
frequency of the tensile failure ( seismic source at the crack tip only)
only shear failure is observed by microseismic monitoring in many
instances. However, hydofracturing is such a small perturbation it is
rarely a hazard when it is intentionally used to enhance permeability.
To our knowledge hydrofracturing to intentially create permeability
rarely creates induced unwanted seismicity. In those other cases of
induced seismicity the issue has been successfully dealt with or
mitigated. In the case of geothermal induced seismicity withdrawal of
fluids as well as injection of fluids can cause seismicity among many
other factors, there is not a strict one to one correlation with
injection ( Allis, 1981, Oppenheimer, 1986).

Seismicity also occurs at many different time and spatial scales. Creep
on a fault could be considered seismicity just as a much as a sudden
loss of cohesion on a fault. Growth faults in the overpressurized zones
of the Gulf Coast is an example. As defined here we will only deal with
events that are sudden and cause "earthquakes". The reason for this
sudden movement (the reason we do not use "slippage" rather than
movement is that slippage may imply that a fault plane already exists,
in some cases new faults or fractures may be created) is that an
imbalance of stresses have been accumulated and the forces holding the
earth in place are not strong enough to prevent failure. If one examines
the subsurface in enough detail one can find fractures, joints, and/or
faults almost anywhere in the world. A fault is not defined in terms of
size, (definition of a fault is a displacement across a fracture or
fracture zone), however, most mapped faults range in size from very
small (few meters) to very large (hundreds of kilometers long). The size
of an earthquake (or how much energy is released) depends upon how much
slip occurs on the fault, how much stress there is on the fault before
slipping, how fast it fails, and over how large an area it occurs.
Damaging earthquakes (usually greater than magnitude 4 or 5) require the
surfaces to slip over relatively large areas (kilometers). For slip to
occur there must also be an imbalance in the stresses and forces acting
within the earth. In other words, if there is not an imbalance in the
forces in the subsurface then there is no net force available to cause
slip, i.e., a sudden release of stored energy. The forces that are
acting to deform the earth and result in an excess energy accumulation
are of course forces that are fundamentally generated by the dynamic
nature of the whole earth. In most regions where there are economic
geothermal resources there is usually tectonic activity, such as in the
western United States. These areas are more prone to induced seismicity
than in more stable areas of the U.S. such as the central U.S. (It must
be noted, however, that one of the largest earthquakes ever to occur in
the U.S. was the New Madrid series of events in the early 1800's in
Missouri, it rang church bells in Boston). It must also be noted that
seismic activity is only a hazard if it occurs above a certain level. At
some level there is seismic activity almost everywhere.

Another factor to consider is that the earth is not a homogeneous
medium. Over the millions of years of movement the surface of the earth
has been deformed and broken into many different patterns. In some areas
where there has been consistent movement, large fault systems have
formed. If the forces are still present then there is a potential for
earthquakes to occur. The San Andreas Fault system in California is one
example. As pointed out above, however, the slip does not have to occur
in discrete or sudden jumps. For example, there are many places along
the San Andreas Fault where the fault is creeping, rather than jumping
in a "stick-slip" type of movement. This partially accounts for the high
level of seismicity in some areas of California, and the low level in
other areas. Figures 1 and 2 are maps of California showing the location
of earthquakes with magnitudes greater than 5.0 (northern California and
part of Nevada only) and less than 3.0, respectively. A significant
feature to note in Figures 1 and 2 is the relative number of events
greater than 5.0 (this includes magnitude 6's and 7's also) compared to
the number of events less than 3.0. The significant factor being that in
general, for each point increase in magnitude, there are about ten times
fewer earthquakes, i.e., ten times fewer 5's than 4's, and one hundred
times fewer 6's than 4's, etc. Also, large or damaging earthquakes tend
to occur on developed or active fault systems. In other words, large
earthquakes rarely occur where there is not a fault large or long enough
to release enough energy. Also it is difficult to create a large new
fault, because there is usually a pre-existing fault that will slip
first, rather than a new fault being created. As can be seen in Figures
1 and 2, all significant historical activity above magnitude 5.0 that
has been observed in California has occurred on preexisting faults. One
last important feature to note regarding earthquake activity is that the
size of the fault, in addition to the forces available, and the strength
of the rock, determine how large an event may potentially be. It has
been shown, that in almost all cases, large earthquakes start at depth.
It is only at depth (five to ten kilometers) where there can be enough
stored energy to provide the adequate amount of force to move the large
volumes of earth required to create a large earthquake.

# Induced Seismicity

Seismicity has been correlated with a number of human activities. For
example, mining activities in the deep gold mines of South Africa have
produced large "rock bursts" when the removal of rock relieves the
stress. Seismicity also occurs due to a volume reduction in the
subsurface, i.e., material is removed and a collapse occurs. Seismicity
is also associated with the collapse of the cavity created as a result
of an underground nuclear explosion. Fluid extraction is also a cause of
induced activity. The most famous case being the Wilmington, California
oil field events in the 1940's and 1950's. A third type of induced
seismicity has been associated with fluid injection. The most notable
being the seismicity associated with the fluid disposal operations at
the Rocky Mountain Arsenal near Denver, Colorado. In that case
seismicity increased as the rate of fluid injection increased. A more
complicated situation of induced seismicity is the case of geothermal
fields. For example, seismicity has steadily increased as production has
increased at The Geysers geothermal field in northern California. In
most geothermal fields fluids are being withdrawn and injected in the
same region, thermal and chemical effects are also occurring, and all
these effects combine to produce a variety of mechanisms for inducing
seismicity. Lastly, an increase in seismicity has also been observed
when reservoirs are impounded behind dams.

Water injection seems to be a common cause of induced seismicity. The
Denver disposal operations and some geothermal cases of induced
seismicity are closely related to the phenomenon of the effect of the
pore pressure in the earth reducing the "effective strength" of the
rocks in the subsurface. Pore pressure is the value of the pressure of
the fluid within the pores and fractures of the rock matrix in the
subsurface. The magnitude of the pressure is usually just the weight of
the water column at any particular location and depth. The deeper one
goes in the earth the higher the pore pressure. As pointed out before, a
fault will slip (an earthquake) when the forces acting to cause slip are
greater than the forces keeping the fault together. The forces keeping
it together are friction, the inherit strength of the rock, and the
pressure (from the weight of earth above) holding the surfaces together.
Pore pressure acts where the rock is most permeable, i.e. usually the
faults. The role of pore pressure in earthquake generation is that it
tends to push apart the fault surfaces (in effect reduces the pressure
from the weight of the rock), reducing the amount of force needed to
cause the rock to slip on the fault. This is where fluid injection plays
a role. Because the earth is not a perfectly homogeneous material that
will "take" fluids at any rate, as one tries to inject water into the
earth, there will be a pressure build up around the point of injection.
In a very porous, permeable material the fluid will flow easily and the
pressure build up will be small, i.e., the tendency will be for the
fluids to disperse quickly, resulting in a small increase in the pore
pressure. In some cases, where the rock is less porous and less
permeable, it may take a great amount of pressure to inject fluids,
causing a large pore pressure build up along the fluid pathway
(sometimes a fault). If there are forces acting in a direction to cause
an earthquake, the pore pressure will reduce the forces holding the
fault together, and the surfaces will then slip, causing an earthquake.
In the case of injected fluids, the size, rate and manner of seismicity
is controlled by the amount of fluid injected in the subsurface, the
orientation of the stress field relative to the pore pressure increase,
how extensive the local fault system is, and last but not least the
deviatoric stress field in the subsurface, i.e., how much excess stress
there is available to cause an earthquake. If there is no net force
available to push the rocks in any preferred direction, then a pore
pressure buildup would have to be so great as to literally push the
fault surfaces apart so that they were not touching, even then they may
not move. An indication of how much excess force is available in the
subsurface is the amount of historical seismicity there has been in an
area. In almost all cases of induced seismicity there has not been any
induced seismicity (greater than magnitude 4.0) where there has not been
historical seismicity of the same size or larger. In other words it is
difficult to induce seismicity of a large magnitude where the faults are
small, the rocks are weak (large amounts of energy can not be built up),
and/or where there is not sufficient energy (stress build up) to cause
an earthquake.

The rate of fluid injection also has an impact on seismicity. In
geothermal regions one tends to focus on the volume of injections rather
than on the rate. For example, there are two main mechanisms that have
been hypothesized to cause induced seismicity due to reservoir
impoundment, rapid stress buildup due to reservoir loading, and an
effective reduction in strength due to pore pressure build up. In
general the first effect is characterized by a rapid response to
reservoir filling. Once the load is increased through a large body of
water sitting on the surface, the earth will usually respond in a
relatively quick fashion. The seismicity in most of these cases is
shallow, small magnitude, spatially related to the reservoir, and
usually subsides after the earth has adjusted to the load, i.e, a
temporary redistribution of the stress field. In the second case of
increased pore pressure it is usually characterized by a delayed effect,
because it takes time for the pore pressure to diffuse to depth. This
effect is similar to seismicity caused by fluid injections. Water
pressure is built up due to a large, deep, body of water sitting on top
of the earth, i.e., the water pressure at depth is increased causing an
effective reduction in the forces holding the faults together. Unlike
water injection through a pipe, however, there is no direct connection
to the subsurface, the only connection being the pathways that faults
and fractures provide. Also, unlike water injection in a well, the paths
to the subsurface depend on where the reservoir sits relative to the
faults and fractures providing the path to the subsurface. The amount of
pressure built up also depends upon the height of the water column,
i.e., the depth of the reservoir. Therefore, it is easy to see why large
magnitude event is not a common phenomenon, a variety of factors must
come together at the right time (enough energy has been stored up by the
earth to be released), and in the right place (on a fault large enough
to produce a large event), for a significant earthquake to occur.

Another controlling factor in induced seismicity is the regional stress
field. For example,

although California is on the edge of a plate, the geology and stress
distribution is such that some areas of California are more prone to
earthquakes than others. Figures 3 is a map of California showing where
the seismicity greater than a magnitude of 3.0 and less than 5.0
occurred between 1900 to 2004. As stated above, several conditions must
be met for significant earthquakes to occur (damaging events). There
must be a large enough fault system so that there is a fault to slip,
there must be forces present to cause slip along the fault (as opposed
to some other direction) and these forces must be greater than the
forces holding the fault together (the sum of the forces perpendicular
to the fault plus the strength of the material in the fault). Also, as
pointed out above, larger earthquakes usually only occur at depth (ten
kilometers or greater) where there is a chance to build enough elastic
energy to cause a large event.

## Geothermal Induced Seismicity 

Water injection into geothermal systems has become a nearly universal
and often required strategy for extended and sustained production of
geothermal resources. For example, to reduce a trend of declining
pressures and increasing non-condensable gas concentrations in steam
produced from The Geysers, operators have been injecting steam
condensate, local rain and stream waters, and most recently treated
wastewater piped to the field from neighboring communities. Monitoring
of microearthquakes related to production and injection has been
conducted since the mid 1970's. MEQ has been applied as a general
indicator of fluid paths and general response to injection at The
Geysers for over 20 years (Majer, 1978, Majer and McEvilly 1979, Marks
et al., 1978; Ludwin and Bufe, 1980; Peppin and Bufe, 1980; Bufe et al.,
1981; Allis, 1982; Denlinger and Bufe, 1982; Ludwin et al., 1982;
Eberhart-Phillips and Oppenheimer, 1984; Oppenheimer, 1986; Stark, 1992,
Stark and Majer, 1989, Beall et al., 1999, Smith et al., 2000). A
dramatic increase in planned injection rates and spatial extent of
injection due to the recent completion of a wastewater pipeline (from
the city of Santa Rosa) has raised concerns regarding the societal and
economic impact of injection related seismicity. It is possible that the
rate of MEQ events may place an upper bound on injection at The Geysers.
Already the operators are evaluating a 50 percent increase over the
initial Santa Rosa injection. Without this injected water the thermal
capacity of The Geysers will be underutilized and The Geysers will not
be able to provide California with as much low cost electricity as
possible. Vapor-dominated and very hot "tight" geothermal reservoirs
such as The Geysers by their very nature are water-short systems. These
are prime candidates for enhanced geothermal activities. If The Geysers
were produced without simultaneously injecting water, reservoir
pressures and flow rates from production wells would decline fairly
rapidly, and would reach uneconomically small levels while enormous heat
reserves would still remain in the reservoir rocks. Furthermore, in some
of these systems a significant portion of the recoverable geothermal
energy is currently underutilized due to high concentrations of
non-condensable gas and corrosive HCl. Mitigation of these deleterious
components through water injection would significantly increase the
resource.

Water injection is not automatically beneficial. Injected water may
migrate along major fractures and quickly reach production wells, which
may degrade production by lowering fluid enthalpy and temperature. At
its best, injected water will be completely vaporized by contact with
hot rocks before it reaches production wells, supplying additional
steam, and increasing reservoir pressures and production well flow rates
with minimal or even positive societal impact. Injection can also
improve the quality of produced fluid from a chemical viewpoint, by
reducing concentrations of non-condensable gases such as CO~2~ and
corrosive gases such as HCl.

Several studies have demonstrated that MEQs at The Geysers geothermal
area are associated with both water injection and steam extraction
(Majer and McEvilly 1979; Eberhart-Phillips and Oppenheimer 1984; Enedy
et al. 1992; Stark 1992; Kirkpatrick et al. 1999; Smith et al. 2000;
Stark 2003; Mossop and Segall 2004). These studies include correlation
of spatial and temporal MEQ distributions with injection/production
data. In a recent paper, Mossop and Segall (2004) make a comprehensive
correlation study based on induced seismicity and operational data from
1976 to 1998. They found three types of induced seismicity at high
significance: i) Shallow, production-induced seismicity that has a long
time lag on the order of 1 year; ii) deep, injection-induced seismicity
with short time lag, \< 2 months; iii) deep, production-induced,
seismicity with short time lag, \< 2 months that appears to diminish in
the late 1980s. For each of these three types of induced seismicity they
also proposed failure mechanisms based on analytical modeling and
reasoning.

For shallow induced MEQs, Mossop and Segall (2004) found that MEQ
distribution closely matches mapped low pressures in the reservoir and
the areas of maximum volume strain inferred from surface deformation
data, suggesting that these events are caused by poroelastic stressing.
The observations are consistent with a contracting reservoir, which as
it shrinks, induces stresses and strains in the surrounding crust. Shear
stresses on faults outside the reservoir can increase, causing
subsidence. However, Mossop and Segall's (2004) suggestion that shallow
earthquakes are production-induced is in contrast with results of
Rutledge et al. (2002). Studying one specific case in detail, they found
that shallow MEQs are well correlated to injection, rather than
production, and with a relatively short time lag of about 1 week. For
shallow MEQs there might be a long-term effect caused by the overall
steam-production and local short-term responses related to injections.
In addition, Parotidis et al (2004) hypothesized that there is a back
front of seismicity produced that will cause extended periods of
seismicity after injection has ceased. This was found during hydraulic
fracturing cases not located at The Geysers.

For deep induced MEQs occurring after the 1980s there seems to be a
consensus that these are correlated to local injection rates with some
time lag (Stark 1992; Enedy et al. 1992;Romero et al. 1995, Kirkpatrick
et al. 1999; Smith et al. 2000; Stark 2003). For example, Stark (1992)
showed that plumes of MEQs are clustered around many injection wells,
and the seismic activity around each injection well correlates with its
injection rate. Mossop and Segall (2004) hypothesized that
injection-induced MEQs are probably caused by thermo-elastic
perturbation due to cold-water injection into a hot reservoir. When cool
water flows into hot rock fractures, the fracture faces contract by
cooling, loosening the frictional forces across the fractures and
thereby allowing stress release by seismic slip. Although Mossop and
Segall (2004) studied other mechanisms (e.g. loss of effective stress
due to hydraulic pressure in the fracture), they concluded that it is
the temperature contrast between the injected water and the hotter rock
fracture surfaces that is probably the dominant mechanism driving
Geysers injection-induced seismicity. Finally, Mossop and Segall (2004)
attributed deep production-induced seismicity to thermo-elastic
stressing caused by evaporative cooling. They concluded that an
evaporative-thermoelastic model could explain why deep production
correlated seismicity declined in the mid 1980s as the reservoir dried
out and evaporative cooling diminished. The most likely large induced
event in a geothermal region is the delayed type, the type due to
increased water pressure diffusing to depth, or thermal-mechanical
effects. It should be noted that there has been no large seismicity of
the rapid response type associated with any reservoir impoundment in
California, possibly because the rapid type is caused by changing the
near at depth surface (within a few kilometers) loads and stress fields,
thus not affecting the stresses where the potential larger events occur.

### Potential uses of MEQ activity

Stark (1992) found that where clusters extend some distance from the
injectors, the production wells tend to show "heavy" isotopic signature
of flashed injectate. Stark (1992) therefore hypothesized that MEQs are
induced where injected water is present as liquid. He suggested that the
MEQs occurring in this liquid zone might be a result of the effects of
hydraulic head and/or cooling due to the injected water. Recently, Stark
(2003) used this hypothesis to explain the vertical pattern of induced
seismicity in the Northern Geyser reservoir. Historic Geysers
earthquakes and injection data shows an area of approximately 8 km^2^
underlain by a cluster of MEQs in the depth range of 3 to 5 km below sea
level. The cluster lies far below the normal 240 °C isothermal reservoir
and is in the underlying High Temperature Zone (HTZ), where temperature
gradients can exceed 100°C per km. Above this cluster there is a gap,
0.5 to 1 km thick, where few MEQs occur. Above the gap is a more typical
pattern of the Geysers seismicity, including plumes of MEQs associated
with injection wells. Stark (2003) used a conceptual model to show that
this pattern could be governed by the temperature contrast between
injected water and the rock, and would imply that significant volumes of
injected water have descended into the HTZ reaching a depth as great as
5 km below sea level. Furthermore, Stark (2003) studied monthly
injection and seismic data from 1983 to 2002 and found that the deep
injection induced seismicity was lagging behind by 3 months suggesting
that it would take about 3 months for the injected water to descend to
depths of 3 to 5 km.

The above studies have made progress in showing a general correlation of
liquid injection and steam production with various types of induced
MEQs. Furthermore, several plausible hypotheses have been proposed to
explain the mechanisms producing those MEQs. In general most geothermal
regions in the US are subject to active tectonic forces associated with
the strike-slip relative motion between the North-American and Pacific
plates or the tensional forces in the Great Basin (Stark, 2003,
Oppenheimer 1986). Many naturally occurring fractures may be stressed to
near the failure point, so a small perturbation in the stress field
could lead to failure. However, it is not at all certain that most MEQs
are produced by shear slip along pre-existing fractures (Julian et al.
1993; Kirkpatrick et al. 1999; Ross et al 1999). Ross et al. (1999)
conducted highly accurate moment tensor analysis for thirty recorded
earthquakes in The Geysers area and showed that most of the earthquakes
have a non-shear component in their focal mechanisms. They suggested
that sources may be explained by combinations of tensile cracks and
shear movements accompanied by fluid flow. Cracks open in the presence
of high-temperature and pressure fluids, rapid flow in the new void,
possibly accompanied by water flashing to steam. In general, rapid
cooling along a fracture is capable of creating thermally induced
fractures (TIFs) in the rock matrix adjacent to the fracture (Perkins
and Gonzalez, 1985). In any case, it is likely that thermo-elastic
responses, induced by rapid cooling, play a major role in inducing MEQs.

## Examples of Induced seismicity

### The Geysers 

One of the prime examples of induced seismicity due to production as
well as injection is The Geysers geothermal field in Northern
California. Seismicity has been monitored for a number of years at this
location and provides one of the most complete data sets available. In
addition two large injection projects over the last seven years has
provided the opportunity to examine the seismicity and changes in
seismicity due to a large influx of water. Last but not least the
seismic arrays have been deployed over the entire Geysers field rather
than just the planed injection region to examine the field -wide
response to injection rather than just in the injection area. The
hypothesis being that the increased microearthquake activity is due to a
diverse set of mechanisms. That is, there is not one "triggering"
mechanism but a variety of mechanisms in operation that may work
independently, together, or superimpose to enhance or possibly reduce
seismicity. For example, as one injects water into the reservoir there
is obviously cooling, a change in pore pressure (at least locally around
the well) and possibly wider ranging stress effects. There has always
been a debate about the relation between the location of the
microearthquakes and the location of the fluids. If the events are due
to thermal contraction from cooling the rock matrix one would assume
that would take a very long time, i.e. the thermal front travels orders
of magnitude slower than the fluid front. As it is, the fluid front does
not travel in one continuous manner but it fingers it way through the
fractures in a lace-like manner. Unlike the rock matrix, fracture
surfaces can cool very quickly as they are contacted by the fluid front.
By examining the spatial and temporal rate of change in seismicity one
may be able rule out or confirm certain mechanisms. Also, as the
injections proceed effects may be felt on a field wide basis. As the
local stresses change around each injection well they may superimpose
upon the existing regional stresses or link up to form a larger local
effect that in turn may affect a wider region within the field. Figure 4
shows The Geysers field, the location of injection wells, the injection
pipelines and wells for the two large injection projects over the last
seven years, and the location of the various seismic arrays, i.e. the
USGS array, the geothermal operator's array (Calpine), and the newly
installed Lawrence Berkeley National Laboratory (LBNL) array. Each array
was designed for a different sensitivity and purpose. Figure 5 shows the
rate of seismicity from 1965 to the presentat The Geysers. The data are
for magnitudes above 1.5 as determined from the USGS data set at the
Northern California Earthquake Data Center. As can be seen, as the
injection increases the seismicity increases, but not at all levels. If
one only looks at the larger events (magnitude 3's) the seismicity has
stayed fairly constant since 1985. There is also no clear relation
between total injection and seismicity, except if one looks at all
events above 1.5. As can be seen there are peaks in seismicity in 1986
and again in 1998, where there were peaks in injection. It is also
important to point out that as steam production has decreased since 1986
the overall seismicity has remained fairly constant.

If one considers the energy release over time, rather than just a count
of earthquakes, then one gets a different picture. One can take the same
information in Figure 5 from 1984 to the present and looked at the
energy release over time by using an energy-magnitude relation Log 10 E
= 11.4 + 1.5 M, (E = energy in ergs, M = magnitude of the event). Figure
6 shows the rate of seismicity (total events above M = 1.5) for The
Geysers area since 1984. If one converts the magnitudes to energy one
obtains the results in Figure 7. As can be seen the rate of energy
release is actually decreasing as a function of time.

If one looks at the SE Geysers a slightly different picture emerges. The
SE Geysers had an increase in injection in 1997 from the Lake County
pipeline. Seismicity has been steadily increasing since the mid-1980's,
as can be seen in Figure 8, as has the energy release (Figure 9). In
recent years there has been a leveling-off of the energy release, and,
to a lesser extent, the seismicity.

In terms of data from the new LBNL array no definite conclusions can be
drawn yet due to the short time of monitoring of the effects of the
Santa Rosa pipeline. Also, the LBNL array has had different times of
monitoring with different modes of operation during the startup. A
constant, however, which one can possibly normalize to is the number of
triggers on the new array, the 10 stations that have been operating
almost continuously since early 2003. Figure 10 shows the number of
triggers (6 stations had to detect and event in a one second window to
be a trigger). Triggers are not as susceptible to number of stations as
are locations, that is, many events that are triggered on are not
located due to signal to noise ratio. As can be seen the number of
triggers in 2003 are fairy constant. Shown in Figure 11 and 12 are the
events located by the LBNL array for 10 days in March of 2004 and all of
March 2004. Figure 13 are all of the events located by the LBNL array in
October of 2003, i.e. one month prior to the start of injection. Also
shown in these figures is the location of the magnitude 4.4 on February
18, 2004. The October and March time periods were chosen because the
seismic array was fully operational during these times and the October
period is before the injection and the March period is after the
injection start up in December of 2003. These plots clearly show there
is an increase in overall seismicity in the injection area. As stated
before this is typical of seismicity at The Geysers, and some or all of
the increase may just be normal seasonal variation as the non-Santa Rosa
water injection ramps up. Low-magnitude seismicity increased in the SE
Geysers when supplemental injection began there (Kirkpatrick et al,
1999; Beall et al, 1999; Smith et al, 2000) and it is not surprising
that is occurring now. If past experience is any indication the system
will reach an equilibrium as time proceeds and seismicity will level off
and possibly decrease. It has been our experience that the initial
injections will perturb the system, cause an increase in seismicity,
then level off and/or decrease. The time period will be a function of
the size of the disturbance and the volume of the affected area. Rate
seems to be an important factor also. One hypothesis worth considering
is that if the rate of increase injections is varied (give the system a
chance to equilibrate) there may be less initial seismicity. Also, as
pointed out about the historical seismicity at The Geysers, the yearly
energy release is actually decreasing. The recent injections may reverse
this trend but it is to early in the monitoring process to determine.
Last but not least, what will be the impact of the maximum event size?
The maximum event at the Geysers was back in 1982 (4.6), but in the past
year there have been 3 events of magnitude greater than 4.0 (see Figure
5). The maximum event will depend upon the size of the fault available
for slippage as well as the stress redistribution due to injection and
production. To date there has been no faults mapped in The Geysers which
would generate a magnitude 5.0 or greater. This is not an absolute
guarantee that one would not happen, but does lower the likelihood.

All of the above issues will be addressed over the next several years as
the network continues to operate and the injections continue and
possibly increase the then NW.

## Benefits of Induced Seismicity Monitoring

In order to realistically examine the overall benefit of Enhanced
Geothermal Systems one must look at both the public and private sectors.
Access to high quality, state-of-the-art seismic information will be
important for both public acceptance and industry reservoir management.
For example, at The Geysers related geothermal industry benefits will
include establishment of an non-industry monitoring and reporting system
capable of providing the high quality, publicly credible, seismic data
base needed to gain public acceptance of wastewater injection; and the
basic scientific knowledge regarding the relations between seismicity
and fluid movement in the crust. It is worthwhile to have public and
private researchers access to the data. It is believed that ready
availability of these data to a broad spectrum of researchers could
result in an increased understanding of the fundamental processes
involved in fluid movement in the Earth\'s crust. This information may
find application in several disciplines including geothermal energy
production, non-geothermal electrical energy production, petroleum
recovery, and earthquake studies.

The most established use of earthquake data in geothermal regions, the
tracking of strain release and presumably injection flow paths, could be
greatly enhanced if the many theories describing how earthquakes and
injectate are related were better constrained by observation. This
requires an improved understanding of the \"triggering\" mechanisms of
both the injection and the production related induced seismicity and of
any source mechanism peculiarities that naturally occurring earthquakes
may have in geothermal regions. The locations of the earthquakes have
also been used to characterize patterns of permeability in reservoirs.
However, this is a very complex issue since in different circumstances
earthquakes can be more closely associated with either relatively low or
relatively high permeability. Because characterizing permeability of
geothermal reservoirs is of great importance in targeting wells and
predicting overall reservoir performance, reducing the uncertainty in
such earthquake interpretations would have great value.

A recent success (Julian et al., 1996, Foulger et al, 1997) has been
reported in using microearthquakes as illumination sources to image
physical properties within The Geysers reservoir area. For instance,
\"tomographic\" imaging of seismic wave velocity can be periodically
repeated to map temporal changes in water saturation. A decline in water
saturation is often accompanied by a decline in production pressure and
an increase in non-condensable gas concentrations. Therefore, the
existing earthquake array was designed to also provide the needed data
to address such issues.

Seismicity associated with wastewater injection will become an
increasingly important issue at The Geysers. The City of Santa Rosa is
evaluating the possibility of expanding their disposal from the current
11 million gallons per day of water from their treatment plant to as
much as double that volume. As with the Lake County project, Santa Rosa
liquids will, over the 20-year life of the project, contribute to
equivalent low air pollution generation increases of up to 85 megawatts,
serving up to 115,000 residential customers. The City of Santa Rosa
selected The Geysers option as the lowest cost beneficial use
alternative, and to overcome the intense water quality-based opposition
to increased wastewater disposal into the Russian River. Use of The
Geysers will eliminate the need for disposal of the treated water in the
Russian river. Thus, Geysers injection of treated Santa Rosa wastewater
addresses environmental issues related to water quality, and reclaims
the water for the beneficial use of California electricity ratepayers.

It is also likely that during the monitoring of the seismicity
information will be gained which will be the prime motivator for
operational decisions which will increase net production. For example in
many geothermal reservoirs there is a large untapped portion which could
be exploited if proper injection and production strategies are designed.
Due to concerns regarding MEQ generation one must also take into account
the impact of injection on seismic as well as reservoir conditions. If
injected under the right conditions and rates wastewater may mitigate
deleterious high non-condensable and corrosive gas concentrations in the
reservoir. *In situ* mitigation will alleviate the economic and
technological issues presently preventing exploitation of much of a high
temperature reservoir characterized by high concentrations of CO~2~,
H~2~S, and HCl in the vapor contaminates the production stream,
requiring costly surface mitigation strategies, diminished well life
times and retrofitting of power plants to handle the high gas contents.
.

# Recommendations/Suggestions for Future Work

In January of 2004 the US Department of Energy funded an extension of
this work to keep the array running and expand the array to coverage of
the Aidlin area, a region northwest of the current main production area.
This area is the target of an Enhanced Geothermal Systems project by DOE
to maximize the steam out put. The current array installed by this
project is state-of-the-art in every way: station electronics,
digitization, GPS, radio link (spread spectrum) and central data
processing. A possible enhancement to use several wide bandwidth
stations (30 seconds to 100 hertz) throughout The Geysers. These wide
bandwidth stations would be interspersed with the existing stations to
provide spatial coverage of The Geysers. This would provide broadband
coverage as well as high frequency coverage of the entire field. The
advantage would be a more complete coverage of data for understanding
source mechanisms. The reason that this is important is that one
hypothesis of the induced seismicity is stress induced seismicity from
long period data (distant large magnitude events). The long period data
would provide information to address this hypothesis.

Another possible future activity would be to apply advanced analysis of
the MEQ data. The original work with the Geysers data (Kirpatrick et al,
1999) was motivated primarily by the desire to have a processing
capability that would allow the thousands of seismic events per month to
be analyzed in real time and extract any general seismic parameters that
could be associated with production and injection data. The primary
emphasis was on seismic parameters such as event locations that could be
associated with fluid migration and second-order moment tensors that
could be associated with mode of failure in terms of opening, closing,
or sliding of cracks. In order to meet the requirements for rapid
automatic processing with the computers available at the time, it was
necessary to make a number of simplifying assumptions in setting up the
processing software, such as a simple velocity model,
frequency-independent ray theory, and characterization of a seismic
phase with the two parameters of arrival time and amplitude. By taking
advantage of improvements in computer speed, developments in the theory
of elastic wave propagation, and new ideas about seismic sources, it is
now possible to make a number of significant improvements in the methods
that were developed for processing MEQ data.

The state of the art is using the first arrival amplitudes to compute
moments of the events. Improvements in computer speed and new
theoretical methods make it feasible to locate seismic events and
estimate moment tensors by processing the complete waveforms recorded on
seismograms. This avoids most of the sources of uncertainty associated
with identifying phases, measuring arrival times, and estimating
amplitudes that are contained in the current processing methods. This
part of the research would be closely connected to the estimation of an
improved velocity models which means that there would be a continuous
monitoring of any temporal change in the material properties of the
reservoir.

Another improvement over the last two years is the interpretation of
source data in terms of an asperity model for an earthquake. Recent
analyses of small earthquakes along the San Andreas Fault have resulted
in the development of an asperity model of an earthquake that provides
an alternative to the conventional model that has dominated the
interpretation of seismic data for the past forty years. Given the large
number of small seismic events in some geothermal areas and the
opportunity to estimate stress changes caused by the withdrawal and
injection of fluids, these areas appear to be an ideal site for applying
some of the techniques that were developed for the study of the small
San Andreas events. Should the data indicate that the asperity model
helps to explain the seismic events our understanding of why these
events are occurring could be significantly advanced.

Furthermore, with the density of stations and large number of events it
will be possible to interpret velocity anomaly data in terms of fracture
density. Composite medium theory is a useful method of explaining the
material properties in complex near-surface sites where rocks of various
types, voids, and fluids are all present. Efforts are now underway to
extend this theory so that it more accurately incorporates the effects
of fractures and any fluids that they may contain. An important property
of this theory is that it produces frequency-dependent velocities, which
means that some of the uniqueness problems that are encountered when
interpreting seismic velocities in terms of material properties can be
addressed, particularly with the high-frequency data that are recorded
by the seismic arrays at the Geysers. A particularly attractive scenario
that exists is the possibility of interpreting in a uniform manner any
temporal changes in event location, moment tensor, and material
properties in terms of fracture density, orientation, and fluid content.

# Conclusions 

One cannot assume that induced seismicity in geothermal regions is
automatically linked on a one to one basis with injection. There are
many different mechanisms within geothermal regions that are interacting
to affect the seismicity. In most areas, which are prime candidates for
enhanced geothermal activities, the potential reservoir possesses one or
all of the following characteristics; it is either hot, has a low fluid
content, low permeability, has a high content of undesirable gases, and
is in a tectonically active area. All of these properties may either
interact to increase the seismicity as the system is perturbed by
injection or withdrawal, or may constructively interact to have a
smaller effect on seismicity. For example, seismicity at The Geysers has
been shown to increase at the outset of injection. As time has proceeded
the overall energy release is actually decreasing. This is not
surprising if the system is coming to an equilibrium after each
injection perturbation. Induced seismicity has been shown to occur in
many geothermal areas as injection is started or increased. However, as
been observed at The Geysers this seismicity is on a field wide basis as
well as associated with individual injection wells.

The majority of negative aspects seem to be associated with the impact
of seismicity on the surrounding community. Other effects such as well
failure due to subsidence, well bore damage and damage of surface
facilities are minimal or has not significantly impacted the cost
benefit ratio of the geothermal operations. In a number of geothermal
fields and potential geothermal fields in the U.S. the induced
seismicity activity or potential for seismicity seems to be below the
significant damage potential (less than 5.0). This is for several
fundamental reasons, 1. there are no faults close enough to create a
large damaging event, 2. if there are large faults near by then it is
usually the case that large events are initiated at depth (5 to 10
kilometers), and most geothermal production and injection activities are
shallower than 5 kilometers, thus making it difficult to trigger a large
event, 3. in many cases there may be no negative effects if the
seismicity is small or that the geothermal area is in a remote area.

The impact of the seismicity on enhanced geothermal activities, however,
is positive as well as negative. The increased seismicity is being used
to increase the understanding of the dynamics of the reservoir as well
as monitor the flow of fluids, maximize injection for optimal heat
extraction and imaging the reservoir in general. An increased overall
understanding of the mechanisms causing the seismicity may lead to
greatly increased recovery from geothermal reservoir.

Overall the impact of induced seismicity on the implementation of
various different enhanced geothermal activities will depend on the risk
associated with the activity and the cost benefit ratio. All experience
to date has shown that the risk, while not zero, has been either minimal
or can be handled in a cost effective manner.

# ACKNOWLEDGEMENTS

I would like to thank all of the people who contributed to this report
through discussions or material, i.e., Mitch Stark, Bill Smith, David
Oppenheimer, John Peterson, Art Protacio, Greg Nordquist, and Allan
Jelacic. This work was funded by the office of Geothermal Energy, U.S.
Department of Energy through contact number DEAC0376SF00098

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# FIGURES

![](media/image1.png){width="6.09375in" height="6.979166666666667in"}

**Figure 1** Map of earthquake locations with magnitudes greater than
5.0 in northern California from 1900 to the present.( source the
Berkeley Seismographic Lab)

![](media/image2.png){width="6.09375in" height="6.979166666666667in"}

**Figure 2** Map of the locations of events less than 3.0 in northern
California from 1900 to the present.(source the Berkeley Seismographic
Lab)

![](media/image3.png){width="6.09375in" height="6.979166666666667in"}

**Figure 3** Map of the locations of events less than 5.0 and greater
than 3.0 in northern California from 1900 to the present.( source the
Berkeley Seismographic Lab)

# ![](media/image4.wmf){width="7.625in" height="6.003472222222222in"}

**Figure 4** Location of USGS stations, Current Calpine array, and the
new LBNL stations. Also shown are the locations of the pipelines used
for the water from Santa Rosa.

![](media/image5.wmf){width="6.666666666666667in"
height="3.7694444444444444in"}

**Figure 5.** Historical seismicity from 1965 to the present at The
Geysers. Data are from the NCEDC.

![](media/image6.wmf)

**Figure 6**. Rate of seismicity for the entire Geysers area since 1984.
The line fitted to the data is a linear fit of a 12-month running
average.

![](media/image7.wmf)

**Figure 7**. Energy release over time since 1984, the line is a linear
fit to a 12-month moving average.

![](media/image8.wmf)

**Figure 8.** Rate of seismicity for the SE Geysers since 1984. Straight
line is a liner fit to the 12 month moving average.

![](media/image9.wmf)

**Figure 9.** Rate of energy release for the SE Geysers, the black line
is a 12-month moving average.

![](media/image10.wmf){width="6.454861111111111in"
height="4.749305555555556in"}

**Figure 10.** The number of triggers from the LBNL array as a function
of time (2003 -- 2004). Of note is the gradual rise in seismicity after
the start of Santa Rosa injection in December of 2003.

![](media/image11.wmf){width="6.4590551181102365in"
height="4.900787401574803in"}

**Figure 11.** Event distribution for 10 days in March of 2004. The
Injection wells are the blue squares.

![](media/image12.wmf){width="6.2492125984251965in"
height="4.885433070866141in"}

**Figure 12.** . The seismicity in all of March 2004. The blue squares
are the injection wells; yellow star is the magnitude 4.4 that occurred
on Feb 18, 2004.

![](media/image13.wmf){width="6.478346456692913in"
height="5.082677165354331in"}

**Figure 13.** Location of all events in October of 2003, two months
prior to Santa Rosa injection. Blue squares are the location of the
injection wells. The yellow star is the approximate location of the
magnitude 4.4 on February 18, 2004.