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Use of Geothermal Resources: Environmental Considerations 235

For example, Stark (2003) has documented that during injection of recharge waters into The
Geysers geothermal reservoir, the temperature difference between the reservoir and the injected
fluid will be in excess of 100°C. Stark (2003) noted that microseismic events are temporally and
spatial associated with fluid injection (see Figure 9.16). He concluded that the thermoelastic effects
associated with injecting cool water into this geothermal system are the likely cause of this relation-
ship. The volume changes associated with this thermoelastic process are most likely accommodated
by the growth of existing fractures and the formation of new microcracks.
Normally, such processes cannot lead to earthquakes beyond a magnitude of about 2.5. The
reason for this can be understood by considering the size of area affected by the cooling process
associated with water injection. The volumetric changes indicated in Figure 12.4 would amount to
a slip of 10–15 cm if uniform contraction of a rock volume took place. If the contraction were con-
strained to be solely in one plane, the maximum amount of contraction would be 1–2 m. Since the
area affected is restricted to the immediate vicinity of the injection well, and since the amount of
slip is small, the amplitude of motion propagated to the ground surface would be very small. Such
seismic signals can be detected using sensitive seismometers and can be useful for mapping where
fracturing is occurring, but would have little consequence otherwise.
The main exception to this is when there exists a significant component of natural stress. As
noted previously in Chapter 2, the Earth is a heat engine that drives movement of tectonic plates.
These movements stress the plates to varying degrees and in different ways. High stress conditions
can occur where two plates slide against each other, as is the case with the San Andreas fault. High
stress can also be generated where plates converge, such as at subduction zones. Finally stress may
accumulate in the interior of plates due to changes in plate motion and other effects that slightly
warp the plates. Hence, in many settings there will exist some preexisting state of stress, also
called in situ stress. The magnitude of the in situ stress is normally well below the value of μ for
f
the rocks, fractures, and faults that compose the local geology. Under those conditions there will
be no seismic activity. However, if the in situ stress field results in the ratio of shear stress to nor-
mal stress being close to values of μ for local fractures and/or faults, it is possible that thermoelas-
f
tic effects associated with an injection of cool water could trigger larger than anticipated slip.
This consequence could result either from an increase in the total stress due to the thermoelastic
effects, or the thermoelastic effects could perturb the orientation of the in situ stress field. If either
situation results in a stress condition that results in failure, the magnitude of the resulting ground
motion will depend on, among other things, the dimensions of the area over which rupture or slip
occurs.
rupture area and magnitude
An empirical relationship between magnitude and the area over which rupture occurs was devel-
oped by Wells and Coppersmith (1994)

M = 4.07 + 0.98 × log (A), (12.5)

where M is the magnitude of the event and A is the rupture area, in km . Shown in Figure 12.5 is the
2
relationship between magnitude and rupture area. Most seismic events associated with an injection
of cool water have magnitudes less than 2.5 (shaded box in Figure 12.5), suggesting rupture areas
of less than 0.2 km . If such ruptures occur on surfaces that are more or less equant in form, the slip
2
areas would have dimensions of ca. 100–150 meters on a side or less.
To avoid the possibility of inducing larger, damaging seismic events through geothermal energy
development, these relationships and observations suggest several important attributes to evaluate
for a site. Most areas of interest for geothermal applications have been studied by geologists. The
research results often include evaluation of the seismic history and some consideration of the local
stress field. Data from such studies should be used to evaluate the magnitude and orientation of the
local stress field, how it changes with depth and the uncertainties associated with those evaluations.