Induced Seismicity and Energy Technologies - Induced Seismicity Potential in Energy Technologies - page 10

a hydraulic fracturing operation) would not be considered the cause of a M 7 earthquake
that was initiated at 10 km depth, even if the hydraulic fracturing and earthquake were
close in space and time.
The earthquake history of a region also plays a role in inferring whether a particular
earthquake was induced. If a certain earthquake appears to be related to human activity, but
similar earthquakes have occurred in the past in that region, the connection with human
activity is more tenuous than if the correlation between earthquake and human activity
occurred in a previously aseismic region. In the latter case, an important indicator might be
the rate of occurrence of multiple earthquakes, compared to the historical rate (Ellsworth
et al., 2012). The important point is that there often is no definitive proof that a particular
earthquake was induced; conclusions are usually based on inference.
ENERGY TECHNOLOGIES AND INDUCED SEISMICITY
Geothermal Energy
Geothermal energy production captures the natural heat of the Earth to generate steam
that can drive a turbine to produce electricity. Geothermal systems fall into one of three
different categories: (1) vapor-dominated systems, (2) liquid-dominated systems, and
(3) enhanced geothermal systems (EGS). Vapor-dominated systems are relatively rare. A
major example is The Geysers geothermal field in Northern California. Liquid-dominated
systems are used for geothermal energy in Alaska, California, Hawaii, Idaho, Nevada, and
Utah. In both of these types of hydrothermal resource systems, either steam or hot water
is extracted from naturally occurring fractures within the rock in the subsurface and cold
fluid is injected into the ground to replenish the fluid supply. EGS are a potentially new
source of geothermal power in which the subsurface rocks are naturally hot and fairly im-
permeable, and contain relatively little fluid. Wells are used to pump cold fluid into the hot
rock to gather heat, which is then extracted by pumping the fluid to the surface. In some
cases a potential EGS reservoir may lack sufficient connectivity via fractures to allow fluid
movement through rock. In this case the reservoir may be fractured using high-pressure
fluid injection in order to increase permeability. Permeability is a measure of the ease with
which a fluid flows through a rock formation. (See Chapter 2 for detailed discussion of
permeability and its relevance to fracture development and fluid flow.) In each of these
geothermal systems, the injection or extraction of fluid has the potential to induce seismic
activity. Further description of these technologies and examples of induced seismic activity
are provided in Chapter 3.