Types and Causes of Induced Seismicity - Induced Seismicity Potential in Energy Technologies - page 15

Types and Causes of
Induced Seismicity
INTRODUCTION
Energy technology activities known to have produced induced seismicity, whether
significant enough to be felt by humans or so small as to be detected only with sensitive
monitoring equipment, are fluid injection and withdrawal as well as purposeful fracturing
of rocks. For each of these activities the critical components required to produce induced
seismicity are the presence and orientation of existing faults, the state of stress of the Earth's
crust, the rates and volumes of fluid injection or withdrawal, and time. Understanding these
components gives some confidence in being able to draw conclusions about what seismicity
might be induced in the future, and under what conditions. The physical mechanisms 1
responsible for inducing seismic events are discussed here with reference to specific energy
technologies; detailed explanations of these technologies and their relationship to induced
seismic events are presented in Chapter 3.
FACTORS AFFECTING INITIATION AND MAGNITUDE OF A
SEISMIC EVENT
Shallow earthquakes result from slip (movement) along a preexisting fault. Two critical
questions concerning such earthquakes are (1) which factors are responsible for the initia-
tion of a seismic event and (2) which factors control the magnitude of the event.
Initiation of a Seismic Event
The Earth's crust is crossed by a network of preexisting fractures and faults of various
sizes. Any of these faults could, in principle, be activated if the shear stress (τ) acting on
the fault overcomes its resistance to slip or movement of the adjacent rock blocks (called
“shear resistance”). In most cases, the shear resistance (or shear strength) is due to friction.
In other words, the shear strength is proportional to the difference between the normal
stress (σ) acting on the fault and the pressure (ρ) of the fluid permeating the fault and the
surrounding rock. The fault remains stable (does not slip) as long as the magnitude of
1 Although hydromechanical coupling is the dominant mechanism responsible for inducing seismic events, other
coupling mechanisms (e.g., thermomechanical and chemomechanical) could also play a role.