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Additional resources

What is induced
seismicity?

[1] Induced seismicity refers to seismic events (usually earthquakes) caused partially or completely by human (anthropogenic) activities.

What is the difference between induced and triggered seismicity?

[1] Some workers distinguish between “induced” and “triggered” seismicity according to whether all or only part of the strain energy released was anthropogenic, using the term “induced” when most of the energy was of anthropogenic origin, and “triggered” when it was predominantly of natural origin.

[2] Determining the proportion of anthropogenic to naturally stored energy is very difficult, although in the case of larger earthquakes it is clear much of the energy is of natural tectonic origin. We use the term “induced” for both cases, which is in keeping with the convention of the Committee on Induced Seismicity Potential (National Research Council, 2013).

What human activities
can induce earthquakes?

[1] A variety of human activities have been suggested to have induced seismicity. These include impounding surface water reservoirs, erecting tall buildings, engineering coastal land accretion, and removing mass from the surface by quarrying. The subsurface extraction of resources, including groundwater, coal, hydrocarbons and geothermal fluids, as well as tunnel excavation have also been reported to induce earthquakes, as have injection activities. Injection activities include waste fluid disposal, hydrofracturing (otherwise known as hydraulic fracturing or fracking), research experiments, gas storage, enhanced oil recovery, and carbon sequestration. Nuclear tests often induce local earthquakes. Chemical explosions have occasionally been reported to induce earthquakes.

What are the mechanisms
of induced seismicity?

[1] Shear-slip on fault planes is the most common earthquake source process. The factors involved in nucleation can be related by the “Mohr-Coulomb Theory”:

Tcrit = μ(σn - P) + So

where “Tcrit” is the critical shear stress needed for failure, “μ” is the coefficient of friction of the fault plane, “σn” is the applied normal stress across the fault, “P” is the pore fluid pressure in the fault zone, and “So” is a constant related to the cohesive strength of the material or sliding surface. Summarised below are the most common anthropogenic induction mechanisms:

Increased pore fluid pressure: Increases in pore fluid pressure reduce the effective stress (σn - P) and consequently lower the shear stress needed for failure. Pore fluid pressure can be increased by the injection of fluid and the presence of surface water impoundments.

Reduced normal stress: The removal of overlying mass can reduce the normal stress (σn) across a fault. Since the normal stress acts to clamp a fault “shut”, reductions in normal stress reduce the shear stress required for failure. This mechanism has been proposed for quarrying, mining and fluid extraction.

Increased shear stress: For faults that dip at particular angles, increases in vertical load (i.e. mass) increase the shear stress on the fault plane. This increase in shear stress may exceed the critical shear stress (Tcrit) needed for failure. Mass loading as an inducing mechanism has been proposed for skyscrapers, coastal management and surface water impoundments.

How can we mitigate
against induced
seismicity?

[1] Earthquakes can pose obvious threats to engineering works and to human life. In order to protect people, infrastructure and the environment, a number of common practices can be employed to reduce the risk:

Baseline studies: Temporary local seismometer networks deployed before a project can improve earthquake-detection thresholds and provide more complete assessments of natural seismicity to inform safety strategies.

Numerical modelling: Computer models can estimate the likely effects of a project on, for example, the stress field, subsurface faults and fractures, and fluid flow. Such models are simplified scenarios of reality and require input parameter values that may be poorly known, however their results may still be useful.

Seismic reflection surveys: Using artificial acoustic signals (e.g., explosions or vibration trucks) transmitted into the ground and recorded reflected signals, geophysicists can create a picture of the subsurface. These pictures may be two-, three-, or even four-dimensional (the fourth dimension being time), and may reveal potentially unstable geological features such as faults.

Pre-fracture injection tests: For fluid injection projects it is common to inject a small volume of fluid prior to the commencement of full-scale injection. This smaller injection test provides information on how the subsurface may respond to larger volume injection.

“Traffic light” systems: Local seismic monitoring can be continued throughout a project’s lifetime. When combined with pre-defined thresholds for seismic magnitudes or peak ground accelerations (PGA), projects can be modified or terminated based on any detected earthquakes. A user-friendly format commonly employed is a traffic light system, where the predefined seismic magnitudes or PGA correspond to traffic light colours. Typically, green means the project continues as planned, amber means the project continues with modification (e.g., reduced injection rate), and red means an immediate halt in the project, potentially permanently.