What is induced seismicity?
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?
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 anthropogenic, and “triggered” when it was predominantly of natural origin.
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 (Hitzman, 2013).
What human activities can induce earthquakes?
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, though we know of no reported cases of seismicity caused by chemical explosions.
What are the mechanisms of induced seismicity?
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. Below we summarise 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?
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 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 provides information on how the subsurface may respond to larger volume injection.
“Traffic light” systems
As well as local baseline seismic monitoring prior to the start of a project, seismic monitoring can also be undertaken throughout a project’s lifetime. A user-friendly format is the traffic light system, where predefined seismic magnitudes correspond to the traffic light colours. If these magnitudes are exceeded this may result in the modification or termination of a project. Green means the project continues as planned, orange may mean the project continues at reduced rates (modification), and red may mean an immediate halt in the project. An example is the UK traffic light system for shale gas hydrofracturing.
