As defined here, induced seismicity is earthquake activity resulting from human activity that causes a rate of energy release, or seismicity, which would be expected beyond the normal level of historical seismic activity. For example, if there is already a certain level of seismic activity before human activities begin, one would expect that this “historical” seismic activity would continue at the same rate in the future. If, however, human activity causes a concurrent increase in seismic activity, this increase in seismic activity would be considered “induced.” In addition, if the seismic activity returns to background activity after the human activity stops, that would be another sign that the seismic activity was induced.
Although research is still being carried out on the detailed causes of induced seismicity, there are many different applications associated with induced seismic activity. In addition to the subsurface stresses, fluid pressures play a key role in causing seismicity. Explained in simple terms, fluids can play a major role in controlling the pressures that are acting on the faults. The fluid pressure in the pores and fractures of the rocks is called the “pore pressure.” The pore pressure acts against the weight of the rock and the forces holding the rock together (stresses due to tectonic forces). If the pore pressures are low (especially compared to the forces holding the rock together), then only the imbalance of natural in situ earth stresses will cause an occasional earthquake. If, however, pore pressures increase, then it would take less of an imbalance of in situ stresses to cause an earthquake, thus accelerating earthquake activity. This type of failure, illustrated in Figure 1, is called shear failure. Injecting fluids into the subsurface is one way of increasing the pore pressure and causing faults and fractures to “fail” more easily, thus inducing an earthquake. (Figure 1 shows this process in graphical form.) Thus, induced seismicity can be caused by injecting fluid into the subsurface or by extracting fluids at a rate that causes subsidence and/or slippage along planes of weakness in the earth.
Figure 1. Concept of effective stress. The diagonal line is the plane of slippage or failure (fault). The two opposing arrows are the forces keeping the fault from slipping (normal stress, sigma). Tau is the value at which failure (slippage on a fault plane) occurs. As the pore pressure, P, rises the normal stress decreases because the pore pressure acts against the normal stress, resulting in a lower “effective stress,” thus allowing seismicity to occur at lower shear stresses. It should be noted that the coefficient of friction and rock strength are usually constant, but in a small minority of cases, if thermal and chemical conditions are changing, the rising or lowering of these two properties will either increase or lessen the seismicity.
Figure 2, which shows an example of induced seismicity being caused by water injection, is a cross section of the earth showing the location of earthquakes (green dots), as well as the locations of injection wells (thick blue lines) and production wells (thin lines, these wells extract fluid). Note the large number of events associated with the injection wells.
Figure 2. Example of injection related seismicity; note the close correlation between water injection wells and the location of the seismicity.
Other factors thought to be responsible may be thermal changes and/or chemical changes caused by fluid movement and injection. This type of induced seismicity has been noted not only in geothermal reservoirs but in reservoir impoundment (water behind dams), waste injections, oil and gas operations, and underground injection of fluids for waste disposal. Almost all of the significant events (recorded activity and in some cases felt activity) are associated with shear failure. These types of earthquakes can be very small or large, depending on the geologic environment and available forces to cause an earthquake. Mining (creating cavities in the subsurface) also cause shear failure along planes of weakness, but that is usually caused by relieving stress or subsidence.
Another type of induced seismicity is that which is associated with “hydrofracturing.” Hydrofracturing is done by injecting fluid into the subsurface to create distinct fractures in order to link existing fractures together. This activity creates additional permeability in the subsurface, which facilitates extraction of in situ fluids (such as oil and gas). Hydrofracturing is distinct from many types of shear-induced seismicity, because hydrofracturing by definition occurs only when the forces applied create a type of fracture called a tensile fracture, or “driven” fracture. Shear failure has been associated with hydrofracturing operations, as the fluid leaks off into existing fractures, but due to the very-high-frequency nature of tensile failure (seismic source at the crack tip exclusively), only the associated shear failure is observed by microseismic monitoring. However, hydofracturing is such a small perturbation, it is rarely, if ever, a hazard when used to enhance permeability in oil and gas or other types of fluid-extraction activities. To our knowledge, hydrofracturing to intentionally create permeability rarely creates unwanted induced seismicity that is large enough to be detected on the surface—even with very sensitive sensors—let alone be a hazard or an annoyance. In fact, the very small seismic shear events created from the shear failure associated with the hydrofracture process are used to map the location of the induced permeability and as management tools to optimize fluid production. If not for the very small shear events, it would be much more difficult to understand the effect of hydrofracturing, because the seismic energy created from the “main fract” is too low to be detected, even by the most sensitive instruments at the surface of the earth. Figure 3 is an example of how seismicity is used to map these hydrofractures.
Last but not least, another reason that the seismic risk associated with hydrofracture operations is low is that such operations are of relatively low volume and short duration (hours or days at the very most), compared to months and years for the other types of fluid injections described above.
Figure 3. Cross section through a stimulation well showing six different stages of hydrofracture stimulation and the associated seismicity (magnitude -1.0 to -2.5) during the entire hydrofracture (less than 24 hours) Warpinski et al 2005.
Seismicity occurs over many different time scales and spatial scales. Creep on a fault could be considered seismicity just as a much as a sudden loss of cohesion on a fault. Growth faults in the overpressurized zones of the Gulf Coast are an example. As defined here, we will only deal with events that are sudden and cause “earthquakes.” If one examines the subsurface of the Earth in enough detail, one can find fractures, joints, and/or faults almost anywhere. A fault is not defined in terms of size (a fault is defined as a displacement across a fracture, joint, or fracture zone). However, most mapped faults range in size from very small (a few meters) to very large (hundreds of kilometers long). The size of an earthquake (or how much energy is released) depends on how much slip occurs on the fault, how much stress there is on the fault before slipping, how fast it fails, and over how large an area the slip occurs. Damaging earthquakes (usually greater than magnitude 5) require fault surfaces to slip over relatively large areas (kilometers). For slip to occur, there must also be an imbalance in the stresses and forces acting within the earth. In other words, if there is not an imbalance in the forces in the subsurface, then there is no net force available to cause slip, i.e., a sudden release of the stored energy. The forces acting to deform the earth (resulting in an excess of energy accumulation) are of course forces that are fundamentally generated by the dynamic nature of the whole earth. In most regions where there are economic geothermal resources, there is usually tectonic activity, such as in the western United States. These areas are more prone to induced seismicity than in more stable areas of the U.S. such as the central U.S. (It must be noted, however, that one of the largest earthquakes ever to occur in the U.S. was the New Madrid series of events in the early 1800s in Missouri, it rang church bells in Boston). It must also be noted that seismic activity is only a hazard if it occurs above a certain level, and is large enough and/or close enough to inhabited areas. At some level, there is seismic activity almost everywhere.
Another factor to consider is that the earth is not a homogeneous medium. Over the millions of years of movement, the surface of the earth has been deformed and broken into many different patterns. In some areas where there has been consistent movement, large fault systems have formed. If the forces are still present, then there is a potential for earthquakes to occur. (The San Andreas Fault system in California is one example.) As pointed out above, however, slip does not have to occur in discrete or sudden jumps. For example, there are many places along the San Andreas Fault where the fault is creeping, rather than jumping in a “stick-slip” type of movement. This partially accounts for the high level of seismicity in some areas of California, and the low level in other areas. Although some people think that there are large earthquakes everywhere in California, records of historical activity since 1900 show that such events are mainly confined to distinct zones. These zones of weakness tend to fail and cause earthquakes much more often than zones away from faults.
One last important feature to note regarding earthquake activity is that the size of the fault (in addition to the forces available) and the strength of the rock determine how large an event may potentially be. It has been shown, that in almost all cases, large earthquakes start at depth (five to ten kilometers). It is only at depth where there can be enough stored energy to provide an adequate amount of force to move the large volumes of earth required to create a large earthquake. This implies that if seismicity is induced at shallow depths (less than 5 kilometers), seismic events might be numerous, but no one event would be large.
To realistically examine the overall impact (benefits as well as risks) of induced seismicity, one must look at both public and private sectors. Access to high quality, state-of-the-art seismic information will be important for both public acceptance and industry response. For example, in the energy industry, benefits will include establishment of a non-industry monitoring and reporting system capable of providing the high quality, publicly credible seismic database needed to gain public acceptance of wastewater and fluid injection; and the basic scientific knowledge regarding the relationships between seismicity and fluid movement in the crust. It is worthwhile to have both public and private research access to the data. Availability of these data to a broad spectrum of researchers could result in an increased understanding of the fundamental processes involved in fluid movement within the Earth's crust. This information may find application in several disciplines including geothermal energy production, non-geothermal electrical energy production, petroleum recovery, CO2 sequestration, and earthquake studies.
The most established use of earthquake data, the tracking of strain release and presumably injection flow paths, could be greatly enhanced if the many theories describing how earthquakes and injectate are related were better constrained by observation. This requires an improved understanding of the "triggering" mechanisms of both injection- and the production-related induced seismicity, and of any source-mechanism peculiarities that naturally occurring earthquakes may have. The locations of earthquakes have also been used to characterize patterns of permeability in reservoirs. However, this is a very complex issue, since in different circumstances, earthquakes can be associated with either relatively low or relatively high permeability. Because characterizing the permeability of geothermal reservoirs is of great importance in targeting wells and predicting overall reservoir performance, reducing the uncertainty in such earthquake interpretations would have great value.
Most of the negative aspects associated with induced seismicity seem to be associated with the impact of seismicity on the surrounding community. Other effects, such as well failure due to subsidence well bore damage and damage of surface facilities, are minimal, or have not significantly impacted the cost-benefit ratio of industrial operations.
Overall, the impact of induced seismicity on the implementation of various different energy recovery and/or disposal activities will depend on the risk associated with the activity and the cost-benefit ratio. All experience to date has shown that the risk, while not zero, has been either minimal or can be handled in a cost effective manner.