Induced seismicity

The column of water in a large and deep artificial lake alters in-situ stress along an existing fault or fracture. In these reservoirs, the weight of the water column can significantly change the stress on an underlying fault or fracture by increasing the total stress through direct loading, or decreasing the effective stress through the increased pore water pressure. This significant change in stress can lead to sudden movement along the fault or fracture, resulting in an earthquake.[22] Reservoir-induced seismic events can be relatively large compared to other forms of induced seismicity. Though understanding of reservoir-induced seismic activity is very limited, it has been noted that seismicity appears to occur on dams with heights larger than 330 feet (100 m). The extra water pressure created by large reservoirs is the most accepted explanation for the seismic activity.[23] When the reservoirs are filled or drained, induced seismicity can occur immediately or with a small time lag.

The first case of reservoir-induced seismicity occurred in 1932 in Algeria's Oued Fodda Dam.

The largest earthquake attributed to reservoir-induced seismicity occurred at Koyna Dam

The 6.3 magnitude 1967 Koynanagar earthquake occurred in Maharashtra, India with its epicenter, fore- and aftershocks all located near or under the Koyna Dam reservoir.[24] 180 people died and 1,500 were left injured. The effects of the earthquake were felt 140 mi (230 km) away in Bombay with tremors and power outages.

During the beginnings of the Vajont Dam in Italy, there were seismic shocks recorded during its initial fill. After a landslide almost filled the reservoir in 1963, causing a massive flooding and around 2,000 deaths, it was drained and consequently seismic activity was almost non-existent.

On August 1, 1975, a magnitude 6.1 earthquake at Oroville, California, was attributed to seismicity from a large earth-fill dam and reservoir recently constructed and filled.

The filling of the Katse Dam in Lesotho, and the Nurek Dam in Tajikistan is an example.[25] In Zambia, Kariba Lake may have provoked similar effects.

The 2008 Sichuan earthquake, which caused approximately 68,000 deaths, is another possible example. An article in Science suggested that the construction and filling of the Zipingpu Dam may have triggered the earthquake.[26][27][28]

Some experts worry that the Three Gorges Dam in China may cause an increase in the frequency and intensity of earthquakes.[29]

Mining affects the stress state of the surrounding rock mass, often causing observable deformation and seismic activity. A small portion of mining-induced events are associated with damage to mine workings and pose a risk to mine workers.[30] These events are known as rock bursts in hard rock mining, or as bumps in underground coal mining. A mine's propensity to burst or bump depends primarily on depth, mining method, extraction sequence and geometry, and the material properties of the surrounding rock. Many underground hardrock mines operate seismic monitoring networks in order to manage bursting risks, and guide mining practices.[31]

Seismic networks have recorded a variety of mining-related seismic sources including:

• Shear slip events (similar to tectonic earthquakes) which are thought to have been triggered by mining activity. Notable examples include the 1980 Bełchatów earthquake[32] and the 2014 Orkney earthquake.
• Implosional events associated with mine collapses. The 2007 Crandall Canyon mine collapse and the Solvay Mine Collapse[33] are examples of these.
• Explosions associated with routine mining practices, such as drilling and blasting, and unintended explosions such as the Sago mine Disaster.[34] Explosions are generally not considered "induced" events since they are caused entirely by chemical payloads. Most earthquake monitoring agencies take careful measures to identify explosions[35] and exclude them from earthquake catalogs.
• Fracture formation near the surface of excavations, which are usually small magnitude events only detected by dense in-mine networks.[30]
• Slope failures, the largest example being the Bingham Canyon Landslide.[36]

Cumulative number of earthquakes in the central U.S. The red cluster at the center of the map shows an area in and around Oklahoma which experienced the largest increase in activity since 2009.

Injecting liquids into waste disposal wells, most commonly in disposing of produced water from oil and natural gas wells, has been known to cause earthquakes. This high-saline water is usually pumped into salt water disposal (SWD) wells. The resulting increase in subsurface pore pressure can trigger movement along faults, resulting in earthquakes.[37][38]

One of the first known examples was from the Rocky Mountain Arsenal, northeast of Denver. In 1961, waste water was injected into deep strata, and this was later found to have caused a series of earthquakes.[39]

The 2011 Oklahoma earthquake near Prague, of magnitude 5.8,[40] occurred after 20 years of injecting waste water into porous deep formations at increasing pressures and saturation.[41] On September 3, 2016, an even stronger earthquake with a magnitude of 5.8 occurred near Pawnee, Oklahoma, followed by nine aftershocks between magnitudes 2.6 and 3.6 within 3 1/2 hours. Tremors were felt as far away as Memphis, Tennessee, and Gilbert, Arizona. Mary Fallin, the Oklahoma governor, declared a local emergency and shutdown orders for local disposal wells were ordered by the Oklahoma Corporation Commission.[42][43] Results of ongoing multi-year research on induced earthquakes by the United States Geological Survey (USGS) published in 2015 suggested that most of the significant earthquakes in Oklahoma, such as the 1952 magnitude 5.5 El Reno earthquake may have been induced by deep injection of waste water by the oil industry.[5] Prior to April 2015 however, the Oklahoma Geological Survey's position was that the quake was most likely due to natural causes and was not triggered by waste injection.[44] This was one of many earthquakes which have affected the Oklahoma region.

Since 2009 earthquakes have become hundreds of times more common in Oklahoma with magnitude 3 events increasing from 1 or 2 per year to 1 or 2 per day.[45] On April 21, 2015, the Oklahoma Geological Survey released a statement reversing its stance on induced earthquakes in Oklahoma: "The OGS considers it very likely that the majority of recent earthquakes, particularly those in central and north-central Oklahoma, are triggered by the injection of produced water in disposal wells."[46]

Large-scale fossil fuel extraction can generate earthquakes.[47][48] Induced seismicity can be also related to underground gas storage operations. The 2013 September–October seismic sequence occurred 21 km off the coast of the Valencia Gulf (Spain) is probably the most known case of induced seismicity related to Underground Gas Storage operations (the Castor Project). In September 2013, after the injection operations started, the Spanish seismic network recorded a sudden increase of seismicity. More than 1,000 events with magnitudes (M_L) between 0.7 and 4.3 (the largest earthquake ever associated with gas storage operations) and located close the injection platform were recorded in about 40 days.[49][50] Due to the significant population concern the Spanish Government halted the operations. By the end of 2014, the Spanish government definitively terminated the concession of the UGS plant. Since January 2015 about 20 people who took part in the transaction and approval of the Castor Project were indicted.

The changes in crustal stress patterns caused by the large scale extraction of groundwater has been shown to trigger earthquakes, as in the case of the 2011 Lorca earthquake.[51]

Enhanced geothermal systems (EGS), a new type of geothermal power technologies that do not require natural convective hydrothermal resources, are known to be associated with induced seismicity. EGS involves pumping fluids at pressure to enhance or create permeability through the use of hydraulic fracturing techniques. Hot dry rock (HDR) EGS actively creates geothermal resources through hydraulic stimulation. Depending on the rock properties, and on injection pressures and fluid volume, the reservoir rock may respond with tensile failure, as is common in the oil and gas industry, or with shear failure of the rock's existing joint set, as is thought to be the main mechanism of reservoir growth in EGS efforts.[52]

HDR and EGS systems are currently being developed and tested in Soultz-sous-Forêts (France), Desert Peak and the Geysers (U.S.), Landau (Germany), and Paralana and Cooper Basin (Australia). Induced seismicity events at the Geysers geothermal field in California has been strongly correlated with injection data.[53] The test site at Basel, Switzerland, has been shut down due to induced seismic events. On November 2017 a Mw 5.5 struck the city of Pohang (South Korea) injuring several people and causing extensive damage, the proximity of the seismic sequence with an EGS site, where stimulation operations has taken place few months prior the earthquake raised the possibility that raises the possibility that this earthquake was anthropogenic. According to two different studies it seems plausible that the Pohang earthquake was induced by EGS operations.[54][55]

Researchers at MIT believe that seismicity associated with hydraulic stimulation can be mitigated and controlled through predictive siting and other techniques. With appropriate management, the number and magnitude of induced seismic events can be decreased, significantly reducing the probability of a damaging seismic event.[57]

Induced seismicity in Basel led to suspension of its HDR project. A seismic hazard evaluation was then conducted, which resulted in the cancellation of the project in December 2009.

Hydraulic fracturing is a technique in which high-pressure fluid is injected into the low-permeable reservoir rocks in order to induce fractures to increase hydrocarbon production.[58] This process is generally associated with seismic events that are too small to be felt at the surface (with moment magnitudes ranging from −3 to 1), although larger magnitude events are not excluded.[59] For example, several cases of larger magnitude events (M > 4) have been recorded in Canada in the unconventional resources of Alberta and British Columbia.[60]

Operation of technologies involving long-term geologic storage of waste fluids have been shown to induce seismic activity in nearby areas, and correlation of periods of seismic dormancy with minima in injection volumes and pressures has even been demonstrated for fracking wastewater injection in Youngstown, Ohio.[61] Of particular concern to the viability of carbon dioxide storage from coal-fired power plants and similar endeavors is that the scale of intended CCS projects is much larger in both injection rate and total injection volume than any current or past operation that has already been shown to induce seismicity.[62] As such, extensive modeling must be done of future injection sites in order to assess the risk potential of CCS operations, particularly in relation to the effect of long-term carbon dioxide storage on shale caprock integrity, as the potential for fluid leaks to the surface might be quite high for moderate earthquakes.[13] However, the potential of CCS to induce large earthquakes and CO₂ leakage remains a controversial issue.,[63][64][65]

Since geological sequestration of carbon dioxide has the potential to induce seismicity, researchers have developed methods to monitor and model the risk of injection-induced seismicity, in order to better manage the risks associated with this phenomenon. Monitoring can be conducted with measurements from an instrument like a geophone to measure the movement of the ground. Generally a network of instruments around the site of injection is used, though many current carbon dioxide injection sites do not utilize any monitoring devices. Modeling is an important technique for assessing the potential for induced seismicity, and there are two primary types of models used: physical and numerical. Physical models use measurements from the early stages of a project to forecast how the project will behave once more carbon dioxide is injected, and numerical models use numerical methods to simulate the physics of what is occurring inside the reservoir. Both modeling and monitoring are useful tools to quantify, and thus better understand and mitigate the risks associated with injection-induced seismicity.[12]

To assess induced seismicity risks associated with carbon storage, one must understand the mechanisms behind rock failure. The Mohr-Coulomb failure criteria describe shear failure on a fault plane.[66] Most generally, failure will happen on existing faults due to several mechanisms: an increase in shear stress, a decrease in normal stress or a pore pressure increase.[12] The injection of supercritical CO2 will change the stresses in the reservoir as it expands, causing potential failure on nearby faults. Injection of fluids also increases the pore pressures in the reservoir, triggering slip on existing rock weakness planes. The latter is the most common cause of induced seismicity due to fluid injection.[12]

The Mohr-Coulomb failure criteria state that

${\displaystyle \tau _{c}=\tau _{0}+\mu (\sigma _{n}-P)}$

with ${\displaystyle \tau _{c}}$ the critical shear stress leading to failure on a fault, ${\displaystyle \tau _{0}}$ the cohesive strength along the fault, ${\displaystyle \sigma _{n}}$ the normal stress, ${\displaystyle \mu }$ the friction coefficient on the fault plane and ${\displaystyle P}$ the pore pressure within the fault.[12][67] When ${\displaystyle \tau _{c}}$ is attained, shear failure occurs and an earthquake can be felt. This process can be represented graphically on a Mohr's circle.[12]

While there is risk of induced seismicity associated with carbon capture and storage underground on a large scale, it is currently a much less serious risk than other injections. Wastewater injection, hydraulic fracturing, and secondary recovery after oil extraction have all contributed significantly more to induced seismic events than carbon capture and storage in the last several years.[68] There have actually not been any major seismic events associated with carbon injection at this point, whereas there have been recorded seismic occurrences caused by the other injection methods. One such example is massively increased induced seismicity in Oklahoma, USA caused by injection of huge volumes of wastewater into the Arbuckle Group sedimentary rock.[69]

It has been shown that high energy electromagnetic pulses can trigger the release of energy stored by tectonic movements by increasing the rate of local earthquakes, within 2–6 days after the emission by the EMP generators. The energy released is approximately six orders of magnitude larger than the EM pulses energy.[70] The release of tectonic stress by these relatively small triggered earthquakes equals to 1-17% of the stress released by a strong earthquake in the area.[71] It has been proposed that strong EM impacts could control seismicity as during the periods of the experiments and long time after, the seismicity dynamics were a lot more regular than usual.[72][73]

Risk is defined as the chance/probability of being exposed to hazard. The hazard from earthquakes depends on proximity to potential earthquake sources, their magnitudes, and rates of occurrence and is usually expressed in probabilistic terms. Earthquake hazards can include ground shaking, liquefaction, surface fault displacement, landslides, tsunamis, and uplift/subsidence for very large events (M_L > 6.0). Because induced seismic events, in general, are smaller than M_L 5.0 with short durations, the primary concern is ground shaking.[74]

Ground shaking can result in both structural and nonstructural damage to buildings and other structures. It is commonly accepted that structural damage to modern engineered structures happens only in earthquakes larger than M_L 5.0. The main parameters in structural damage is peak ground velocity (PGV). Ground shaking is usually measured as peak ground acceleration (PGA) in seismology and earthquake engineering. When PGA is greater than 18-34% of g (the force of gravity), moderate structural damage is possible, and very strong shaking can be perceived.[75] In rare cases, nonstructural damage has been reported in earthquakes as small as M_L 3.0. For critical facilities like dams and nuclear plants, it is crucial to ensure the ground shaking is not able to cause any unaffordable damages.

Human anxiety is another factor in determining the risk of induced seismicity. Anxiety refers to the human concern created by low-level ground shaking. Because injection-induced seismicity is generally of a small magnitude and short duration, human anxiety is often the only or primary hazard associated with felt events.

Extended reading - An Introduction to Probabilistic Seismic Hazard Analysis (PSHA)

Probabilistic Seismic Hazard Analysis (PSHA) is aimed to quantify the possibility of the ground motion reaching certain arbitrary levels or thresholds at a site when taking all the possible earthquakes (both natural and induced) into consideration.[76][77][78][79] It is used for building codes in both United States and Canada, as well as protecting dams and nuclear plants from the damage of seismic events.[76][80]

Understanding the geological background on the site is a prerequisite for seismic hazard analysis. Parameters that contribute to possible seismic events should be understood before the analysis. Formations of the rocks, subsurface structures, locations of faults, state of stresses and other parameters that contribute to possible seismic events are considered. Records of past earthquakes of the site are also required.

The magnitudes of all earthquakes that occurred at the studied site can be utilized in the Gutenberg-Richter relation, as shown below,

${\displaystyle \log N(\geq M)=a-bM}$

where ${\displaystyle M}$is the magnitude of seismic events, ${\displaystyle N}$is the number of events with magnitudes bigger than ${\displaystyle M}$, ${\displaystyle a}$ is the rate parameter and ${\displaystyle b}$ is the slope. ${\displaystyle a}$ and ${\displaystyle b}$ vary at different sites. By studying the catalogs of previous earthquakes, ${\displaystyle a}$ and ${\displaystyle b}$ for one specific site can be interpreted, hence the number (probability) of earthquakes exceeding a certain magnitude can be predicted.[76][81]

Ground motion consists of amplitude, frequency and duration of the shaking. PGV (peak ground velocity) and PGA (peak ground acceleration) are often used in describing ground motion. By combining PGV and PGA parameters with Modified Mercalli intensity (MMI) for a certain site, ground motion potential equations can be utilized to estimate the ground motions related to induced seismic events, particularly at close distances.[76]

The standard PSHA uses the distributions of different inputs to generate various models for prediction.[82] Another way is to combine Monte-Carlo simulation in PSHA.[76][14] By considering all the parameters, as well as the uncertainties in these parameters, seismic hazards of the interested sites can be described statistically.

In the end, PSHA is able to give an estimation of the potential damages from induced seismicity in both magnitudes and distances. In the analysis, damage thresholds can be set by MMI, PGA or PGV. Probabilistic hazard analyses indicate that hazards cannot be mitigated effectively within 5 km, that is to say, no operations should be performed (an exclusion zone) within 5 km of the site.[76] It is also suggested that real-time monitoring and immediate response protocol be required within 25 km from the site.[76]

Induced seismicity can cause damage to infrastructure and can also lead to brine and CO2 leakages.[83] It is easier to predict and mitigate seismicity caused by explosions. Common mitigation strategies include constraining the amount of dynamite used in one single explosion and the locations of the explosions. For injection-related induced seismicity, however, it is still difficult to predict when and where induced seismic events will occur, as well as the magnitudes. Because induced seismic events related to fluid injection are unpredictable, it has garnered more attention from the public. Induced seismicity is only part of the chain reaction from industrial activities that worry the public. Impressions toward induced seismicity are very different between different groups of people.[84] The public tends to feel more negatively towards earthquakes caused by human activities than natural earthquakes.[85] Two major parts of public concern are related to the damages to infrastructure and the well-being of humans.[84] Most induced seismic events are below M 2 and are not able to cause any physical damage. Nevertheless, when the seismic events are felt and cause damages or injuries, questions arise from the public whether it is appropriate to conduct oil and gas operations in those areas. Public perceptions may vary based on the population and tolerance of local people. For example, in the seismically active Geysers geothermal area in Northern California, which is a rural area with a relatively small population, the local population tolerates earthquakes up to M 4.5.[86] Actions have been taken by regulators, industry and researchers. On October 6, 2015, people from industry, government, academia, and the public gathered together to discuss how effective it was to implement a traffic light system or protocol in Canada to help manage risks from induced seismicity.[87]

To mitigate the possible consequences of induced seismicity, hazard and risk assessment is essential. A Traffic Light System (TLS), also referred to as Traffic Light Protocol (TLP), is a calibrated control system served as a direct mitigation method for induced seismicity. Its merits consist of providing continuous and real-time monitoring and management of ground shaking of induced seismicity for specific sites. TLS was first implemented in 2005 in an enhanced geothermal plant in Central America. For oil and gas operations, the most widely implemented one is modified by the system used in the UK. Normally there are two types of TLS - the first one sets different thresholds, usually earthquake local magnitudes (ML) or ground motion (PGV) from small to large. If the induced seismicity reaches the smaller thresholds, modifications of the operations should be implemented by the operators themselves and the regulators should be informed. If the induced seismicity reaches the larger thresholds, operations should be shut down immediately. The second type of traffic light system sets only one threshold. If this threshold is reached, the operations are halted. This is also called a "stop light system". Thresholds for the traffic light system vary between and within countries, depending on the area. Risk assessment and tolerance for induced seismicity, however, is subjective and shaped by different factors like politics, economics, and understanding from the public.[88]