Induced seismicity

Induced seismicity is typically earthquakes and tremors that are caused by human activity that alters the stresses and strains on Earth's crust. Most induced seismicity is of a low magnitude. A few sites regularly have larger quakes, such as The Geysers geothermal plant in California which averaged two M4 events and 15 M3 events every year from 2004 to 2009. The Human-Induced Earthquake Database (HiQuake) documents all reported cases of induced seismicity proposed on scientific grounds and is the most complete compilation of its kind.

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 5.7 El Reno earthquake may have been induced by deep injection of wastewater by the oil industry. A huge number of seismic events in oil and gas extraction states like Oklahoma is caused by increasing the volume of wastewater injection that is generated as part of the extraction process. "Earthquake rates have recently increased markedly in multiple areas of the Central and Eastern United States (CEUS), especially since 2010, and scientific studies have linked the majority of this increased activity to wastewater injection in deep disposal wells."

Induced seismicity can also be caused by the injection of carbon dioxide as the storage step of carbon capture and storage, which aims to sequester carbon dioxide captured from fossil fuel production or other sources in Earth's crust as a means of climate change mitigation. This effect has been observed in Oklahoma and Saskatchewan. Though safe practices and existing technologies can be utilized to reduce the risk of induced seismicity due to injection of carbon dioxide, the risk is still significant if the storage is large in scale. The consequences of the induced seismicity could disrupt pre-existing faults in the Earth's crust as well as compromise the seal integrity of the storage locations.

The seismic hazard from induced seismicity can be assessed using similar techniques as for natural seismicity, although accounting for non-stationary seismicity. It appears that earthquake shaking from induced earthquakes may be similar to that observed in natural tectonic earthquakes, or may have higher shaking at shorter distances. This means that ground-motion models derived from recordings of natural earthquakes, which are often more numerous in strong-motion databases than data from induced earthquakes, may be used with minor adjustments. Subsequently, a risk assessment can be performed, taking into account the increased seismic hazard and the vulnerability of the exposed elements at risk (e.g. local population and the building stock). Finally, the risk can, theoretically at least, be mitigated, either through reductions to the hazard or a reduction to the exposure or the vulnerability.

There are many ways in which induced seismicity has been seen to occur. In the 2010s, some energy technologies that inject or extract fluid from the Earth, such as oil and gas extraction and geothermal energy development, have been found or suspected to cause seismic events. Some energy technologies also produce wastes that may be managed through disposal or storage by injection deep into the ground. For example, waste water from oil and gas production and carbon dioxide from a variety of industrial processes may be managed through underground injection.^{[citation needed]}

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. 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. 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 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. 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.