| Part of a series on earthquakes |
|---|
| Types |
| Foreshock • Aftershock • Blind thrust Doublet • Interplate • Intraplate Megathrust • Remotely triggered • Slow Submarine • Supershear Tsunami • Earthquake swarm |
| Causes |
| Fault movement • Volcanism • Induced seismicity |
| Characteristics |
| Epicenter • Hypocenter • Shadow zone Seismic wave • P-wave • S-wave |
| Measurement |
| Mercalli scale • Richter scale Moment scale • Surface wave magnitude scale Body wave magnitude scale • Seismometer Earthquake duration magnitude |
| Prediction |
| Coordinating Committee for Earthquake Prediction Earthquake sensitive |
| Other |
| Shear wave splitting • Adams–Williamson equation Flinn-Engdahl regions • Earthquake engineering Seismite • Seismology |
Induced seismicity refers to typically minor earthquakes and tremors that are caused by human activity that alters the stresses and strains on the Earth's crust. Most induced seismicity is of an extremely low magnitude. A few sites regularly have larger quakes, such as The Geysers geothermal plant, which in the past 5 years has averaged 2 M4 events and 15 M3 events every year.[1]
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There are a number of ways in which induced seismicity has been seen to occur.
The mass of water in a reservoir alters the pressure in the rock below and through fissures in the rocks, lubricates the fault[2], which can trigger earthquakes. Reservoir-induced seismic events can be relatively large compared to other forms of induced seismicity. The first case of reservoir induced seismicity occurred in 1932 in Algeria’s Oued Fodda Dam. Unfortunately, understanding of reservoir induced seismic activity is very limited. However, it has been noted that seismicity appears to occur on dams with heights larger than 100 meters. The extra water pressure created by vast reservoirs is the most accepted explanation for the seismic activity.[3] Induced seismicity is usually overlooked due to cost cutting during the geological surveys of the locations for proposed dams. Once the reservoirs are filled, induced seismicity could occur immediately or with a small time lag. 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.[4] 180 people died and 1,500 were left injured. The effects of the earthquake were felt 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 massive earth-fill dam and reservoir recently constructed and filled there.
The filling of the Katse Dam in Lesotho, and the Nurek Dam in Tajikistan is an example.[5] 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.[6]. However, researchers have been denied access to seismological and geological data to examine the cause of the quake further.[7][8][9]
Some experts worry that the Three Gorges Dam in China may cause an increase in the frequency and intensity of earthquakes.[10]
Mining leaves voids that generally alter the balance of forces in the rock. These voids may collapse producing seismic waves and in some cases reactivate existing faults causing minor earthquakes.[11] Natural cavern collapse forming sinkholes would produce an essentially identical local seismic event.
Fossil fuel extraction can generate earthquakes. [12][13] Hydraulic fracturing of natural gas wells produces large amounts of waste water. This contaminated water is often pumped into salt water disposal (SWD) wells. The weight and lubricity of this waste water has been shown to trigger earthquakes. [14] [15]
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.[16]
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.[17] The test site at Basel, Switzerland, has been shut down due to induced seismic events.
| Site | Maximum Magnitude |
|---|---|
| Cerro Prieto, Baja California, Mexico[19] | 7.2 |
| The Geysers, United States | 4.6 |
| Cooper Basin, Australia | 3.7 |
| Basel, Switzerland | 3.4 |
| Rosemanowes Quarry, United Kingdom | 3.1 |
| Soultz-sous-Forêts, France | 2.9 |
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.[20]
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.