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In 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.

Contents

Causes

There are a number of ways in which induced seismicity has been seen to occur:

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Reservoirs

The mass of water in a reservoir alters the pressure in the rock below, 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 Quedd Fodda Dam. Unfortunately, understanding of reservoir induced seismic activity is very limited. However, it has been noted that semisicity 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.[1] 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.

In 1967 a 6.3 magnitude earthquake occurred in Maharashtra, India with its epicenter, fore and aftershocks all located near or under the Konya Dam reservoir.[2] 180 people died and 1,500 were left injured. The effects of the earthquake were felt 230 Kilometers 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. With scares of landslide emerging, the dam was drained and consequently seismic activity was almost nonexistent. On August 1, 1975, a large magnitude 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.[3] In Zambia, Kariba Lake may have provoked similar effects.

Some experts worry that the Three Gorges Dam in China may cause an increase in the frequency and intensity of earthquakes.[4]

Buildings

The construction of some supertall buildings can produce induced seismicity. There were several small earthquakes in the area surrounding the Taipei 101 tower following its construction.

Mining

Mining leaves voids that can 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[5]. Natural cavern collapse forming sinkholes would produce an essentially identical local seismic event.

Extraction of fossil fuel and groundwater

Subsidence caused by fossil fuel (oil and/or natural gas) and groundwater extraction can generate seismic waves and minor earthquakes.[6]

Geothermal energy

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.[7]

HDR and EGS systems are currently being developed and tested in Soultz-sous-Forêts (France), Desert Peak and the Geysers (U.S.), Paralana and Landau (Germany), and Cooper Basin (Australia). The test site at Basel, Switzerland, has been shut down due to induced seismic events.

Largest Events at EGS Sites Worldwide[8]
Site Maximum Magnitude
Cooper Basin, Australia 3.7
Basel, Switzerland 3.4
Rosemanowes, 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.[9]

Induced seismicity at 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.[10] Although Basel is in a known earthquake zone (most of the city was destroyed in a magnitude 6.5 earthquake in 1356), the Basel project had not performed a thorough seismic risk assessment before starting geothermal stimulation.[11]

Seismic events in Basel reached the trip point of Richter Magnitude ML 2.9 six days after the main stimulation was started on December 2, despite precautionary reduction of the injection rate earlier that same day upon reaching earlier "soft" thresholds.[12] Even after injection was suspended at 11:34 am, tremors continued to grow, including a 2.7 ML event followed by a 3.4 ML event later that day. After that, the well was bled off as soon as practicable.[13] However, further tremors exceeding magnitude 3 were recorded on 6 January (measuring 3.1),[14] 16 January 2007 (3.2), and 2 February 2007 (3.2).[15][16][17]

In all, between December 2006 and March 2007, the six borehole seismometers installed near the Basel injection well recorded more than 13,500 potential events connected with the geothermal project. The 200 largest were between magnitudes 0.7 and 3.4. Nine of these events had an ML of 2.5 or larger. The remainder were too small to be observed or felt at the surface. Between December 2 and January 24, 168 seismic events with magnitude greater than 0.6 occurred within 1 km of the wellbore, at depths of 4-5 km, near the well bottom.[18].

External links

http://www.internationalrivers.org/en/node/3972 A map of reservoir-induced earthquakes (RIS).

http://tais.iaspei.net IASPEI's Triggered and induced seismicity working group website.

References

  1. ^ http://internationalrivers.org/de/node/1477
  2. ^ Reservoir-Induced Seismicity
  3. ^ International Rivers
  4. ^ Chen, L. & Talwani, P. (1998) Seismicity in China, Pure and Applied Geophysics, 153, 133-149.
  5. ^ Redmayne, D.W. (1988) Mining induced seismicity in UK coalfields identified on the BGS National Seismograph Network. Geological Society, London, Engineering Geology Special Publications; vol. 5; pp. 405-413
  6. ^ Van Eijsa, R.M.H.E, Muldersa, F.M.M, Nepveua, M, Kenterb, C.J, Scheffers, B.C. (2006) Correlation between hydrocarbon reservoir properties and induced seismicity in the Netherlands, Engineering Geology, 84, 99-111
  7. ^ Tester, Jefferson W. (Massachusetts Institute of Technology) et al (2006) (14MB PDF). The Future of Geothermal Energy - Impact of Enhanced Geothermal Systems (EGS) on the United States in the 21st Century. Idaho Falls: Idaho National Laboratory. pp. 4-10. ISBN 0-615-13438-6. http://geothermal.inel.gov/publications/future_of_geothermal_energy.pdf. Retrieved 2007-02-07.  
  8. ^ Bromley, C.J. & Mongillo, M.A. (February 2007), "All Geothermal Energy from Fractured Reservoirs - Dealing with Induced Seismicity", IEA Open Journal 48 (7): 5, https://byustudies.byu.edu/http://www.iea.org/impagr/cip/pdf/Issue48Geothermal.pdf  
  9. ^ Tester 2006, p. 5-6
  10. ^ Glanz, James (2009-12-10), "Quake Threat Leads Swiss to Close Geothermal Project", The New York Times, http://www.nytimes.com/2009/12/11/science/earth/11basel.html?_r=1  
  11. ^ Bromley 2007, p. 5
  12. ^ ^ a b c d Dyer, B. & Schanz, U. et al. (2008), "Microseismic imaging of a geothermal reservoir stimulation", The Leading Edge 27 (7): 856–869, doi:dx.doi.org/10.1190/1.2954024
  13. ^ Dyer 2008.
  14. ^ "Geothermal project shakes Basel again". Swissinfo.ch. 2007-01-06. http://www.swissinfo.ch/eng/front/detail/swissinfo.html?siteSect=105&sid=7407138. Retrieved 2009-07-04.  
  15. ^ Ladner, Florentin (2007-11-16). Geological and hydraulic evaluation of the Basel 1 geothermal reservoir. Geneva. http://www.geothermal.ch/fileadmin/docs/downloads/genf_2007pp.pdf. Retrieved 2009-07-04.  
  16. ^ Mukuhira, Yusuke et al. (2008). "Characterization of Microseismic Events with Larger Magnitude Collected at Basel, Switzerland in 2006". GRC Transactions 32: 87–94. http://niweb.kankyo.tohoku.ac.jp/PDF/GRCVol32MUKUHIRA.pdf.  
  17. ^ Schanz, Ulrich; Ben Dyer, Florentin Lander, Markus Haring (2007). Part 2: Microseismic aspects of the Basel 1 geothermal reservoir Presentation at the 5th Swiss Geoscience Meeting. http://www.geothermal.ch/fileadmin/docs/downloads/geoschanztalk.pdf.  
  18. ^ Swiss Federal Institute of Technology, Zurch (2007-02-06). DHM-Basel: Statement 1. http://www.seismo.ethz.ch/basel/articles/Pressekonf_Basel_20070125.pdf.  

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