Induced seismicity is the general term for any seismic activity caused by human activities. It is highly relevant to the question of global energy because it is a problem that arises in at least four major sources of energy: it has been linked to coal mining, oil drilling and reservoir impoundment for hydroelectricity. [1-5] It also plays a particularly interesting role in recent developments in geothermal energy. [6-8] In general, the mechanisms for induced seismicity are not completely understood; it seems as though several different mechanisms may be involved in each of these situations. A better understanding of these processes could have important implications for the direction of development of renewable energy in the future.
A geothermal heat or electricity plant works by brining hot fluids from deep in the earth up to a heating plant or turbine above ground. In traditional geothermal systems, the fluids already exists in pores in the earth's rock, and flow through fissures to the plant's intake. Thus, traditionally geothermal plants were built near geysers or other places where hot fluids flow easily. Recent developments have allowed this method to be applied even in places where the rock is not naturally permeable and porous. An enhanced geothermal system (EGS) is one that facilitates the flow of these fluids through the rock by pumping cooler fluids into the earth, increasing the pressure and cooling the rock, thereby creating microfissures through which fluid can more easily flow. [8]
An EGS by its very nature induces seismicity. Very sensitive seismographs can detect the microfissuring induced by the local pressure and temperature changes in the rock. However, the seismic activity caused directly by pumping fluids into the rock does not generally seem to be hazardous. [7] Instead, studies of active EGS systems reveal that the most dangerous mechanism of seismicity is shear failure. [8] Shear failure comes into play when there is already a shear fault present on the location of the plant. A shear fault is when one mass of rock rests on the sloped top on another mass of rock, held there by static friction. If fluid is injected into the earth, it can lubricate the fault, causing slippage and potentially a severe earthquake. In the language of poromechanics, the pore pressure increases along the fault, approaching the overburden stress, which weakens the rock. [9]
Whether these shear-failure earthquakes might become large enough to endanger surrounding communities is still an open question. [6,8]
An act which has been shown conclusively to induce damaging seismicity is the damming of rivers. Reservoir induced seismicity has been recorded at over seventy locations worldwide, including damaging earthquakes at Koyna, India; Hsingengkiang, China; Kariba, Zimbabwe and Kremasta, Greece. [5]
Both of the mechanisms involved in EGS induced seismicity appear to effect reservoirs as well. First, the pressure on the rock varies with the water level of the reservoir, and this can create enough stress to cause an earthquake. Second, over time water from the reservoir seeps into the rock below it, which can lubricate shear faults as described above. [4] Talwani has shown that seismicity around reservoirs tends to increase either immediately after changes in water level or delayed by months to years. [5] He attributes this bimodal distribution to the play of both of these mechanisms. He also finds that in most cases, seismicity returns to normal levels after five or ten years.
Induced seismicity in coal mining is generally minor, resulting from rockbursts or pillar collapses which are of concern mainly in the mine itself. [1,2] Seismographic measurements have been used to study structural mine safety but the issue is not generally of safety to the surrounding community. [1]
However, there have historically been several earthquakes associated with oil and gas drilling. In the 1920s, seismicity increased around the Goose Creek oil field in Texas, causing minor damage to houses. Correlation has been measure in many studies, mostly with earthquakes of magnitude at most 4.0. There is a possible association of a 7.0 earthquake at Gazli in Soviet Uzbekistastan with a major gas field. It seems counterintuitive that fluid extraction should lead to earthquakes since fluid extraction is inherently stabilizing, just as fluid injection is inherently destabilizing. P. Segall at Stanford has proposed a model of fluid-extraction induced seismicity that leads to the surprising conclusion that fluid extraction creates stresses in places where there is no change in pore fluid content. [3]
A better understanding of induced seismicity would have important political and environmental implications. It is still an open question whether the 2008 earthquake in Sichuan province in China that left 80,000 people dead or missing was related to the nearby Zipingpu Reservoir. [10,11] If the earthquake was definitively linked to the reservoir, that could be damaging to the Chinese government. [10] Such evidence might also limit where reservoirs can be impounded in the future - the Zipinpu Reservoir is within a mile of a major fault line. [10]
For EGS, further understanding of the danger of induced seismicity seems necessary to the success of the industry. [6] Perhaps because of this, many reports on induced seismicity in EGS seem to view it as essentially a public relations and public education problem. [7,8] Protests about induced seismicity have shut down an EGS plant in Basel, Switzerland, and a proposed plant at the Geysers, California, has also met with public resistance. [6] Majer et. al. point out that all of the induced seismicity from EGS up to this point has caused at most minor damage. [7] Still, only a handful of EGS plants have ever been operational. [8] In his Nature editorial, seismologist D. Giardini argues convincingly that it is too soon to be able to accurately assess the risk of serious earthquakes resulting from EGS. [6]
© Peter Smillie. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] H. Hasegawa et al., "Induced Seismicity in Mines in Canada - An Overview," Pure Appl. Geophys. 129, 423 (1989).
[2] K. Holub, "Predisposition to Induced Seismicity in some Czech Coal Mines," Pure and Applied Geophysics 150 435 (1997).
[3] P. Segall, "Earthquakes Triggered by Fluid Extraction," Geology 17, 942 (1989).
[4] D. Simpson, W. S. Leith and C. H. Scholz, "Two Types of Reser[voir-Induced Seismicity," Bull. Seismol. Soc. Am. 78 2025 (1988).
[5] P. Talwani, "On the Nature of Reservoir-Induced Seismicity," Pure Appl. Geophys. 150, 473 (1997).
[6] D. Giardini, "Geothermal Quake Risks Must Be Faced," Nature 462, 848 (2009).
[7] E. Majer, "Induced Seismicity Associated With Enhanced Geothermal System," Geothermics 36, 185 (2007).
[8] J. Tester et al., "The Future of Geothermal Energy," Massahusetts Institute of Technology and Idaho National Laboraotry, INL/EXT-06-11746, 2006.
[9] B. Bruce and G. Bowers, "Pore Pressure Terminology," The Leading Edge 21, 170 (2002).
[10] S. LaFraniere, "Possible Link Between Dam and China Quake," New York Times, 5 Feb 09.
[11] K. Deng et al., "Evidence that the 2008 Mw 7.9 Wenchuan Earthquake Could Not Have Been Caused by the Zipingpu Reservoir," Bull. Seismol. Soc. Am. 100, 2805 (2010).