Types of induced earthquakes
Five types of main human beings activities may affect the seismotectonic environment in their area of influence, through changes in local seismicity level. They are:
1. Mining and quarries activity;2. Deep fluid injection under high pressure;3. Liquids's extraction;4. Underground explosions;5. Reservoirs' filling during the construction of dams.
The first three types of activities usually produce small seisms, with magnitudes not superior to 5,0 in the Richter Scale. However, as for the IV type, some nuclear tests run 60's, with a power in the neighborhood of tens of megatons (1 megaton = 106t of Trinitrotolueno-TNT) produced artificial seisms of the order of 7.0. The reservoir-induced seisms, even though they have small magnitudes, they might sometimes, reach moderate magnitudes (between 5.0 and 6,5 in the Richter Scale). Seisms this strong can produce severe macrosseismics side effects, with human victims and considerable damages, generating, therefore, a large environmental and social impact (Marza et al., 1999).
i)Seismicity induced by mines and quarries
In this case, the seismicity is induced by variations in the elastic effort, caused by the extraction of great amounts of rocks during the activities of mining and quarries. Two types of induced earthquakes may result form this: those caused by the extraction of materials within deep mines, usually with the occurrence of seisms close to the extraction site (Cook, 1976) and those which are superficial, due to diggings in shallow-depth mines and the removal of materials in quarries (Pomeroy et al., 1976).
II) Seismicity induced by deep fluid injection under high pressure
The fluid injection under high pressure by perforation produced the best documented case of induced seismicity (Simpson, 1986). From April 1962 to September 1963, more than 700 seismic events were registered, with magnitudes between 1.0 and 4.3, in the vicinities of Denver (Colorado), a very stable region, with rare historical incidence of earthquakes (Evans, 1966). The epicenters were situated close to a armory that produced toxic-nature liquids. To avoid environmental and security problems, the fluids kept being pumped to the interior of deep wells, until it was detected a direct correlation between the pumpings and the occurrence of land tremors. This happened because increments in the water pressure, through the pores and tiny-sized fractures of the deep in rocks, reduced the effective tectonic effort, facilitating the displacement of eventual faulted blocks.
Armed with these information, the scientists of the American Geological Service (USGS) decided to put into practice a special test, in an oil-extraction area which had been abandoned, in Rangely, still in Colorado. They took turns, injecting and, then, pumping water, as the changes in the seismicity level were registered by means of a local seismographic network. They found out that, on reaching a given level, the fluid's pressure triggered land tremors, which, then, ceased to be when the pressure was removed (Raleigh et al.,1976).
In another experiment carried out in 1970, in Matsushiro, Japan, the theory that the earthquakes may be induced by the water's pressure in the rocks' pores was also proven. In this case, the fluid was injected under high pressure in a well 1.800 meters deep. These events proved the importance of the water as a sesmicity-triggering mechanism, indicating that it is possible to think about the control of small-scale earthquakes (Ohtake, 1974).
III) Fluid-removal-induced seismicty
There are examples of earthquakes induced by a cause opposite to fluid injection, that is, the fluid-extraction-induced earthquakes. When fluids are extracted from the rocks, through the oil, water or gas explorations, there is a substantial reduction in the rocks' pressure, suggesting, apparently, that the seism-inducement potential will also be reduced. However, this is not true.According to the theory of the poroelasticity, the fluids' extraction might, sometimes, reduce the pressure upon the pores of an area, modifying locally the state of the tectonic forces, leading, in this way, to seisms. Most of the related fault mechanisms are inversed or normal (Davis et al., 1993). Examples of seisms induced by fluid extraction can be found in the U.S. (center-south of the Texas), Canada (Alberta), France (Lacq). IV) Nuclear explosion-induced seismicity
Seisms can also be induced by the running of underground nuclear tests. A nuclear explosion can destabilize the state of the tectonic forces, producing fractures in the rocks and decreasing the resistance to shearing. Thus, an induced seismicity is going to be detected by a local seismographic network, as long as the tectonic forces are not stabilized.
Investigations conducted in the nuclear tests' zone of Nevada (U.S.A.) indicated that underground nuclear detonations were immediately followed by tremors similar to aftershocks or replicas of large natural seisms (Bolt, 1976). An remarkable example is the underground nuclear explosion performed on November 19th 1968, in the testing grounds of Nevada, with an explosive power of 1,1 megaton, matching the seismic energy of an earthquake of mb = 6,5 (Båth, 1979). Soon afterwards, replicas could be observed at distances of up to 15 Km from the detonation site. The geologists determined that aftershocks had been unleashed, mainly, because of the explosion's setting free the existing natural tectonics forces in the area of the explosion (Kisslinger, 1976).
V) Reservoir-Induced Seismicity (RIS)
This is the most common type of induced seismicity and also the one which is understood the least. The reservoir's water mass results in an additional load, which causes a significant growth of the elastic tension, while the growth in the pores' pressure may be generated in two ways: directly, through the infiltration of the reservoir's water and indirectly, through the closing of pores and saturated fractures beneath the reservoir. The superficial load produced by each meter of water is of 0,1 bar or about 20 bars in a reservoir the size of the reservoir of Binational Itaipu (180 meters deep).
What happens to the water of a large reservoir? Building a dam creates a new lake, that is going to modify the static conditions of the rocky formations from point of viwe of both the mechanics (on account of the water mass's own weight) and the hydraulics (as a consequence of the fluid's infiltration, which causes internal pressures inside the deep rocky layers). This twofold combination can unleash tectonic disturbances and, eventually, generate seisms, if the local conditions are propitious (existence of imperfections and tectonic forees conveniently guided).
Even if the water's weight in reservoirs more than 100 meters deep does not suffice to crack the rocks at the base, the water column will exert a hydrostatic pressure, driving the liquid through rocks' pores and preexisting fractures. The following picture schematicaly illustrates a the triggering process for a reservoir-induced mand tremor. It may take months or even years for the pressure effect to reach not-so-long distances. This depends on the ground's permeability and to which extent the rocks are fractured.
Triggering-process of a tremor.
(modified by Veloso, 1992)
However, as soon as the pressure reaches more fractured zones, the water is driven into the rocks, reducing the effective tectonic stress and facilitating shifts of the faulted blocks. The water still has the role of a chemical agent: by hydrating certain molecules, it weakens the material and favorr the formation of new cracks, which lead to the liquid's penetrating even further deep into the interior of the rocky mass.
The RIS (Reservoir-Induced Seisms), therefore, result from the dynamic phenomenon of the complex interaction of the newly lake-induced forces, which begin to interfere with the balance of the pre-existing natural forces.
It is not known, for sure, if the reservoir just foreshadows the occurrence of earthquakes which would happen anyway or if it is also able to modify seisms' magnitude.
The greatest reservoir-induced seisms
All over the world, so far, 8 (eight) earthquakes with magnitudes superior or equal to 4.0 have already been recorded, of which 4 (four) had magnitudes superior to 6.0. The strongest of them, in Koyna, India, in 1965, resulted in 200 deaths, 1500 hurt and severe damages to the neighboring constructions and to the dam's structure itself (Gupta, 1992).
All of them were followed by preshocks and aftershocks and have been reported to be induced by their respective reservoirs (Simpson, 1976 and 1986; Gupta and Rastogi, 1976; Bozovic, 1974; Morrison, 1976; Gough & Gough, 1970; Carder, 1945).
For not having detailed information about the seismicity before these reservoirs' having been filled (with water), it is impossible to set the accurate characteristics of the variation of the seismicity caused by the reservoir.
However, in all cases, the seismicity attested after the the reservoir's being full of water and, especially, the main seism, account for the main change seen in the seismicity of the region around the reservoir.