Each living organism on this planet is intelligent enough to engineer solutions for existence. But they never tread the path and cross their limits. Only humans cross limits.
With the advent of recent industrialization phase, humans are showing arrogance of being superior than nature! And decade after decade, we plan and execute MEGA projects with the help of machines!
Have we ever thought of the impact?
Let us discuss popular way of generating electricity. Dams. And now river linking, new form of Mega dams.
Spend millions of dollar in survey and project planning. Then spend billions in construction with rampant corruption. By the time projects are realized, they not only lose value but also become disaster for ecology.
More than three–quarters of 49 projects assessed in a 1990 World Bank study of hydropower construction costs were found to have experienced unexpected geological problems of some kind. The study concluded that for hydrodams “the absence of geological problems should be treated as the exception rather than the norm.” 🙂
I will quote few references to bring the point to the table that dams increases possibilities of earth-quakes!
Man’s engineering efforts impact the way crustal stresses are released in earthquakes; these includes deep artificial water reservoirs, underground mining, high pressure fluid injection, removing underground fluids like gas, water and oil.
The largest reservoir triggered earth-quake is of magnitude 6.
Other activities triggered earth-quake of magnitude 5
There are more than 70 examples of reservoir induced Seismic Activities.
First known example Hoover Dam, USA.
For a long time, the role of reservoirs in inducing earthquakes was not well understood. Investigation of fluid injection induced earthquakes at the Rocky Mountain Arsenal near Denver, Colorado during the early 1960’s and application of Hubbert and Rubey’s (1959) work by Evans (1966) on the mechanism of triggering earthquakes by increase of fluid pressure, laid the foundation for understanding the phenomenon of reservoir-induced seismicity. Gough and Gough (1970a, b) explained triggering of earthquakes due to incremental stress caused by the load of the reservoir. Gupta et al. (1972a) identified the rate of increase of water level, duration of loading, maximum levels reached, and duration of retention of high water levels among the important factors affecting the frequency and magnitude of earthquakes near artificial reservoirs, The influence of pore fluid pressures in inducing earthquakes in simple reservoir models was investigated by Snow (1972). More sophisticated models of the effects of reservoir impounding on inducing earthquakes based on Biot’s (1941) consolidation theory (Rice and Cleary 1976) are provided by Withers and Nyland (1976) and Bell and Nur (1978). The three main effects of reservoir loading relevant to inducing earthquakes are: (a) the elastic stress increase that follows the filling of the reservoir; (b) the increase in pore fluid pressure in saturated rocks (due to the decrease in pore volume caused by compaction) in response to the elastic stress increase; and (c) pore pressure changes related to fluid migration.
Earthquakes are associated with shear fracturing of rocks. The shear strength of rocks is related to the ratio of the shear stress along the fault to the normal effective stress across the fault. The effective normal stress is equal to the normal stress minus the pore pressure. When the pore pressure increases, the shear stress is not changed, but the effective stress decreases by the amount of the pore pressure. Therefore, the ratio of shear to normal stress increases. If rocks are under an initial shear stress, an increase in fluid pressure can trigger shear failure. At Oroville, Bell and Nur (1978) calculated a maximum drop in strength to be about 40% of the maximum water load. When the fault zone is highly permeable, the strength drop could be as high at 70%. For the Oroville Reservoir, with a water depth of 200 m, these values would translate into drops of 8 and 14 bars. Earthquakes are known to have been triggered consequent to fluid injection and pore pressure changes of 35 bars at Rangely, Colorado (Raleigh et al., 1972, 1976), whereas during a fluid injection experiment only 14 bars pumping pressure was required to trigger earthquakes at Matsushiro, Japan (Ohtake, 1974). Thus the earthquakes at Oroville and other sites of induced seismicity may have been triggered by pore fluid pressure changes.
Read it further and understand the artificial risk for even low seismic activity zones!
While asessing the seismic risk of induced earthquakes near a reservoir, it is not the annual probability of ground shaking, but the acceptable risk in terms of the lifetime of the reservoir, that should be assessed. A more important effect of induced earthquakes is the change in temporal distribution of seismicity (Simpson, 1986). Moreover, induced earthquakes occur in the immediate vicinity of the reservoir. Areas of low natural seismicity are most vulnerable since these are the sites where adequate precautions are not taken to build structures to resist earthquakes; large induced earthquakes have mostly occurred in such areas. In areas of high seismicity, reservoirs may have less impact in changing the seismic regime and civil works are designed to withstand natural earthquakes. In an area of low seismicit where the return period of the maximum expected earthquake may be thousands of years, an increase in the probability of triggering the largest expected earthquake during the lifetime of the reservoir will alter the risk estimate significantly.
Reference: Reservoir Induced Earthquakes By H.K. Gupta
Pore Pressure Diffusion and the Mechanism of Reservoir-Induced Seismicity
The study of reservoir-induced seismicity offers a controlled setting to understand the physics of the earthquake process. Data from detailed investigations at reservoirs in South Carolina suggested that the mechanism of transmission of stress to hypocentral locations is by a process of diffusion of pore pressure (Pp). These results were compared with available worldwide data. The ‘seismic’ hydraulic diffusivity, αs, was estimated from various seismological observations, and was found to be a good estimate of the material hydraulic diffusivity, α. Application of these results to a dedicated experiment to understand RIS at Monticello Reservoir, S.C., suggested that the diffusing Pp front plays a dual role in the triggering of seismicity. The spatial and temporal pattern of RIS can be explained by the mechanical effect of diffusion of Pp with a characteristic hydraulic diffusivity within an order of magnitude of 5 × 104 cm2/s, corresponding to permeability values in the mtl¨¹darcy range. The triggering of seismicity is due to the combined mechanical effect of Pp in reducing the strength and, possibly, the chemical effect in reducing the coëfficiënt of friction between the clays in the pre-existing fractures and the rocks that enclose these fractures.
Excerpt from Silenced Rivers: The Ecology and Politics of Large Dams,
by Patrick McCully, Zed Books, London, 1996
The most widely accepted explanation of how dams cause earthquakes is related to the extra water pressure created in the microcracks and fissures in the ground under and near a reservoir. When the pressure of the water in the rocks increases, it acts to lubricate faults which are already under tectonic strain, but are prevented from slipping by the friction of the rock surfaces.
For most well–studied cases of RIS, the intensity of seismic activity increased within around 25 kilometres of the reservoir as it was filled. The strongest shocks normally occured relatively soon – often within days but sometimes within several years – after the reservoir reached its greatest depth. After the initial filling of the reservoir, RIS events normally continued as the water level rose and fell but usually with less frequency and strength than before. The pattern of RIS is, however, unique for every reservoir.