Abstract

Induced seismicity at Nurek, Tadjik S.S.R., is occurring within the upper plate of the lonakhsh thrust, which passes at depth directly beneath the reservoir. Within the immediate vicinity of the reservoir one can contrast two distinct areas, one seismic and the other aseismic. Both consist of rocks that can be shown to have a highly anisotropic permeability due to alternating permeable and impermeable strata. Water flow is favored along bedding while essentially prohibited across bedding. However, because the two areas differ in structure, they also differ in their vertical component of permeability, as a direct result of the different attitudes of the bedding. In the aseismic area, vertical fluid flow is blocked by a syncline that cups the base of the reservoir. In the seismic area, a plunging anticline that crops out beneath the reservoir allows water to flow to depth in the high-permeability direction–along bedding. Because effective stress changes at depth are dependent on permeability characteristics, knowledge of subsurface permeability or permeability anisotropy can serve as a means by which to predict the distribution of induced seismic activity. Thus, while regional geologic studies may indicate the presence of lithologies that are conducive to the transmission of pore-water pressure, access of water to these units depends on the structural arrangement beneath the reservoir, on a very local scale.

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