| Abstrakt: |
The Koyna region, located in Western India, is a region of recurrent triggered seismicity that started post-impoundment of the Koyna Dam in 1962. Though previous studies have established a relationship between recurrent seismicity and the water level of the Koyna reservoir, little is understood about the possible role of the reservoir water in triggering seismic activity in the region. In the present study, mesoscopic and micro-structural studies of core samples from the basement granitoids provide evidence of fracture networks for fluid percolation and chemical alteration at depth. Salient findings are as follows: (1) presence of brittle deformation features such as fault breccias, fractures, fracture networks, and faults, which can act as water pathways, (2) presence of a cataclastic zone that may act as an impermeable zone and thus prohibit percolation of water thereof, acting as a potential storage area for fluids, in turn promoting dissolution and alteration of minerals, (3) evidence of the presence of fluid such as Fe-staining along fractures and occurrences of secondary precipitation such as calcite, and silica, and alterations such as epidote, chlorite along fractures and networks of mineral veins of epidote and chlorite, (4) low values of Sr and Ba at depth constitute direct evidence of hydrous alteration, (5) presence of fractures and fracture networks in microscopic scale in the thin sections prepared from the apparently intact part of the core signify that fracture networks might be persistent at all depths although it may not appear in mesoscale. Together with the strong correlation between earthquake activity and water levels of the Koyna reservoir and confirmation of the extension of surface fissure and fracture zone to the basement granitoids as brought out by previous studies, the present study provides compelling evidence in support of the percolation of water to the seismogenic depths. So, the weakening of pre-existing fault planes due to the chemical effects of water and an increase in the pore pressure by water infiltration may increase instability that may lead to a movement along the pre-existing faults, and aid repeated seismic slips in the region. Research highlights: The article presents a comprehensive overview of the reservoir-triggered seismicity observed after the impoundment of the Koyna Dam. The study focuses on the role of subsurface infiltration of water from the reservoir to seismogenic depths that may help in facilitating short-term, low-magnitude earthquakes in the region. The design of the article is straightforward; it primarily focuses on reporting the meso- to microstructural observations that bear the evidence of fluid-induced physicochemical alterations of the basement rock, followed by a qualitative discussion on the role of the fluid that may have weakened the pre-existing deformation signatures. The present study provides compelling evidence in support of the percolation of water to the basement granite, thereby increasing pore pressure, and the long-term chemical effects over the fault plane may aid in instability of the fault that may lead to a movement along pre-existing faults, and aid repeated seismic slips in the region. [ABSTRACT FROM AUTHOR] |
