{"title":"Evidence of water percolation in granitoid basement in Koyna seismogenic zone: Implications for reservoir triggered seismicity","authors":"Kunal Modak","doi":"10.1007/s12040-024-02380-6","DOIUrl":null,"url":null,"abstract":"<h3 data-test=\"abstract-sub-heading\">Abstract</h3><p>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.</p><h3 data-test=\"abstract-sub-heading\">Research highlights</h3><ul>\n<li>\n<p>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.</p>\n</li>\n</ul>","PeriodicalId":15609,"journal":{"name":"Journal of Earth System Science","volume":null,"pages":null},"PeriodicalIF":1.3000,"publicationDate":"2024-08-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":null,"platform":"Semanticscholar","paperid":null,"PeriodicalName":"Journal of Earth System Science","FirstCategoryId":"89","ListUrlMain":"https://doi.org/10.1007/s12040-024-02380-6","RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q3","JCRName":"GEOSCIENCES, MULTIDISCIPLINARY","Score":null,"Total":0}
引用次数: 0
Abstract
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.
期刊介绍：
The Journal of Earth System Science, an International Journal, was earlier a part of the Proceedings of the Indian Academy of Sciences – Section A begun in 1934, and later split in 1978 into theme journals. This journal was published as Proceedings – Earth and Planetary Sciences since 1978, and in 2005 was renamed ‘Journal of Earth System Science’.
The journal is highly inter-disciplinary and publishes scholarly research – new data, ideas, and conceptual advances – in Earth System Science. The focus is on the evolution of the Earth as a system: manuscripts describing changes of anthropogenic origin in a limited region are not considered unless they go beyond describing the changes to include an analysis of earth-system processes. The journal''s scope includes the solid earth (geosphere), the atmosphere, the hydrosphere (including cryosphere), and the biosphere; it also addresses related aspects of planetary and space sciences. Contributions pertaining to the Indian sub- continent and the surrounding Indian-Ocean region are particularly welcome. Given that a large number of manuscripts report either observations or model results for a limited domain, manuscripts intended for publication in JESS are expected to fulfill at least one of the following three criteria.
The data should be of relevance and should be of statistically significant size and from a region from where such data are sparse. If the data are from a well-sampled region, the data size should be considerable and advance our knowledge of the region.
A model study is carried out to explain observations reported either in the same manuscript or in the literature.
The analysis, whether of data or with models, is novel and the inferences advance the current knowledge.
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