Deep Borehole Investigations in Koyna-Warna Region

Print

The Koyna Dam located in Maharashtra, western India is the most outstanding example of Reservoir Triggered Seismicity (RTS), where triggered earthquakes have been occurring in a restricted area of 20x30 sq km since the impoundment of Shivajisagar Lake in 1962. These include the largest triggered earthquake of M~6.3 on Dec 10 1967, 22 earthquakes of M>5, about 200 earthquakes of M~4, and several thousand smaller earthquakes since 1962. The RTS was further enhanced by impoundment of the nearby located Warna reservoir in 1993. The seismicity is restricted in depth generally in the top ~10 km, but more commonly in the top 7 km of the Earth’s crust. The site is active. The latest M5.1 earthquake occurred on 12 December 2009. There is no other source of seismic activity within 50 km of the Koyna Dam. This makes it an ideal and natural observatory for earthquake studies. The frequent occurrence of earthquakes including a few with M~5 in a region hosting two important dams, underlines the importance of monitoring and studying this region in detail.

The role of pore fluid pressure changes for RTS has been underlined through several studies and experiments however due to limited direct observations in the near-field of triggered earthquakes; our understanding of these issues is mainly dependent on theoretical computations and modeling. Also, limited data is available to examine the fluid pressure regime, its variations and its correspondence with the occurrence of earthquakes. There is also uncertainty about the relative importance of fluid-driven (hydraulic) fracturing compared to shear failure in different geologic environments, and the interplay of temperature and pore-fluid pressure in reducing the frictional stability.

Super-deep borehole investigations at KTB, Kola, SAFOD and several other locations worldwide have significantly increased our understanding of the processes of the deep continental crust and physics of the Earth’s interior. Useful information has been obtained about fault characterization and fault behavior at depth, transition from brittle to ductile behavior in the crust, fluids in the deep crust, lithospheric dynamics and deformation, impact structures and mass extinctions, volcanism, and nature of thermal transport processes in the continental crust.

Considering the importance of deep borehole investigations, it is proposed to undertake a suite of observations in deep borehole(s) in the area of persistent and focused seismicity. The work will be carried out in collaboration with ICDP and the observations will include stress regime, pore fluid pressure and its variations, heat flow and its variation, orientation of faults, study of chemical properties of fluids, before, during and after earthquake. The proposed boreholes will also facilitate i) observation and analysis of data, generated through the operation of borehole for 4-5 year of time, when it is anticipated that a few earthquakes of magnitude ~3 would occur in the immediate vicinity of borehole, ii) continuous observation to study the data in the far and near field of the earthquake and temporal variation w.r.t. occurrence of earthquake and iii) development of a model of RTS mechanism.

Continuous observations directly within the fault zone at seismogenic depths will help in testing and extending current theories about phenomena that might precede an impending earthquake. Also, the roles of fluid pressure, intrinsic rock friction, chemical reactions and the physical state of active fault zones in controlling fault strength will be evaluated. These studies will also allow for improved models of static stress transfer and earthquake triggering at a regional scale and between specific faults, as needed for intermediate-term seismic hazard forecasting following large