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Abstract:
Knowledge of physical and mechanical properties of subsurface faults is important for understanding the processes responsible for seismogenesis in a region. However, sampling of subsurface faults is quite challenging. Scientific drilling and downhole geophysical logging in active fault zones provide access to fault zone samples from depth as well as direct measurements of in-situ physical properties, leading to substantially improved characterization of such zones. Here, we demonstrate the wealth of information obtained from scientific drilling down to 3 km depth in the Koyna seismogenic zone and their implications for seismogenesis. Geophysical well log data has shown several fault damage zones in the basement which are characterized by low electrical resistivity, low bulk density, high neutron porosity, low Vp and Vs, high Poisson’s ratio and low Young’s modulus compared to the surrounding host rocks, especially below a depth of 2100 m. The data also reveal the presence of water-bearing horizons across most of the fault damage zones, indicating clear evidence of water percolation to deeper levels. Additionally, laboratory derived rock mechanical properties of intact and fault damage zone rocks also show significant variations in strength and static elastic properties of fault zone rocks from the intact host rock. Uniaxial cyclic loading tests on intact granitic rocks, located just above a fault zone, shows that elastic properties corresponding to the loading cycle close to failure is consistent with the elastic properties of the fault damage zone materials. The in-situ and laboratory datasets shed new light on the recurrent RTS in the region.
