[S02-P] Poster

[S02-P-03] Rock strength variations in an active seismogenic zone: evidences from scientific drilling in Koyna, western India

^○Deepjyoti Goswami¹, Vyasulu V. Akkiraju¹, Surajit Misra¹, Sukanta Roy^1,2, Amalendu Sinha³, Harsh Gupta², Brijesh K. Bansal⁴, Shailesh Nayak⁴ (1.Borehole Geophysics Research Laboratory, Ministry of Earth Sciences, Karad 415 114, India, 2.CSIR-National Geophysical Research Institute, Uppal Road, Hyderabad 500 007, India, 3.CSIR-Central Institute for Mining and Fuel Research, Dhanbad 826 001, India, 4.Ministry of Earth Sciences, New Delhi 110 003, India)

Reservoir triggered earthquakes have been occurring in the Koyna region of western India since the impoundment of the Shivaji Sagar reservoir in 1962. The earthquakes are located in the granitic basement rocks underlying a thick pile of Deccan flood basalt covering the region. Triaxial measurements on cores of granitic rocks recovered from recent scientific drilling provide unique opportunity to characterize the effect of ongoing seismic activities on rock strength properties. Nine cored boreholes have been drilled up to a depth of 1522 m in the vicinity of the Koyna seismogenic zone. The boreholes penetrated the total thickness of Deccan flood basalt and passed through a few hundred meters in the underlying granitoids. Laboratory data include measurements on granite gneiss, granite, migmatitic gneiss and mylonitised granite gneiss obtained from four boreholes. Salient results are as follows (i) Increase of rock strength with increasing confining pressure allow determination of the linearized failure envelopes from which the cohesive strength and angle of internal friction are calculated. (ii) Variable differential stress at different depths are the manifestations of deformation partitioning in close association of fault zone(s) or localised fracture zones. (iii) Fractures controlled by naturally developed weak planes such as cleavage and fabric directly affect the rock strength properties, but the majority of failure planes developed during triaxial tests is not consistent with the orientations of pre-existing weak planes. The failure planes may, therefore, represent other planes of weakness induced by ongoing seismic activity. (iv) Stress-strain curves confirm that axial deformation is controlled by the varying intensity of pre-existing shear in the granitoids. (v) Frequent occurrences of low magnitude earthquakes may be attributed to low and variable rock strength of the granitoids, which, in turn, is modified by successive seismic events.