Rock Strength and Elastic Properties of Basement Granitoids from Koyna Region, Deccan Volcanic Province, India

Unconfined compressive strength measurements on 20 borehole core samples constrain the elastic properties of basement granitoids underlying the Deccan basalts in the Koyna region for the first time. Test specimens were taken from the basement section below 1251 m depth in borehole KBH-7, which is located in proximity to the Donichawadi surface rupture associated with the 1967 M6.3 Koyna earthquake. Specimens of granite, granite-gneiss and migmatitic gneiss were tested in the laboratory under dry state and room temperature. Salient results of the study are as follows: (i) The granitoids are characterised by variable compressive strength in the range 22-98 MPa, (ii) Stress-strain response of the basement rock indicate varying degrees of deformation even for the same rock type. (iii) Young's modulus and Poisson's ratio of the granitoids, computed from the linear elastic portion of the stress-strain curves, vary in the range 8-23 GPa and 0.1-0.3 respectively, (iv) The low and variable rock strength of the Koyna granitoids when compared to granitic rocks from aseismic areas indicates their generally weak nature, probably induced by the recurrent seismicity in the Koyna seismogenic zone.