ESSO - Indian National Centre for Ocean Information Services

In-situ stress orientation from 3 km borehole image logs in the Koyna Seismogenic Zone, western India: implications for transitional faulting environment

Deepjyoti Goswami, Pinki Hazarika and Sukanta Roy

Ministry of Earth Sciences, Borehole Geophysics Research Laboratory, Karad 415114, India

Abstract : Knowledge of the in-situ stress regime is critical to investigate the genesis of recurrent triggered seismicity over the past five decades in the Koyna seismogenic zone. Orientations of in-situ horizontal stresses are determined for the first time from analyses of image logs in a 3 km - deep scientific borehole KFD1 in the area. KFD1 passed through 1247 m thick Deccan Traps and continued 1767 m in the granitic basement. Stress induced wellbore failures, breakouts and drilling induced tensile fractures (DITFs), are identified in the acoustic and micro-resistivity images of the granitic basement. Additionally, tightly constrained focal mechanisms of 50 earthquakes of M>=3.6, reported in literature, are inverted to constrain the stress regime. Salient results are as follows: (i) wellbore breakouts and DITFs constrain N9°W+-17° orientation for maximum horizontal principal stress (SHmax); (ii) consistency with the regional NNW-SSE orientation of SHmax from inversion of earthquake focal mechanisms shows that the borehole data are representative for the Koyna region; (iii) breakout rotations at multiple depths below 2100 m indicate that the borehole may have passed through localised fault damage zones; (iv) consistent strike azimuths of steeply dipping fractures with SHmax orientation indicate favourable conditions for strike-slip to normal transitional faulting environment; and (v) stress inversion of 50 well determined earthquake focal mechanisms supports transitional faulting environment in the Koyna seismogenic zone. Thus, stress orientation and fracture information from borehole data, together with stress regime constrained from inversion of earthquake focal mechanisms shed new light on the faulting environment in the region.

Keypoints :

• 1.Stress-induced wellbore failures constrain N9°W+-17° as the mean orientation for SHmax in Koyna seismogenic zone.
• 2.Weak Sea Surface Temperature cooling is observed near the coast despite the passage of an intense cyclone.
• 3.Post-storm thermal recovery of the ocean is modulated by coastal boundary current and associated cyclonic eddy.

Supplementry information of this study

Please contact Dr. Sukanta Roy (sukanta.roy@nic.in; sukantaroy@yahoo.com) for further detail if any.