Investigations of continued reservoir triggered seismicity at Koyna, India

Abstract

Koyna, located in the Deccan Volcanic Province in western India, is the most significant site of reservoir triggered seismicity (RTS) globally. The largest RTS event of M 6.3 occurred here on December 10, 1967. RTS at Koyna has continued. This includes 22 M≥5.0 and thousands of smaller events over the past 50 years. The annual loading and unloading cycles of the Koyna Reservoir and the nearby Warna Reservoir influence RTS. Koyna provides an excellent natural laboratory to comprehend the mechanism of RTS because earthquakes here occur in a small area, mostly at depths of 2–7 km, which are accessible for monitoring. A deep borehole laboratory is therefore planned to study earthquakes in the near-field to understand their genesis, especially in an RTS environment. Initially, several geophysical investigations were carried out to characterize the seismic zone, including 5000 line kilometres of airborne gravity gradiometry and magnetic surveys, high-quality magnetotelluric data from 100 stations, airborne LiDAR surveys over 1064 km2, drilling of 8 boreholes of approximately 1500 m depth and geophysical logging. To improve the earthquake locations a unique network of borehole seismometers was installed in six of these boreholes. These results, along with a pilot borehole drilling plan, are presented here.

Chemical and Noble Gas Isotope Compositions of Formation Gases from a 3 km Deep Scientific Borehole in the Koyna Seismogenic Zone, Western India

“…The Koyna region, situated in western India, is a wellknown site of reservoir-triggered seismicity (RTS), where triggered earthquakes have been occurring in a restricted area of 30 × 20 km 2 since the impoundment of the Koyna reservoir in 1962 [27][28][29][30][31][32][33][34]. There were no earthquakes reported in the Koyna region prior to impoundment of the reservoir within 50 km of the Koyna dam, but the seismic activity started in the months following the impoundment and increased gradually during the next few years culminating in the M~6.3 Koyna earthquake of December 10, 1967 [29,35].…”

“…The region continues to be active with recurrent, low magnitude earthquakes. A strong correlation between earthquake occurrences and the annual loading and unloading cycles of the Koyna and Warna reservoirs has been established from observations during the past few decades [29,34,37]. However, a model to comprehend the genesis of reservoir-triggered earthquakes has remained elusive likely due to lack of direct observations from the near-field of earthquakes.…”

Emplacement history and evolution of the Deccan Volcanic Province, India

“…2b) have shown that paleotopographic undulations of the magnitude of more than 200 m are encountered at several locations (Jain et al, 1995;Kale et al, 2014). The Trap-basement contact shows an elevation difference of almost 600 m within a linear distance of about 20 km across the WGE in deep drilling in the KWSZ (Gupta et al, 2017). Some of the chemostratigraphic formations of the Deccan Traps are of lesser thickness (see: Fig.1 in Schoene et al, 2015) than the amplitudes of the paleotopographic undulation of the basement.…”

“…Geological and geophysical studies (Deshmukh and Sehgal, 1988;Peshwa and Kale, 1997;Raval and Veeraswamy, 2011) in subsequent years brought out the possibility of post-Trappean deformation along the Kurduwadi Lineament (KLZ), Pranhita-Godavari Rift zone (PGRZ) and the Koyna -Warna Seismic Zone (KWSZ). The KWSZ that has suffered sustained low-intensity seismicity for over 50 years (Gupta et al, 2017) is located along the Chiplun -Koyna Lineament. The deep crustal nature of these regional block boundaries has been validated by geophysical studies (e.g.…”

“…The anomalous drainage patterns of mature wide river valleys, beheaded streams (all draining towards the east) located along the west-facing WGE on the crests of the plateaux could not have resulted without structural or neotectonic controls . Although seismicity at Koyna-Warna has been attributed to reservoir-triggered mechanisms (Gupta et al, 2017), microseismicity across the province remains unstudied. Can reactivation of ancient structures explain catastrophic events (e.g.…”