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 km 2 , 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.

Abstract The differences in the tectonic regimes of the western part of the India–Asia collision zone (western Himalayan syntaxis), consisting of the Ladakh–Karakoram, Hindu Kush and Pamir terrains, are examined through the analyses and modelling of gravity anomalies, computed from global gravity model and terrestrial data. Long-wavelength (>450 km) anomalies are related to isostatic compensation and correspond to topographic variations. The short-wavelength gravity highs are primarily due to upthrust blocks and are mostly linked to crustal seismicity. The intermediate depth-focused earthquake region of Hindu Kush–Pamir is characterized by a gravity low probably caused by crustal underthrusting. The deep density structure of the crust and upper mantle constructed from the modelling of gravity anomalies with constraints from seismological information suggests crustal underthrusting of the Indian and the Asian plates in the Hindu Kush–Pamir section unlike that in the Ladakh–Karakoram region, where underthrust Indian lithosphere underplates beneath the Asian plate. The density model supports a hypothesis of slab break-off of Indian and Eurasian plates in the Ladakh–Karakoram segment, and of the Indian plate in the Hindu Kush–Pamir region, whereas the Eurasian plate drastically underthrusts deeper ( c. 200 km), causing deep seismicity in the Hindu Kush–Pamir section.