Yesterday night I caught a Discovery channel program on earthquake monitoring along the San Andreas Fault. A short segment discussed the San Andreas Observatory At Depth a joint effort involving the International Continental Drilling Program (ICDP), NSF and the U.S. Geological Survey (USGS) which involves drilling, recovering core from the fault zone and monitoring seismic activity at a depth of around 3 km or so to better understand the stresses along the fault.
The Indian government has an even more ambitious plan. There is news that a drilling project is being planned along the Koyna fault in southern Maharashtra that will go as deep as 8 km, a hole that will penetrate through the 2 km thick Deccan lava pile and into the underlying Archaean-Proterozoic basement.
The Koyna region has been a locus of seismic activity of > 5 magnitude, the largest being the 6.3 magnitude earthquake of 1967. It is also considered to be one of the better known examples of Reservoir Triggered Sesimicity, the culprit being the Koyna hydroelectric dam.
The map below shows the Koyna fault, which is hypothesized to be an extension of a late Archean -Early Proterozoic shear zone or a zone of weakened crust situated within the Dharwar craton, an Archean continental nuclei. Accumulating stresses along this shear zone periodically break the crust. Another theory is that the Koyna fault coincides with a very deep basement fault oriented roughly with the Western Ghat scarp and is perhaps related to the Mesozoic rifting of India and Cenozoic uplift of the Western Ghat region. The two theories are not mutually exclusive.
The ultimate reason why the Indian crust today is under stress is believed to be the compressional forces generated by the Indian plate colliding with Asia beginning early Cenozoic. These forces combined with those generated by isostatic readjustments due to denudation lead to old zones of crustal weakness getting reactivated and failing along old and new faults. This is the story seen all over the Indian peninsular regions, from the reactivation of Mesozoic faults in the Kutch region (Kutch earthquake 2001) to the Proterozoic central Indian Narmada rift region (Jabalpur earthquake 1997) to southern Maharashtra in the Koyna area and also eastwards near Latur (Killari earthquake 1993).
So far I have not seen any detailed justification why Koyna was chosen for this pioneer project over other regions at equal or greater risk from large earthquakes, for example the region in Gharwal Himalayas along the Main Central Thrust and the Main Boundary Thrust, major structures separating different Himalayan litho-tectonic terrains, or thrust faults along the Himalayan frontal range the Siwaliks which are close to large population centers like Chandigarh and Delhi.
Maybe its because the Koyna region is very well studied and at least one variable influencing the initiation of earthquakes in the Koyna region, the filling and draining of the reservoir and the stresses generated have been well monitored and modeled. These reservoir induced stresses are not nearly big enough to cause earthquakes on their own. They come into play only at a time when faults become critically stressed due to other geological forces, they are the piece of straw that broke the camel's back.
Direct monitoring the scientists are hoping will help understand how these other geological forces cause strain to gradually build up along the fault until breaking point.