Reservoir Triggered Seismicity (RTS): The Role of Reservoir Loading, Fluid Pressure, and Pore Pressure Diffusion

The continual accumulation of strain energy inside the lithosphere due to interplay of body and surface forces leads to seismic failure along the preexisting faults (weak zones), resulting in the generation of earthquakes. This force system, in turn, brings the stress regime of the upper brittle crust beneath the area (which is prone to seismicity) very near to the brittle failure line and it gets triggered by a small perturbation in the local stress regime due to plate tectonic forces, local tectonic forces, reservoir-associated forces, local strength weakening of the crustal material, and the presence of fluid in the crust. The occurrence of earthquakes in the near vicinity of reservoirs were noticed even in late 1930s and early 1940s, when for the first time in 1945 Carder pointed out the generation of earthquakes associated with the Lake Mead reservoir, Colorado, USA. Over the last century, about 90 cases of RTS have been reported. Among those earthquakes, 19 cases have occurred in India e.g. in Koyna, Bhatsa, Idduki, etc. The triggered earthquakes became quite important because these earthquakes can cause severe damages to human life and property. Increase in seismicity following the impoundment of large lakes has been explained in terms of the incremental stresses due to reservoir loading, increased pore fluid pressure, and pore pressure diffusion. Filling of a reservoir built on an infinite half-space has been proved to be conducive to reservoir-induced earthquakes in strike-slip and normal fault environments and inhibitory in a thrust fault environment ( Snow, 1972 ). In all cases, the pore water reduces the normal stress, thereby leading to seismicity. Generally, epicenters of reservoir-triggered earthquakes confine in both space and time. Therefore, they provide an excellent opportunity to study the physics of the earthquake process. In this chapter, various aspects of fluid flow-related seismicity will be discussed.