References
• Talwani, P., 1997. On the nature of reservoir-induced seismicity, Pure and Applied Geophysics, 150, 473-492.
• Talwani, P., Chen, L. & Gahalaut, K., 2007. Seismogenic permeability, k(S), Journal of Geophysical Research-Solid Earth, 112.
• Gupta, H., 2002. A review of recent studies of triggered earthquakes by artificial water reservoirs with special emphasis on earthquakes in Koyna, Earth Science Reviews, 58, 279-310.
Introduction
• Noticed first when Lake Mead (on Arizona-Nevada border) was filled
• We know how and when conditions change so can study response
• Triggered by as little as 20 m filling – crust already close to failure
• M 6.3 triggered by reservoir filling in Koyna, India – killed 200 people.
Three time scales of seismicity
• Initial– Loading from water– increased pore pressure
• Delayed– Diffusion of pore pressure
• Protracted– Subsequent changes in water level
Initial seismicity
• Poroelastic constitutive equations:• – = volume strain increment– B = Skempton’s coefficient– = undrained Poisson’s ratio– G = shear modulus
• – = change in normal stress
2. Delayed seismicity
• Diffusion of pore pressure– – is permeability– is viscosity of pore fluid– is compressibility of bulk rock
Coupled poroelastic response
• Effect of load + diffusion of pore pressure– B Skempton’s coefficient– is the undrained Poisson’s ratio– H(t) is Heaviside unit step function– Assumptions……e.g. isotropic medium
Diffusivity and permeability
• Talwani et al. 2007– Measured hydraulic diffusivity, C, from induced
earthquakes– Found all fractures that produced seismicity had
permeability of 5x10-16 to 5x10-14m-2
– Called this the seismogenic permeability– Assumed seismicity depends on rate of increase of
pore pressure, dp/dt.