HR: 11:50h
AN: T42B-06    [Abstracts]
TI: Seismogenic Permeability and Fluid Flow in Crustal Rocks
AU: * Talwani, P
EM: talwani@geol.sc.edu
AF: Department of Geological Sciences, University of South Carolina, 701 Sumter Street, EWSC Building, Columbia, SC 29208 United States
AB: Pore fluids play both a chemical and a mechanical role in the onset of seismicity. The mechanical role is usually associated with time dependent increases in pore pressures. A study of the temporal and spatial pattern of reservoir and fluid injection induced seismicity, and aftershock patterns of large earthquakes suggest that these pore pressure increases occur by diffusion to hypocentral regions through suitably located fractures. The efficiency of this diffusion depends on the hydraulic diffusivity of the fractures, which in turn is related to their intrinsic permeability, k. I have estimated the permeability from the temporal and spatial pattern of these earthquakes. For 82/84 cases this fracture permeability was found to lie between 0.5x10^-15 m² and 50x10^-15 m² (0.5 to 50 mDarcy), a range that I have labeled seismogenic permeability, k_s. Theoretical modeling shows that when the fracture permeability, k<k_s, the pore pressure increase and fluid flow are negligible. When k>k_s, there is fluid flow and no seismicity. Thus locations of fluid induced seismicity are associated with elevated fluid pressures and seismogenic permeabilities. Fractures associated with aseismic fluid flow are associated with permeabilities that exceed k_s. Support of this concept of k_s was obtained from observations from borehole-injection-induced seismicity. Injection experiments at Nojima fault zone, the location of the 1995 M7.2, Kobe earthquake, showed that the permeability of the Nojima fault zone (NFZ) had been enhanced by that earthquake (k approximately equal to 0.1 to 0.5 Darcy), such that k>k_s. Injection of fluids in NFZ resulted in fluid flow but no earthquakes. Earthquakes occurred in the adjoining volume where the permeability was approximately k_s. At Le Mayet de Montagne experimental site, Soultz, and Hijori hot dry rock sites, seismicity resulted in regions of high pore pressure increases (k approximately equal to k_s) but not in regions of fluid flow (k>k_s). These results suggest that seismogenic permeability is an important parameter in determining pattern of fluid flow in crustal rocks.
DE: 5104 Fracture and flow
DE: 5114 Permeability and porosity
DE: 8010 Fractures and faults
SC: Tectonophysics [T]
MN: Fall Meeting 2005