In situ measurements of hydraulic properties of a shear zone in northwestern South Carolina

Abstract

Abstract. Subsequent to the initial impoundment of the Bad Creek Reservoir in northwest South Carolina in January 1991, lake level fluctuations (up to 33 m d Ϫ1 ) caused delayed (98 hours) correlative water level changes in an observation well (OW3) 250 m away. The bottom of the well is connected to the bottom of the reservoir by a shallowly dipping, 1 m wide shear zone. The amplitude of water level changes in OW3 was 0.19 of that in the reservoir. The amplitude ratio, R, of the water level changes in OW3 to those in the reservoir is a measure of the pore pressure transmitted through the shear zone. Assuming one-dimensional diffusion of pore pressure in the shear zone between the reservoir and OW3, these values of time lag and amplitude ratio of the water level fluctuations changes yield estimates of 0.05 m 2 s Ϫ1 for hydraulic diffusivity and 1.1 ϫ 10 Ϫ15 m 2 for permeability of the shear zone. In subsequent years the time lag decreased to 72 hours and R increased to 0.23, suggesting a nearly 60% increase in the two hydraulic parameters. We interpret the increase to be owing to flushing of fines in the shear zone due to the pumping action of lake level fluctuations or due to dissolution. After 1994, there were no further changes in these parameters suggesting steady state conditions. Our analysis demonstrated that the fluid pressure flow is restricted to the shear zone and the pore pressure transmitted through the shear zone showed a lognormal relationship with the frequency of lake level fluctuations, R ϭ 0.5 exp (Ϫ14.93/T), where T (days) is the period of lake level changes. Spectral analyses of the data were used to obtain a frequency independent hydraulic diffusivity (ϳ0.076 m 2 s Ϫ1 ) of the shear zone after it had reached steady state conditions. Using the undrained response of the reservoir, an in situ estimate value 0.66 was obtained for the Skempton's coefficient. The hydraulic diffusivity ϳ0.1 m 2 s Ϫ1 was found to be lower than that encountered at other locations of induced seismicity in the region.

Hydromechanics of the Koyna–Warna Region, India

“…Assuming B * 0.7 for crystalline rocks (TALWANI et al, 1999) and water height of *70 m, for an earthquake at a depth of 4 km and a radial distance of 2 km, the undrained response is of the order of 20 kPa (Appendix 1), one order of magnitude less than the diffused pore pressure values calculated in this study (see next section). Therefore, we disregard the undrained effect in our calculations and consider only the changes in pore pressure due to diffusion.…”

“…2). Next we describe our understanding of the predominant role of pore pressure diffusion (CHEN and TALWANI, 2001) in isolated, discrete fractures (CORNET and YIN, 1995;TALWANI et al, 1999;EVANS et al, 2005;TALWANI et al, 2007) in the onset of RIS due to lake level fluctuations. We address (a) the role of longperiod lake level fluctuations (ROELOFFS, 1988) on the diffusion of pore pressure to long distances; (b) the increase in pore pressures to threshold levels to induce M 5 earthquakes; (c) the observation that large dp/dt changes help trigger RIS (NUR and BOOKER, 1972;TALWANI et al, 2007) (Sect.…”

Fluid flow and seismicity pattern: Evidence from the 1997 Umbria‐Marche (central Italy) seismic sequence

“…[15] Although the inferred values of hydraulic diffusivity vary over one order of magnitude, they lie within the range of values estimated in other areas: they are slightly larger than those obtained in geothermal and tectonic areas, that range between 10 À2 and 10 m 2 /s [Talwani et al, 1999;Shapiro et al, 1999] and smaller than those estimated by Noir et al [1997] for the central Afar rift (%10 4 m 2 /s). A mean value of 51 m 2 /s for the hydraulic diffusivity gives for the study area, a permeability value of the order of 7.4 Â 10 À12 m 2 [by using C t = 2.9 Â 10 À9 Pa…”

Investigations related to scientific deep drilling to study reservoir-triggered earthquakes at Koyna, India

“…The occurrence of reservoir-triggered seismicity (RTS) has been associated with a number of potential factors including the rate of loading, highest water level reached and the duration of retention of high water levels (Kaiser 1953;Gupta et al 1972a, b). Other studies have investigated the role of reservoir loading (e.g., Gough and Gough 1970a, b;Bell and Nur 1978;Roeloffs 1988) and the influence of pore fluid pressure (Snow 1972;Talwani et al 1999;Do Nascimento et al 2004). While major advancements have been made towards elucidating the role of individual parameters in triggering earthquakes, an integrated model that explains the genesis of RTS is unavailable.…”