A fault can be interpreted as a non-welded interface, causing a discontinuity of displacement in a reflecting wave field that can be described with a linear slip boundary condition. We represent a fault as a layer of cracks; its thickness being small compared to the seismic wavelength. An increase of effective stress can be induced by a drop of pore pressure, either in the fault plane itself or elsewhere, as in a nearby producing hydrocarbon or water reservoir. The changing stress conditions cause crack squeezing and crack closures in the fault plane, thus increasing the portion of welded contact area and strengthening the interface coupling. We investigate the possibility of monitoring these changes from the frequency dependence of time lapse seismic reflection coefficients at the fault plane. Results depend strongly on the mean aspect ratio of the cracks in the fau the presence of long and thin cracks being most visible. Monitoring with S- waves is independent of the fault infill conditions, whereas monitoring with P-wave is only possible for states close to gas saturation.
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