In many sedimentary basins of the world the minimum hori- zontal stress, Sh, is greater in overpressured zones than in normally pressured zones at equivalent depths. A common explanation is that the frictional slip on listric normal faults keeps the difference between vertical stress, Sv and Sh within certain bounds, and the difference is smaller under lower effective stress (i.e., higher pore pressure, Pp). However, in the overpressured parts of the central North Sea graben, United Kingdom, and the Sable subbasin of the Scotian Shelf, Canada, conventional friction envelopes underestimate the magnitude of Sh. These data instead indicate that Sh increases at a rate proportional to but less than the rate of increase of Pp, a condition consistent with a Pp-induced deformation of the rock called poroelastic behavior. This paper argues that, whereas friction may govern Sh in normally pressured basins, poroelastic behavior is responsible for the unusually high Sh in the overpres- sured parts of these same basins. Data on the Pp and Sh gradients from these basins suggest that ΔShPp ∼ 0.7.

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