We assume that the subsurface at the site of a newly impounded reservoir has a small volume of rock with porous-elastic properties significantly different from those of the other rocks in the area. A normal fault passes through this rock volume. We adopt the following implications to quantify reservoir influence at different points of the fault in such a case. The reservoir is circular and of uniform depth. The small rock volume is spherical in shape and embedded in an otherwise homogeneous half space with values of porous-elastic properties in the range normally observed through laboratory measurements on rocks. We infer from calculated results that the reservoir will promote slip at low water level at points of the fault lying within the small rock volume if its diffusivity, and undrained and drained Poisson’s ratios are significantly lower than those of the other rocks at the site. The reservoir will promote slip on the normal fault at points outside the small rock volume at high water level.

$\bf{Highlights}$

$\bullet$ A 3D model for reservoir promoted slip on a subsurface normal fault at low water is investigated.

$\bullet$ The reservoir is circular. The fault cuts a porous-elastic (PE) sphere in a PE half-space.

$\bullet$ The fault slips at low water at points in the sphere if PE half-space has nominal properties.

$\bullet$ PE sphere has low diffusivity and low undrained and drained Poisson’s ratios.