Friction on faults controls slip distribution in response to tectonic stress: the friction distribution can be simplified by considering locked zones (asperities) surrounded by aseismic slipping zones. The aseismic slip of fault sections has an important role in concentrating stress on the asperities and in producing their failure. The slow ground displacement in fault zones is measurable through classic or spatial geodetic techniques and may help to localize the greater asperities on faults. Therefore accurate geodetic measurements in fault zones may be used to evaluate the seismic hazard in the region. We represent the Earth’ s crust by an elastic, homogeneous and isotropic half-space, including a plane normal fault. A locked asperity is considered on the fault, while the surrounding area of thy fault surface undergoes a uniform slip. The surface displacement field is analyzed in the presence and in the absence of the asperity; the influence of the asperity shape, size and depth is studied also varying the dip angle of the fault. We conclude that an asperity, whose area is about 1 km2, determines a surface displacement of mm order, when its centre is placed at depths ranging from 5 to 10 km and the surrounding fault area slips by tens of centimeters: in this case an asperity with an area of about 5x5 km2 could be reasonably localized by current geodetic measurements.