A fully probabilistic formulation is proposed to quantify seismic hazard rates in regions where seismic monitoring has been defective, and as a consequence the catalogue is incomplete in a wide range of magnitudes. Catalogue incompleteness is always accompanied by an imperfect seismic source characterization, the wider the region the more defective its description. To overcome these issues, the approach presented herein is associated with three probabilistic assumptions: (1) the total seismicity of the region is figured out from the incomplete seismic catalogue; (2) the probability that the next seismic event might occur in a specific part of the region is quantified from the spatial distribution of recorded earthquakes; (3) schemes 2 and 3 are naturally incorporated in the classical probabilistic seismic hazard analysis and the seismic hazard curve is computed. The approach is applied to the Gulf of Mexico. It is shown that the Gulf of Mexico should not be considered as a homogeneous seismic source, since non uniform spatial distribution of recorded earthquakes significantly impacts on seismic hazard estimates.