Crustal structures with a thick, surficial sediment layer with low seismic wave speeds produce a reversal in the polarity of the shear stress eigenfunctions of long-period Rayleigh waves at shallow depth. Consequently, seismic disturbances with a strong vertical dip-slip component that are within or just below the sediment layer should generate Rayleigh waves that show a polarity reversal when compared with Rayleigh waves from the same source in a more typical crustal structure. Here the first observation of this unusual behavior is presented by modeling surface waves from the 7 May 2001 North Sea earthquake. A previous study finds a focal mechanism close to vertical dip slip for this earthquake, and suggests that the source is within the 6 km thick sediment layer found in this region. An appropriate structural model is used to generate synthetic seismograms and estimate a double-couple focal mechanism for the source. The orientation of the fault plane determined here is similar to that found by the previous study; however, the slip direction is opposite, demonstrating that the use of an incorrect structural model has a profound effect on focal mechanism determination for this type of seismic source.