Period of sdof on uniform soil modelled with quad elements

[mapak]

Hello,

can anyone please tell me how i can get the period of sdof which is resting on a uniform soil mesh modelled with quad elements?? (2D mesh: 200x50 with 1m thickness) I am working on soil structure interaction problem and i have modelled the same structure on a full soil mesh as well as simplified models with springs (impedance factors). When i run an eigen analysis for the simplified spring models, i get one period (the sdof's period), no problem. But when i run the model with the soil mesh i get a number of periods since every soil element has 8 dof with correspondig masses. Taking into account the mesh, the total number of periods wounds up to a few hundred (!) i cant tell which one is the sdof's...

Any help would be deeply appreciated
Thank you