I think Ru is the interface strength factor in PLAXIS software. Since, soil is liquefying during the earthquake, loose all its strength due to liquefaction. Therefore, it is obvious to have lower value of Ru factor. I suggest you to read PLAXIS manual or refer to similar kind of published work based on PLAXIS software. All the best in your research.

Check you define right ground water table. Also in initial state of stress, make sure that you generated PORE WATER PRESSURE, by clicking on double plus button PORE PRESSURE GENERATION.

Thank you Mr. Karthigyen and Mr. Sadiq

Mr. sadiq, when selecting the dynamic boundaries (tied,viscous or free field) are we supposed to draw an interface? if yes, how are we supposed to draw that? and what will be the material?

Lateral boundries in PLAXIS you can select absorbing boundaries, interface you do not assign at boundaries. In case if you have soil and structure, then you can assign interface, i think in plaxis Mohr–Coulomb interface model is available, you can assign tangential stiffness based on friction angle.