Soil damping

Hello sir,

I am working on the model contains linear elastic soil (100x50 m ) and concrete mat (20 m width and 0.5 thickness) subjected to an impulse laod. Models have been done, the first one with Rayielgh damping ( ξ=5%) and the second one without damping ξ=0.
I used the boundary condotions as shown in the attachment.

The picture shows the relative dispalcement between two points the rock surface (at -50) and at mat level(0).


The problem is: I got a high damping as shown in the picture, I do not where this dampig coming? is there any setting I have to do to remove this damping?