Yes, I have applied velocity history that correspond to sinusoidal acceleration with magnitude of 1.0 m/s2.But your input (time series assigned to the absorbing element) should be the velocity associated with the acceleration you want.

This is interesting. Why does a material property of the absorbing boundary affect the acceleration applied at the bottom of the soil? That means that there should be some kind of relationship between the input and the output, but per the previous quote, it is just a velocity and acceleration, without considering properties of the boundary at the bottom. Maybe I am missing something. Also, please see the explanation about the model at the bottom.If the material properties (G, Poisson, and rho) of your absorbing boundary at the bottom match the ones of the soil model, you should have exactly the input you want.

OK, so just to triple check. When using absorbing boundary at the bottom to apply ground motions by applying tractions via velocity, you get absolute accelerations as output. Please confirm.In this case, since you are applying the input as a boundary traction (your input velocity transformed to boundary traction by the ASDAbsorbingElement) the accelerations will be absolute, that's why you should see your acceleration at the base and not zero.

If your share your model here I will check what's wrong.

**Thank you so much. Attached is the same model as the big domain (240m x 240m), but I reduced it only to 2mx2m because of the runtime (big model takes an entire day on 25 cores). I checked and the results are identical between to big domain model and the small domain model, which is good news meaning that absorbing boundaries work the same way for both models. If you can find a mistake in the small model, it should work for the big model as well.**

**Please note the following regarding the model attached: it has two materials for absorbing boundary at the bottom: a) same as absorbing boundary on the sides, and b) the one called "bedrock", which is stiffer and corresponds to rock at the base of the domain. Interestingly, input motion at the bottom of the soil column is not the same when I change between these two materials assigned to the base absorbing boundary element. (see attached picture).**

It would be great if you can figure out what is the issue. I spent a lot of time playing with this. I just trying to apply 5Hz sinusoidal motion to the soil and compare it with theoretical solution.

Looking forward to hearing from you. Thank you!