Quiet boundaries do not prevent model from drifting in X & Z

We are working on a FLAC3D model where a blast is being simulated with the model. The blast is applied internally (not on the boundaries of the model) and is applied as a stress. All gridpoint fixities are removed before the blast is applied and replaced with quiet boundaries. No free field boundaries are applied as the blast stress is applied internally within the model.

We are finding that the entire model is ‘drifting’ (only a few millimetres, but still a concern) during the application of the blast.

It there a way to prevent this from happening?