FLAC3D manual (Page 1-4: Dynamic Analysis):

"The method follows any prescribed nonlinear constitutive relation. If a

hysteretic-type model is used, and no extra damping is specified, then the

damping and tangent modulus are appropriate to the level of excitation at each

point in time and space, since these parameters are embodied in the constitutive model. By default, if Rayleigh or local damping is used, the associated damping coefficients remain constant throughout shaking and throughout the grid. Spatially varying damping can also be specified."