Seismic dispersion: What does it really mean and what causes it?
Frequency- or wavelength-dependent are indeed equally valid terms but frequency is in practice more measurable than wavelength, so much so that seismic wavelength is calculated not measured. Dispersion is caused by material density, the material the waves pass through will, depending on density, pass certain waves, and wavelengths of those waves, better than other waves and wavelengths.

Describing my visualisation of the effect is difficult, the best way to look at the effect is to find seismic traces of the same earthquake from various ranges. This will show the different wave fronts spreading out as they travel. This is largely due to their compatibility with the material they pass through. I find it comparable to structural resonance there are frequencies that interact well with the rock they're travelling through, they get a boost, and there are those that interact less well, they get held back by the material they're travelling through.