Seismic dispersion: What does it really mean and what causes it? Body waves (to some extent) and especially surface waves show dispersive characteristics during propagation. In Dr. Christopher Liner's book [Elements of Seismic Dispersion (2012)], this phenomenon is quickly attributed to propagating seismic energy being frequency-dependent. I've heard this phrase for a long time, and I am still unsure how to visualize and/or quantify it for the sake of really understanding this phenomenon.

Indeed, why can't one phrase it as being wavelength-dependent as well? Also, for both surface and body waves, what are the primary factors that cause/generate seismic dispersion?
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.