Does the shape of the fault that the earthquake originated from determine its ratio of high frequency to low frequency waves?
What about shallow-fault vs deep subduction zone earthquakes?
There are several factors that determine the spectrum of wave frequencies of earthquakes. They include the following:
1) Size of the earthquake. All other things being equal, the larger the earthquake, the larger the spectrum of frequencies.
2) The texture of the rocks or other materials. Hard materials have a wider range of frequencies than soft materials.
3) The ratio of "body" to surface waves. Waves emanating from the body of the earthquake have higher total frequencies (and greater variation), than waves emanating from the surface of the ground.
I would imagine two factors are involved. The source and the media. The vibration of source and its spectra has to do with its mass and stiffness. The propagation of the seismic wave and its attenuation has to do with media. Admittedly both hard to quantify, but with known geology of local and surrounding area it is possible to estimate the frequency and acceleration.