What determines the spectrum of high and low frequency waves emitted by an earthquake?
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.