What happens to the higher frequency content of the tremors associated with a volcanic eruption?
It appears that, particularly for volcanos expelling andesitic magmas (those having a higher silica content, among other characteristics), the higher frequencies are more prevalent during the explosive portions of eruptions and tend to be absorbed by the more viscous magma during the tremors.

From the conclusion section of [1]:


  The principal distinction between the basaltic and andesitic cases can be identified in the difference of the magma-gas flux between the basic state and the explosive events. In other words, the high viscosity of the andesitic magmas reduces the permanent degassing, thus producing a low-energy volcanic signal (volcanic tremor) and inhibiting higher vibration modes, whereas the explosions are induced by strong
  emission of magmatic fluids, whose ascent along the conduit
  produces Vulcanian-style eruptions and the excitation of several vibration modes.


Some eruptions, e.g., the 1930 eruption of Stromboli are considered to be Vulcanian and would probably have slightly more energy in the higher frequency bands.


  Reference:
  
  
  De Lauro, E., De Martino, S., Falanga, M., Palo, M. (2011) Self-sustained vibrations in volcanic areas extracted by Independent Component Analysis: a review and new results. Nonlin. Processes Geophys., 18, 925–940  [DOI] [PDF] (freely available)