What happens to the higher frequency content of the tremors associated with a volcanic eruption?

Question

From [1]:

The main characteristics of volcanic tremor depend strongly on whether a volcano is erupting explosively and on the intensity of the event. Long before an eruption, tremor is ‘narrow-band’ (about 0.5–2 Hz and sometimes monochromatic or harmonic). On transitions to active volcanism, however, the maximum frequency can climb to 5–7 Hz (refs 1–4) (Fig. 1c and d). Moreover, whereas tremor related to low-intensity volcanism remains narrow-band, tremor associated with intermittent or protracted explosive behaviour is ‘broadband’, characterized by power distributed over the full 0.5–7 Hz tremor bandwidth.

When I read the above, I was not surprised that the tremors associated with volcanic activity were mainly comprised of low frequency components. What did surprise me was that there isn't more high frequency energy present in the measurements.

Is there a noticiable amount of high frequency activity in these tremors that is not significant enough to report, or are the higher frequency components absorbed by the crust surrounding the volcano?

References:

1. Jellinek, A.M. and Bercovici, D. (2011). Seismic tremors and magma wagging during explosive volcanism. Nature, 470, 522-526. [DOI]

Answer 1

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:

1. 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)