Decoupling the volcano infrasound source from the crater acoustic response

Napoli (Italia)

Volcano infrasound is an important component of multi-disciplinary volcano geophysics and has proven
utility for tracking eruptive activity and quantifying eruption dynamics. Unfortunately, a major limitation in
our interpretation of volcano infrasound is that it is critically affected by the morphology of the volcanic
crater, which can transform potentially simple source-time functions occurring within the crater into a signal
that is substantially more complex. If infrasound waveforms are to be used to recover important physical
parameters about an eruption source, then a robust understanding of the acoustic response of the crater is
required. In many cases, and especially for large deep craters, the acoustic response function acts as a severe
filter. For example, at Cotopaxi Volcano (Ecuador) infrasound ‘tornillos’ with an impulsive onset and peaked
spectra at 0.2 Hz decaying for more than 90 s are part of the source response due to the crater’s steep-walled,
deep crater.
We analyze broadband infrasound data from open-vent volcanoes with a wide variety of crater geometries
and jointly calculate their crater acoustic response using 1-D (axisymmetric) and 3-D morphologies derived
from structure-from-motion digital terrain models. We analyze both explosion and lava lake infrasound from
Villarrica (Chile), Stromboli (Italy), and Nyiragongo (Democratic Republic of the Congo) to demonstrate a
broad spectrum of volcano infrasound, whose attributes are heavily influenced by crater shape. We
demonstrate how some differences between simulations and recorded explosion are influenced by sourcetime functions, which may range from brief and impulsive to complicated or extended in time. Numerical
modeling shows that each volcanic crater has a unique impulse response and that deconvolving this acoustic
response is vital for estimating important eruption parameters including the size of volcanic explosions.