Infrasonic Resonance of Volcanic Craters

Harmonic infrasound, characterized by an impulsive onset and a gradually decaying coda, has been recorded at open-vent volcanoes, including Villarrica (Chile), Cotopaxi (Ecuador), and Mount Etna (Italy). The observed infrasound signal can be compared to an “old Western bar door that once opened, swings back and forth several times before coming to rest.”

The harmonic infrasound signals are due to resonance of the air mass within a volcanic crater, similar to a how a brass instrument, such as a trombone, works. The observed infrasound is modulated by the crater properties, primarily by the geometry and depth of the crater.

I developed a numerical model of that solves for wave propagation inside a volcanic crater. We were able to use this model to investigate how infrasound signals depend on crater geometry and showed how harmonic infrasound observations could be inverted for crater geometry.

MOUNT ETNA

After several months of increased activity, Mount Etna erupted in December 2018 with explosions at the summit and a fissure eruption on the flank. The peak frequency of summit infrasound signals decreased while resonance increased after the flank eruption. We inverted infrasound observations for crater geometry and showed that the crater depth and radius increased during the eruption, which suggests that the flank eruption drained magma from the summit and that eruptive activity led to erosion of the crater wall. This work illustrates how harmonic infrasound observations can be used to obtain high-temporal-resolution information about crater geometry and can place constraints on complex processes occurring in the inaccessible crater region during volcanic activity.

VILLARRICA

At Villarrica, infrasound is generated by the roiling of the lava lake and heavily modulated by the crater geometry. The character of the infrasound changed in the lead-up to the eruption. The pitch of the infrasound increased and the duration decreased. Using numerical modeling, we were able to attribute this change due to the lava level rising in the crater.

Villarrica volcano can be thought of as analogous to a trombone. Similar to a person blowing into a trombone, explosions from gas bubbles rising and then bursting at the surface of the lava lake create sound waves. Just as the shape of a trombone can change the pitch of the notes it produces, the geometry of the crater that holds the lava lake modulates its sounds. When the lava lake is deep down in the volcano’s crater, the sound registers at a lower pitch or frequency just like when a trombone is extended. When the lava lake rises up in the crater the pitch or frequency of the sound increases, just like when the trombone is retracted.




This study demonstrated the utility of infrasound monitoring at open-vent volcanoes. Monitoring infrasound resonant modes could be effectively used for anticipating future eruptions at both Villarrica and other basaltic volcano analogs, including Kilauea (Hawaii), Shishaldin (Alaska), Nyiragongo (Democratic Republic of Congo), Piton de la Fournaise (Reunion), Masaya (Nicaragua), Etna and Stromboli (Italy), and Ambrym and Yasur (Vanuatu).


COTOPAXI

Harmonic infrasound signals of exceptionally low frequency and long duration were observed at Cotopaxi Volcano (Ecuador) after a sequence of eruptions in 2015 changed the shape of the crater. The infrasound was remarkably stable over a period of six months providing an opportunity to understand the unique “voiceprint” of Cotopaxi. We were able to use infrasound observations to constrain the geometry of the immense crater at the summit.