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We characterize and interpret a new type of infrasound signal originating from the summit of Volcán Cotopaxi (Ecuador) that was primarily observed between September 2015 and March 2016, following the 2015 eruptive period. This infrasound waveform is a slowly decaying sinusoid with exceptional low-frequency (fp = 0.2 Hz) and high quality factor (Q = ~10) and resembles the shape of tornillo seismic waveforms. The repeating events, occurring about once per day in early 2016, are stable in frequency content, and we attribute them to excitation of a vertical-walled crater, with radius of about 125mand length of 300 m. Spectral properties of the tornillo permit constraints on crater sound speed (335 m/s ± 6%) and temperature (4–32°C). The initial polarity of the tornillos is predominantly a rarefaction and could reflect repeating crater bottom collapse events (implosions) or explosion sources whose infrasound is heavily modulated by the crater’s pipe-like geometry.

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Active volcanoes produce intense infrasound, or low-frequency sounds below 20 Hz, which may be recorded with specialized microphones many kilometers from a volcanic crater. An objective of volcano infrasound research is to infer volcano source processes, such as explosions, and the modulating influences of topographic effects and atmospheric transmission. This study reports on a novel type of signal recorded at Cotopaxi Volcano (in Ecuador) where the infrasound possesses a remarkably low frequency and reverberates for many tens of seconds. We attribute the form of these infrasound events to the geometry of Cotopaxi’s crater, which is a deep, steep-walled cylinder about 300-m deep and with a diameter of approximately 125 m and acts like a gigantic pipe resonator.

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This document was originally published in Geophysical Research Letters by Wiley on behalf of the American Geophysical Union. Copyright restrictions may apply. doi: 10.1029/2018GL077766

This document was originally published in [JOURNAL TITLE] by Wiley on behalf of the American Geophysical Union. Copyright restrictions may apply. doi: [DOI #]