Infrasound Tornillos Produced by Volcán Cotopaxi's Deep Crater

We characterize and interpret a new type of infrasound signal originating from the summit of Volcan 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 (f(p)=0.2 Hz) and high quality factor (Q=similar to 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 125m and length of 300m. Spectral properties of the tornillo permit constraints on crater sound speed (335m/s +/- 6%) and temperature (4-32 degrees 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.