Plugs and chugs—seismic and acoustic observations of degassing explosions at Karymsky, Russia and Sangay, Ecuador

Frequent degassing explosions, occurring at intervals of minutes to tens of minutes, are common at many active basaltic and andesitic volcanoes worldwide. In August 1997, April 1998, and September 1998 we recorded seismic and acoustic signals generated at two andesitic volcanoes with ‘Strombolian-type’ activity. Despite variations in explosion frequency (5–15 h−1 at Karymsky as opposed to 1–3 h−1 at Sangay), the signatures of the explosions are remarkably similar at these two, diverse field sites. In all explosions, gas emission begins rapidly and is correlated with an impulsive acoustic pressure pulse. Seismic waveforms are emergent and begin 1–2 s before the explosion. We classify explosion events at the two volcanoes as either short-duration (less than 1 min) simple impulses or long-duration (up to 5 min) tremor events. Many tremor events have harmonic frequency spectra and correspond to regular 1 s acoustic pulses, often audible, that sound like chugging from a locomotive. Chugging events are intermittent, suggesting that the geometry or geochemistry of the process is variable over short time scales. We attribute the 1 Hz periodic chugs to a resonant phenomenon in the upper section of the conduit.