Drumbeat LP "Aftershocks" to a Failed Explosive Eruption at Tungurahua Volcano, Ecuador

Highly periodic, repetitive long-period (LP) earthquakes, known as "drumbeats," have been observed at a range of volcanoes, typically during the ascent of degassed magma. Accelerating rates of drumbeats have been reported before explosions and potentially offer forecasts of future activity. However, the broader phenomenology of drumbeats is poorly understood. Here we describe an episode of over 900 LP earthquakes recorded in November 2015 at Tungurahua Volcano, Ecuador, that we believe are associated with a failed explosion. Rates of LP drumbeats accelerated for 10 hr, consistent with an inverse Omori's law. Before any explosion occurred, seismicity decreased following Omori's law, over a further 6 days. Despite earthquake rates decelerating, amplitudes, spectral peaks,Qvalues, and periodicity remain constant, suggesting there is little change in the source process with time. We argue that the decelerating seismicity is a result of progressive reduction of gas flux, unable to provide sufficient overpressure for explosion.