Very-long-period seismicity associated with the 2009–2015 reawakening of Cotopaxi Volcano, Ecuador

Cotopaxi is a large, ice-capped stratovolcano located in the Ecuadorian Andes. After 72 years of repose, Cotopaxi erupted on August 14, 2015. The precursory activity included long-period (LP) events followed by volcano-tectonic (VT) earthquakes, very-long-period events accompanying LP signals (VLP/LP events), tremor, deformation and SO₂ emissions. VLP/LP events were first observed at Cotopaxi in 2002, and persistently occurred from 2009 to 2014 and during the 2015 eruptions. Previous studies of the VLP/LP seismicity suggested that these events originated by repetitive volume changes in a crack due to degassing of water from magma at a depth of 2–3 km beneath the NE flank. Based on this interpretation, we estimated the magma volumes related to individual VLP/LP events from 2009 to 2015, which were systematically extracted from continuous seismic records of the Cotopaxi broadband seismic network. Based on the accumulated magma volume and the VLP/LP activity, our study is divided into seven periods (phases A − G), during which the magma supply rate significantly fluctuated. In phase E (June 1–July 27, 2015), before the eruptions, the magma supply rate increased. Degassing at the VLP source generated gas flows in the conduit and pre-eruptive tremor, gradually drying out a shallow hydrothermal system. In phase F (July 28–September 15, 2015), we estimated the highest magma supply rate, leading to magma fragmentation at the VLP source and eruptions accompanied by tremor. In phase G (September 16–December 29, 2015), the magma supply rate decreased, and overall eruptive activity, VLP/LP events, and tremor gradually waned. These results indicate that the VLP/LP events were likely generated by degassing from magma supplied to the VLP source through an intruded dike before and during the eruptions. The VLP/LP activity provides critical useful information about the magma supply rates that controlled eruptive and gas emission activity at Cotopaxi during this period and may help to constrain magma volumes during future reactivations.