Explosive processes during the 2015 eruption of Axial Seamount, as recorded by seafloor hydrophones

Following the installation of the Ocean Observatories Initiative cabled array, the 2015 eruption of Axial Seamount, Juan de Fuca ridge, became the first submarine eruption to be captured in real time by seafloor seismic and acoustic instruments. This eruption also marked the first instance where the entire eruption cycle of a submarine volcano, from the previous eruption in 2011 to the end of the month-long 2015 event, was monitored continuously using autonomous ocean bottom hydrophones. Impulsive sounds associated with explosive lava-water interactions are identified within hydrophone records during both eruptions. Explosions within the caldera are acoustically distinguishable from those occurring in association with north rift lava flows erupting in 2015. Acoustic data also record a series of broadband diffuse events, occurring in the waning phase of the eruption, and are interpreted as submarine Hawaiian explosions. This transition from gas-poor to gas-rich eruptive activity coincides with an increase in water temperature within the caldera and with a decrease in the rate of deflation. The last recorded diffuse events coincide with the end of the eruption, represented by the onset of inflation. All the observed explosion signals couple strongly into the water column, and only weakly into the solid Earth, demonstrating the importance of hydroacoustic observations as a complement to seismic and geodetic studies of submarine eruptions. Plain Language Summary Axial Seamount, a submarine volcano on the Juan de Fuca ridge, erupted in 2015. This eruption was recorded in real-time by an array of seafloor seismometers and hydrophones located on the volcano, and connected to shore by a power and data cable. Hydrophones recording the sounds generated by the eruption reveal several different types of explosions, including short bursts interpreted as lava-water interactions, and prolonged signals thought to be due to explosive ejection of gas and ash. These signals provide a window into the dynamics of the undersea eruption and are an excellent complement to other types of data including earthquakes and ground deformation.