Acoustic and tephra records of explosive eruptions at West Mata submarine volcano, NE Lau Basin

West Mata is a 1200 m deep submarine volcano where explosive boninite eruptions were directly observed in May 2009. Here we present long-term acoustic and tephra records of West Mata explosion activity from three deployments of hydrophone and particle sensor moorings beginning on 8 January 2009. These records provide insights into the character of explosive magma degassing occurring at the volcano's summit vent until the decline and eventual cessation of the eruption during late 2010 and early 2011. The detailed acoustic records show three types of volcanic signals, 1) discrete explosions, 2) diffuse explosions, and 3) volcanic tremor. Discrete explosions are short duration, high amplitude broad-band signals caused by rapid gas bubble release. Diffuse signals are likely a result of 'trap-door' explosions where a quench cap of cooled lava forms over the magmatic vent but gas pressure builds underneath the cap. This pressure eventually causes the cap to breach and gas is explosively released until pressure reduces and the cap once again forms. Volcanic tremor is typified by narrow-band, long-duration signals with overtones, as well as narrow-band tones that vary frequency over time between 60-100 Hz. The harmonic tremor is thought to be caused by modulation of rapid, short duration gas explosion pulses and not a magma resonance phenomenon. The variable frequency tones may be caused by focused degassing or hydrothermal fluid flow from a narrow volcanic vent or conduit. High frequency (>30 Hz) tremor-like bands of energy are a result of interference caused by multipath wide-band signals, including sea-surface reflected acoustic phases, that arrive at the hydrophone with small time delays. Acoustic data suggest that eruption velocities for a single explosion range from 4-50 m s^-1, although synchronous arrival of explosion signals has complicated our efforts to estimate long-term gas flux. Single explosions exhibit ~4-40 m³ s^-1 of total volume flux (gas and rock) but with durations of only 20-30 ms. Interestingly, explosion activity increased at West Mata for several months, observed at more distant hydrophone stations, following the September 2009 8.1 Mw Samoan earthquake. The tephra and hydrophone data were only synchronously recorded from January to May 2010, but these data indicate a repeated record of summit explosions followed by down flank debris flows, an important process in the construction of the volcanic edifice. Bathymetric differencing between 2010 and 2011 shows two large negative anomalies at the summit and a broad positive anomaly on the east flank, interpreted as a major slump that removed part of the summit during the final magma withdrawal related to formation of the summit pit crater.