Seismic and acoustic observations at Mount Erebus Volcano, Ross Island, Antarctica, 1994–1998

Abstract

Volcanic activity at Mount Erebus is dominated by eruptive activity within a phonolitic summit lava lake. Common eruption styles range from passive degassing to Strombolian explosions, which typically occur several times daily, and occasionally in swarms of up to 900 per day. Shallow explosions, although generally the result of steady exsolution of volatiles from depth, can be triggered by surficial input of H2O through mass wasting of rock, snow and ice from the crater walls. Broadband observations of Strombolian explosions document very-long-period (VLP) signals with strong spectral peaks near 20, 12 and 7s, which are polarized in the vertical/radial plane. These signals precede lava lake surface explosions by ∼1.5s, are highly repeatable, and persist for up to 200s. First motions indicate a deflationary source, with any precursory inflation being below the ∼30s passband of our instruments. Particle motions suggest a VLP source residing up to 800m below the lava lake surface; however, this depth could be exaggerated by near-field radial tilt.Seismic and acoustic signals associated with lava lake explosions commonly show evidence for multiple bubble bursts in corresponding complexity features resulting from varying time delays and relative sizes of superimposed bursts. A systematic decrease in seismic/acoustic ratio for smaller surface explosions suggests that either the seismic energy from the smallest, shallowest bubble bursts experiences much greater seismic attenuation than energy arising from larger events which may involve a deeper, less attenuative portion of the magma column, and/or that the shallowest layer is seismically isolated from deeper parts of a stratified magma column, which are not excited by the smallest explosions due to sharp impedance contrasts across distinct layers.Tremor at Erebus is uncommon, with only a few isolated instances identified in five years of monitoring. Some tremor events are nearly monochromatic, and some exhibit numerous gliding harmonic spectral lines.