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

Volcanic activity at Mount Erebus is dominated by eruptive activity within a phonolitic summit lava lake. Common eruption Styles range from passive de gassing to Strombolian explosions, which typically occur several times dail y, and occasionally in swarms of up to 900 per day. Shallow explosions, alt hough 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 explos ions document very-long-period (VLP) signals with strong spectral peaks nea r 20, 12 and 7 s, which are polarized in the vertical/radial plane. These s ignals precede lava lake surface explosions by similar to 1.5 s, are highly repeatable, and persist for up to 200 s. First motions indicate a deflatio nary sourer, with any precursory inflation being below the similar to 30 s passband of our instruments. Particle motions suggest a VLP source residing up to 800 m below the lava lake surface; however, this depth could be exag gerated by near-field radial tilt. Seismic and acoustic signals associated with lava lake explosions commonly show evidence for multiple bubble bursts in corresponding complexity featur es resulting from varying time delays and relative sizes of superimposed bu rsts. A systematic decrease in seismic/acoustic ratio for smaller surface e xplosions suggests that either the seismic energy from the smallest, shallo west bubble bursts experiences much greater seismic attenuation than energy arising from larger events which may involve a deeper, less attenuative po rtion of the magma column, and/or that the shallowest layer is seismically isolated from deeper parts of a stratified magma column, which are not exci ted by the smallest explosions due to sharp impedance contrasts across dist inct 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. (C) 2000 Elsevie r Science B.V. All rights reserved.