Seismic, Acoustic, and Video Observations of Conduit Processes During Strombolian Explosions at Mount Erebus, Antarctica

Aster, R, Mah, S Y, Johnson, JAF, Kyle, P, McIntosh, W, Dunbar, N, Ruiz, M, Desmarais, E

Department of Earth and Environmental Science, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801 United States

Mount Erebus hosts a persistently active, open-conduit magmatic system that produces 2-10 similar Strombolian eruptions per day from a persistent lava lake. The lava lake, an open skylight to a shallow magma chamber, is composed of phonolitic magma with a near-surface viscosity 10^3 Pa-s. Since 1996, seasonal broadband seismic and infrasound data have been collected from the summit plateau and crater rim at vent distances between 0.66 and 2.5 km. Acoustic signals have very simple associated Green's functions and readily discriminate multiple and single gas slug eruptions. Acoustic observations show that typical Erebus Strombolian explosions consist of single gas slugs with mass flux rates of ~5 times 10^3 Kg/s and cumulative gas fluxes of ~2 times 10^3 Kg. Seismic signals show a complex short period (>1 Hz) signature dominated by lava lake refraction and seismic scattering in the upper volcano, accompanied at some stations by a ground-coupled airwave. Strombolian explosions are ubiquitously accompanied by oscillatory very-long-period (VLP) signals observed in the near-field by broadband seismometers. VLP signals have origin times that precede seismoacoustic origin times by approximately 4 s, persist for up to 6 minutes, and dominate the near-field Strombolian explosion displacement signal. VLP signals have modal spectra with principal periods near 20.7, 11.8, and 7.7 s and $Q$ values around 10. Stacking of similar VLP signals significantly decreases noise (especially the strong microseism near 7 s), enabling the detection of higher VLP modes, particularly at periods near 4.8 s, 3.5, and 2.6 s. VLP particle motions are approximately vent-radial, but show significant azimuthal and dip inconsistencies, suggestive of an appreciably asymmetric quasistatic elastic response for the summit crater region due to strength and/or topographic variation. We postulate that Erebus VLP signals represent either a long period mechanical resonance of the conduit system during refilling or internal gravity waves excited in the pycnocline of the vesiculated lava lake system by the buoyant ascension of eruptive gas slugs. Comparing seismograms from 1996-1997 with more recent examples shows that the VLP signature of Erebus is temporally varying and is thus sensitive to shallow conduit conditions.