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.
UR: http://www.ees.nmt.edu/Geop/Erebus/erebus.html