STROMBOLIAN ERUPTION-ASSOCIATED VERY LONG PERIOD OSCILLATIONS OF MOUNT EREBUS, ANTARCTICA

2002 Seismological Society of America Annual Meeting Abstract Volume.

ASTER, R.C., MAH, S., RUIZ, M., FROSTENSON, D., DESMARAIS, E., KYLE, P., Earth and Environmental Science Department, New Mexico Institute of Mining and Technology, Socorro, NM 87801, aster@dutchman.nmt.edu; McIntosh, W., and Dunbar, N., New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, Socorro, NM 87801.

Strombolian explosions from the persistent phonolitic lava lake of Mount Erebus have been intermittently observed with PASSCAL and (more recently) with permanent broadband seismometers at vent-radial distances from 0.7 to 2.5 km since 1996. Since 2000 a time-stamped infrared-sensitive camera on the crater rim has also provided video observations. Strombolian explosions are also ubiquitously accompanied by oscillatory very-long-period (VLP) signals observed in the near-field by broadband seismometers. VLP onsets precede seismoacoustic onsets by approximately 4 s, persist for up to 6 minutes, and dominate near-field displacements with approximate moments (up to 1013 N-m) that exceeds explosion short-period moments by approximately 2 orders of magnitude. VLP signals have modal spectra with principal periods near 20.7, 11.8, and 7.7 s and Q values of approximately 10. Stacks of similar VLP events show numerous higher modes, particularly at periods near 4.8 s, 3.5, and 2.6 s. VLP particle motions are approximately vent-radial, but show azimuthal and dip inconsistencies, probably due to an asymmetric quasistatic elastic response due to strength and/or topographic variations. We postulate that Erebus VLP excitation represents conduit constriction-induced resonance during refilling and/or lossy internal gravity waves excited in the strong pycnocline expected in a highly vesiculated upper conduit system. The moment discrepancy between the short-period and VLP signals indicates a gravitational energy source for VLP excitation from gravitational potential energy released during the buoyant ascension of the eruptive gas slug and subsequent refilling of the eviscerated lava lake. Seasonal variations in the time- and frequency-domain signature suggest that VLP excitation at Erebus is sensitive to progressive changes in shallow conduit conditions.