After twenty years of seismic observations on Mt. Erebus with only rare incidences of sustained tremor-like signals being recorded, tremor activity became conspicuous around June 2000. More than 310 tremor episodes were recorded through 2004, with three main periods of activity (February 2001, May-June 2002 and January-June 2003), as documented by Ruiz, 2003. These tremor episodes showed a variety of waveforms, from chaotic to highly harmonic. Harmonic tremor episodes commonly exibit a fundamental frequency near 1 Hz with up to 28 harmonics. Episodes showing gliding (proportional shifting of all spectral peaks) are commonly seen. Rapid-fire tremors, a sequence of many equally spaced short tremor events, have also been recorded. This increase of tremor activity was not accompanied by increased activity at the crater of Erebus, where an open lava lake and persistent low-level Strombolian activity have long been observed. Instead, the onset of tremor has corresponded with the calving of several megaicebergs from the Ross Ice Shelf, and their drifting into the Ross Island area. The tremor source has been identified as ice-ice and ice-ground collisions involving these icebergs, and is currently a topic of interdisciplinary research between Erebus researchers and several other groups of glaciologists and oceanographers.

MacAyeal, D. R., Okal, E., Aster, R., Bassis, J., Seismic and hydroacoustic tremor generated by colliding icebergs, J. Geophys. Res., 113, F03011, doi:10.1029/2008JF001005, 2008.

Martin, S., Drucker, R., Aster, R., Davey, F.,, Okal, E., Kim, Y.-J., Scambos, T., MacAyeal, D., Kinematic and Seismic Analysis of Giant Tabular Iceberg Breakup in the Coastal Regime of Antarctica, submitted, 2009.

Seismic instrumentation at Mt. Erebus volcano. Short period stations, primarily sensitive to frequencies above 1 Hz, and having relatively low dynamic range (so that large events may be clipped) are represented by green square and Broadband, high-dynamic range seismic stations are denoted by red circles. The E1S broadband station has been operating since 1999; other broadband stations were installed in December, 2002.

Monthly number of iceberg tremor episodes recorded at Mt. Erebus since June 2000. The dashed curve shows the relative radiation of seismic energy associated with tremor activity.

Histograms showing distribution of fundamental frequency of iceberg harmonic tremors recorded on Mt. Erebus volcano. The fundamental frequency is defined as the lower frequency value of a harmonic spectrum. Usually this value is taken in the portion of the seismogram with the largest number of harmonics. Large numbers of tremor episodes reported with frequencies smaller than 0.5 Hz likely reflect the increment of MEVO broadband seismic stations operating since January 2003.

Distribution of the number of overtones associated with iceberg tremor episodes recorded on Mt. Erebus volcano since June 2000. Less than 10% of the events do not exhibit any significant peak in the signal spectrum. Approximately 20% of the events are monochromatic, meaning they have a spectrum with a single peak. More than 70% of the tremor episodes have at least two harmonic peaks.

Introduction

Representative Spectrograms