Mount Erebus Volcano Observatory

Lava lake explosion seismograms exhibit similar waveforms up to frequencies of several Hz, suggesting consistent source mechanisms, locations, and ray path parameters. Shown above are seismograms from a typical explosion as recorded by the MEVO short-period (SP) network (left side) along with spectrograms (time-varying spectral amplitude plots) (right side) for two of the channels. Traces are self-scaled but maximum ground velocity (micrometers/second) is given for each trace. Note that the signal energy is largely confined between 2 and 8 Hz.

Explosions exhibit highly repeatable signals up to 20 seconds into their codas, indicating a very high degree of similarity in source and raypath parameters and a nondestructive source mechanism. Shown above are seventeen recordings of explosions from January, 1998, as seen at station ABB, 10 km from the lava lake, illustrating the overall similarity in gross waveform features.

This repeatability confirms that explosion events occur within a very limited source region, i.e. within the upper few meters of the lava lake. Theoretical calculations including parameters such as volatile content and magma viscosity predict that the dynamics of vesiculation and bubble growth within the phonolitic magma at Erebus will yield sufficient gas partial pressures to cause disruption and explosion only within the uppermost 60 m of the magma column (Dibble, 1994), further supporting the seismic observation of clearly repeating source/receiver paths for explosions.

Details of explosion onsets, however, exhibit short-period variability which suggests that the fragmentation mechanism (bubble bursting) is not identically repeated from event to event; rather, variations such as a stuttering source, multiple bursts or other complexity appear to be common. The image above illustrates the onset of seismic (left panels) and acoustic (right panels) signals for two explosion events, demonstrating parallel degrees of apparent simplicity (top event) or complexity (bottom event) at the onset. Recordings were made at seismic station E1S and co-located microphone E1LI, situated 0.7 km from the lava lake; traces show 2 seconds of data.

In December 1996 and January 1997, temporary broadband seismometers were deployed on the summit plateau of Erebus and significant new features of explosion signals were observed for the first time. These new observations are summarized in our recent Geophysical Research Letters publication, Broadband recordings of Strombolian explosions and associated very-long-period seismic signals on Mount Erebus volcano, Ross Island, Antarctica (Rowe et al, 1998).