Sustained Seismic Signals
Two types of sustained seismic signals are observed at Erebus: chaotic disturbances originating in various locations about the volcano and lasting no more than three to five minutes at a time; and occasional episodes of harmonic tremor originating closest to stations ABB and HOO, with durations usually ranging from a few to ten minutes.
Many of the chaotic signals observed appear to originate nearest to side-crater station E1S and may represent degassing episodes from the lava lake. A definitive acoustic signal has not been recorded in association with these events; however, videocamera recordings of the lava lake confirm episodes of extended energetic, non-explosive degassing at the volcano summit. Rock and ice avalanches within the crater provide another potential source for these signals. MEVO personnel on the crater rim in December 1997 reported a large debris and ice avalanche within the crater; however, little simultaneous seismic signal was recorded in association with this avalanche event. We therefore interpret most emergent, extended, chaotic signals originating nearest station E1S as likely lava lake degassing. The figure above illustrates a typical such event recorded by the MEVO network. The right-hand panel shows spectrograms for closest and most distant stations E1S and ABB, respectively; note that they do not contain continuous consistent, extended spectral peaks, hence this would not be considered harmonic tremor. Spectral content more closely resembles that of a series of lava lake explosions with appreciable excitation to 7 Hz, yet the seismograms are far more emergent and sustained than those of discrete explosion events of comparable amplitude.
Chaotic, sustained signals are also observed on the volcano's flanks. These tend to occur near the heavily glaciated portions of the mountain. Because no evidence exists for eruptive activity or fumarolic activity outside of the immediate region of the summit, we interpret these signals as failure within the Ross Island glaciers.