A thorough re-evaluation of local non-explosion events recorded by the MEVO network raises the possibility that a large percentage originate within the glaciers covering Ross Island. The figure right shows a pair of very energetic probable icequakes which occurred approximately 500 m from station ICE, displayed with self-scaled network traces. Characteristics of the seismograms at nearby stations ICE and MAC suggest a volcanotectonic origin; however the impulsiveness and short-period nature of the signal degrade quickly with distance. Further, attenuation is much stronger than that observed for volcanotectonic events, as shown by amplitudes indicated in the figure, ranging from over 50 microns/sec of clipped amplitude for closest stations to about 1 micron/sec at more distance stations.
For energetic ice sources within 300 m of a station, instrumental distortion can dominate the recorded signal. Shown at left in the upper panel is an energetic icequake which occurred very near to station HOO. The strongly monochromatic nature of the signal is thought to result from a 24-25 Hz resonance of the L4C seismometer itself, which has experienced a strong, high-frequency excitation. A similar signal characteristic has been observed on L4C instruments recording icequakes at Redoubt volcano in Alaska, which is also extensively glaciated. This resonance is not clearly observed if an event occurs even a small distance from the nearest seismometer, although significant high-frequency components are visible. The lower panel illustrates a series of likely icequakes which occurred near BOM station within a few minutes of the HOO event in the upper panel. Ice activity appears to be episodic at Erebus. A seasonal trend in total triggered seismicity with events concentrated in the Austral late summer/early fall may reflect seasonal changes in glacial movement.