The figure at lower right shows a high-frequency local event recorded by the MEVO network. The impulsive arrivals at nearby stations and clear evidence of secondary phases in the traces suggest that such events are volcanotectonic in origin.
The calculated hypocenter falls within the perimeter of the network, roughly beneath the summit plateau of Erebus at a depth of 14 km below sea level. Events having these characteristics are rare on Erebus, comprising less than 1% of triggered seismicity. Volcanotectonic (VT) events represent shear slip on fractures within or beneath a volcanic edifice and they often accompany an anomalous influx, migration, or evacuation of material, and corresponding change in the deviatoric stress field, before, during or after eruption. The long-term, relatively steady-state behavior of Erebus suggests, however, that the supply of magma and volatiles is roughly constant and unobstructed, and hence no significant temporal stress changes are occurring. An exception to this steady-state behavior was the October 8-9, 1982 swarm of VT events associated with possible dike injection near Abbott Peak wherein over 700 events were observed to occur within a time span of 48 hours, accompanied by harmonic tremor.
A difficult station distribution, a complex three-dimensional velocity structure and somewhat emergent nature of the first arrivals at more distant stations all tend to confound the location process for many non-explosion events.
Low frequency events at Erebus are routinely noted. A majority of these events exhibit first arrivals at either ABB or BOM, located on the northwest and east sides of the volcano, respectively. Between 1994 and 1998, approximately 670 events previously classified as "volcanic long period" were recorded by the MEVO network. Most of these events are highly emergent, making identification of first arrivals, and hence location, difficult. Unlike the long-period (LP) events observed at other volcanoes, Erebus low-frequency events occurring in the vicinity of ABB and BOM exhibit a range of spectra across the network, from nearly monochromatic to rather broadband. It would thus be difficult to characterize a "typical" event. Further, many of the signals appear to be comprised of multiple superimposed events, or a single source with repeated and growing excitation. Occasionally, such events occur nearest to stations SIS, HOO and MAC. The figure shown right illustrates an example low-frequency event which exhibits a slowly growing signal intensity.
Most such low-frequency events appear to be very shallow in origin, although precise location - especially hypocentral control - is extremely difficult, given the emergent onsets. A small percentage of low-frequency events do resemble volcanic LP signals. These appear exclusively near stations ABB and BOM. The LP nature of this small population and their apparent source locations, coupled with the three-fold symmetry of Ross Island, suggests the presence of active radial dike systems between Mount Erebus and Mounts Bird, to the north, and Terror, to the east. In 1908, members of the Shackleton expedition reported fumarolic activity in the vicinity of Abbott Peak, as observed from some distance down the flanks of the volcano. No evidence has been found for hydrothermal alteration in the surrounding rocks, but in October 1982 a significant swarm of VT events was recorded by the analog seismic network, accompanied by periods of harmonic tremor. This brief but intense activity has been interpreted as a dike injection near Abbott Peak. In subsequent years, occasional LP events have continued to be observed near Abbott Peak (ABB station).