GCMD

Summary

Abstract: Studies of the location of earthquake foci at Mt Erebus found that eruption earthquakes had an apparent range of depths to 4km, but that infrasonic signals were more consistant with a surface origin. Although a possible explanation was time error in picking emergent seismic onsets, and inaccurate modelling of velocity structure, a more attractive option was the triggering of eruptions in the vents by earthquakes at depth. The International Mt Erebus Eruption Mechanism Study (IMEEMS) was initiated to find if the earthquake origin times were earlier than or equal to the times of visible eruption, as required by the triggering hypothesis, using video recordings of eruptions with accurate time display.

The aim was to collect digital and video data on families of explosion earthquakes for the velocity modelling of the volcano and its magma column. TV surveillance equipment and an LPH geophone was added to an existing seismic telemetry net to compare the onset times of signals for each eruption. Each season, TV surveillance equipment was maintained and surveillance initiated, and infrasonic microphones and long period seismographs were serviced. Eruptions were video taped and the recordings from the seismic net were played back and analysed for the eruptions seen to eject bombs. A digital seismic event recording sysem was installed November 25, 1988 using a PC computer based system in parallel with the analoge tape recorder. Changes in the temperature of the crater walls and lava lake was measured with infrared temperature measurements each year.

Overall this project covered the distribution in space and time of volcanic earthquakes, explosion earthquakes, tectonic earthquakes, earthquake swarms and tremors, explosion infrasonic waves, magnetic induction signals from eruptions, infrared temperatures, eruption velocities and volumes of lava bombs, and the velocity structure of the erupting magma columns. A pilot seismic refraction survey was made on the summit cone of Mt Erebus in 1989-1990 along a 200m line between the upper hut and Nausea Knob. Two seismic lines were recorded 180, and 330m long for shallow velocity data. The seismic source was sledge hammer blows for the 180m line and 1.5m lengths of detonating cord laid on the surface for the 330m line. In the following season, to eliminate the possibility that the observed delay to seismic waves from strombolian explosions was caused by thick and/or very low velocity layers under the recording stations on the flanks of the volcano, a seismic refraction line was run between the warm ground in the Side Crater and the somma rim at the edge of the summit plateau.