Sources of Financial support

Proposed Programme

Studies of the physics and mechanisms of volcanic eruptions are not well advanced world-wide due to the rarity and brevity of eruptions and the danger to equipment from the volcano, the weather, and human interference. Yet they are essential if we are to reliably predict disastrous eruptions. Erebus offers a rare opportunity because it is continuously active, has a lava lake acting as a window to the magma chamber, is only mildly dangerous to equipment and personnel, and has no risk of water damage or human theft and destruction. Its situation in an aseismic region ensures that all the earthquakes have volcanic significance, and the relative lack of electronic and atmospheric pollution in Antarctica enable excellent telemetry of data and sampling of gas and heat output. The Antarctic Treaty has enabled International Cooperation and the sharing of costs and data to an extent which would have been next to impossible on most volcanoes. No other active alkaline volcano in the world can be studied so efficiently.

Thus our study of eruption mechanism is important world- wide, as well as because Erebus is a very large volcano of considerable importance for the understanding of the geodynamics and structure of Antarctica.

Our work has covered the distribution in space and time of volcanic earthquakes explosion earthquakes tectonic earthquakes, earthquake swarms and tremor, 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 column. Also our TV surveillance has been of considerable help to S-081 (USAP) and to K092 in their studies of erupted gases.

The principal objectives of the season were to collect digital and video data on families of explosion earthquakes for the velocity modelling of the volcano and its magma column and to determine changes in the temperature of the crater walls and lava lake since 1986 and 1987.

Scientific Endeavours and Achievements

No new equipment was introduced this year. Rather we reluctantly relinquished the Windless Bight Infrasonic Array, in order to return the radio thermoelectric generator (RTG) to NSF. The ever deepening snow covering the microphones was reducing the sensitivity to Erebus eruption signals.

Thanks to the dedication of Howard Nicholson and Bruce McGregor, the Scott Base recording equipment required little servicing on our belated arrival in Antarctica on 28 November (a record 4 turnarounds) and we immediately prepared for put in on Erebus.

We flew to Fang Glacier in 3 lifts (one for the Grizzly) starting mid afternoon on 2 December, as the weather improved. S- 081 had flown up a few hours before us. The advance party of S- 081 came down and drove them up next day, while we waited out our planned 2 day acclimitisation, and drove up to the tower hut with S-081 help on 4 December.

Work on the summit comprised changing the infrasonic microphone battery at Truncated Cones (CONI) and E1 (E1S1), and readjusting the long period horizontal seismometer at E1 (E1LH). We tested and checked the receiver and antenna at MACZ station, newly installed by S-081, to find why it was not receiving and relaying the signal from BOMB, and checked that it was in the best possible site.

We moved up to the upper hut for 2 days, and using the space heater to keep the Nimbus stacking refraction seismograph warm, recorded 2 seismic lines 180 and 330 m long from the hut to Nausea knob for shallow velocity data. The surface velocity was 3000+/− 300 m/s on the undisturbed volcanic cone, and 2100+/−300 m/s on the old slump under the hut. The seismic source was sledge hammer blows for the 180 m line, and 1.5 m lengths of detonating cord laid on the surface for the 330 m line. Electric dets and the Nimbus shot box were used without incident by taking care to earth ourselves before handling detonators, and holding the bare ends of wires in each hand before connecting them, so that static voltages were discharged through the body. The start circuit for the Nimbus was wrapped around the det cord so it was opened by the shot, thus overcoming a fault in the start signal provided by the Nimbus shot box. A minimum of 5 shots had to be stacked to record at 330 m distance, and unluckily the Nimbus double-triggered on the 5th one, and ruined the record. It is estimated that 10 times more charge per shot win be required for the main survey next season.

While Dibble and O'Brien did the seismic survey, Shimizu and Kennedy began infrared temperature measurements at the crater rim, and continued after we all shifted back to the lower hut. To begin with, fume obscured the view inside the crater, and on subsequent attempts the thermometer malfunctioned in that the spot which was being measured could not be seen through the viewfinder. Inspite of the hand warmers strapped around it, the instrument got too cold sitting on its tripod, and became unreliable.

Upon return to Scott Base, Shimizu immediately began playback of analog magnetic tapes containing seismic data. Significant earthquakes since August 1989 were played back to charts from these tapes. He also collected amplitude data from E1SPZ events recorded on the SAN-EI long term chart recorder for that period. O'Brien began continuous video recording of the TV signal from the Erebus summit and took over archiving well recorded earthquakes on the digital seismograph from McGregor. During his stay 270 three hour videotapes were recorded, on which the lava lakes were visible 60% of the time. 16 explosive-type eruptions of the eastern lava lake were recorded on VCR and by the six seismograph stations and two microphones on the volcano. After Dibble and Shimizu's departure for New Zealand, O'Brien continued the magnetic tape playback, printing two sets of charts for study in Japan and New Zealand. 1782 events triggered the digital seismograph during O'Brien's laboratory work, but only 247 were volcanic earthquakes recorded well enough for focal determination.

Publications

Mr O'Brien intends to use the 1989/90 data for an M.Sc. thesis on eruption mechanism, and seismic velocity modelling of Erebus, beginning this year. Dr Dibble will publish the two conference papers presented to IAVCEI and IASPEI in 1989, and contribute to a memoir on Erebus to be published by AGU. Dr Shimizu and Prof Kaminuma will publish their latest seismic results in the Memoirs of the National Institute of Polar Research.

Environmental Impact

The impact on Erebus was reduced this year by the removal of disused microphones from the mountain side, and the return of about 12 reels of spare rope and wire from the upper hut to New Zealand. There is now very little unused equipment or materials on the volcano, and that in use is neat and tidy. The previous transmitter site at the summit of Tenor, which was out of helicopter range from December 1986 until 1989, has been cleared completely and returned to normal.

Future Research

It is anticipated that sufficient digital earthquake recordings will be available by the end of 1990 to define enough earthquake families to establish an approximately correct velocity structure, and focal distribution within the volcano, and the present leader plans to remove the VUW equipment from Erebus by mid January 1991, for re-deployment on some other very active volcano. Also, his compulsory retirement on 31 Dec 1991, will impede further work in Antarctica.

However, I see volcanic seismology as near the end of the pioneering phase, in which researchers settled for minimum equipment to obtain the skeleton of the situation. Already earthquake seismologists are using arrays of 100 to 500 geophones to study earth structures far simpler than a volcano. The ease and reliability with which Erebus has been instrumented may attract more ambitious researchers. Until then, the activity of Erebus is important and unique enough to warrant a minimum monitoring programme, by one or two telemetry seismometer/microphone stations and a two channel San-ei long term ink-chart recorder.

I would recommend Truncated Cones for the telemetry site, because it has very good signal/noise characteristics, and is easy to reach by motor-toboggan from the lower hut on Erebus. Historically, victoria University has refused to support monitoring programs on volcanoes, and handed the equipment at both Ruapehu and White Island volcanoes over to DSIR when the work reached that stage. They may do the same at Erebus, but I consider that the TV surveillance equipment is too precarious for long term monitoring, and that a 16 channel digital seismograph would be wasted on only two channels, and should be removed. I would be happy to consult and cooperate with anyone about this. NIPR and/or NSF may be interested.

Management of Science in the Ross Dependency

Antarctic Division are to be congratulated on the improved management of Scott Base and the field events, and the excellent cooperation with USAP achieved by the new SENSREP system. The cleanliness of the base area is also most impressive.