Immediate Report of Victoria University Of Wellington Antarctic Expedition 1987-88: VUWAE 32
MOUNT EREBUS SEISMIC STUDY (K044)
MOUNT EREBUS SEISMIC STUDY (K044)
TV surveillance of the Erebus lava lake was restored to full operation on 17 November 1987 by replacing the camera box, the window of which had become opaque by etching in volcanic gas. The quality of the pictures was also improved by changing the lens from 16 to 25 mm, and raising the transmitter antenna.
The explosions were weaker than in 1986/7, but the lava lake was larger, and was L-shaped rather than round as before. By 3 January, 108 eruptions had been videotaped at the Scott Base receiving station, and the recordings from the Erebus seismic net had been played back, and later analysed for the 27 eruptions which were seen to eject bombs.
The total TV view time of the crater was 435 hours in 38 days, an average of 11.4±9.2 hours per day, and the average interval between eruptions was 4.03 hours. The eruptions were not randomly distributed, but were clustered in the periods 00-02 and 08-14 hours UT.
As in 1986/7, the TV explosion instant was earlier than the intercept time of P-waves (at zero distance) by 0.69±0.33 second, but the apparent velocity of P was lower, at 1.77±0.42 km/s. This reflects the smaller seismic amplitudes, and consequently later readings of emergent onsets at the more distant stations.
The seismic and infrasonic signals were all of the short period Alpha type, reported by Shibuya in 1984, reflecting the small but increasing area of the lava lake.
Erebus is a unique volcano. It is at higher latitude than any other active volcano above sea level, and is situated in an aseismic intraplate region. At present it has a negligible rate of lava eruption (c. 1 m3 per day) but it has maintained a liquid lava lake in its crater since before 1972. The phonolitic lava has higher silica content (56%) and theoretical viscosity (103 Pas at 1200°C) than any other persistent lava lake, yet it persists in unusually cold conditions at high altitude (3580 m).
Expeditions to the summit have consistently reported an average of several eruptions per day. These are accompanied by explosion earthquakes and as well, more than 100 volcanic earthquakes per day occur in the range ML-2 to +2. The foci are as deep as 15 km, and all the seismologists of the IMESS team (1980-1985) agreed that the foci of explosion earthquakes, identified by accompanying infrasound, extended down to 4 km below the active vent.page 15
The installation of TV monitoring of the lava lake in the 1986/87 season showed that surface explosions of the lava lake were the source of the explosion earthquakes, and suggested that the previously determined focal depths were an artifact of adopting model velocities which were lower than the true velocities in the volcano. Accurate determination of the true velocities is difficult because the onsets are emergent, but the seismic wave-forms of different explosions at the same station are often "identical" (cross correlation coefficients exceeding 0.85), and can be stacked to improve the signal to noise ratio. Volcanological observations around the world are just realising the importance of this, and I co-operated in discovering identical explosion earthquakes by computer search methods at Sakurajima volcano, Japan, in February 1988. The instrumentation at Erebus and recording facilities at Scott Base rival the best volcanological observations in the world.
The principal objectives of research for the 1988/89 season are to make the search for "identical" families of volcano-seismic events more efficient by installing a PC computer based digital seismic event recording system at Scott Base, and by using cross correlation, stack and residual software on all adequately recorded events in a routine manner, assemble enough data for reliable statistics and location of the families.
Scientific Endeavours and Achievements
The work fell into 4 categories:
4-9 November Preparation for ascent of Erebus
As well as the normal preparations for field work in mountainous areas (done by Lassky and Ellis), it was essential for Dibble and Ball to check that the Erebus telemetry equipment at Scott Base was in good condition and adjustment. Otherwise, much time could be wasted on the mountain trying to cure faults later found to be at Scott Base. For instance, the poor locking of the time code on the TV picture, which the 1987 Science lab Technician had blamed on the TV transmitter up Erebus, was found to be caused by the BNC connector to the time code generator being left half undone over more than half the year. Also, the time channel to the 14 channel Sony data recorder had been disconnected since no one knows when!
9-19 November Work on Erebus
A welcome change from the 1986/87 season was that VXE-6 resumed flying helicopters to the lower hut, thus reducing our dependence on toboggans to get equipment up the mountain. In a last-minute switch, however, they delivered our Grizzly to Fang Glacier, when we had all expected to be air-lifted from Fang acclimatisation camp to the hut. After some difficulty starting it because the switch was stiff, the page 16 Grizzly easily climbed the mountain with 2 people and their survival bags aboard. It was also used to take the replacement TV camera box and test equipment to the crater rim. The new box had a 2 watt heated window constructed from a 75 × 50 mm microscope slide with an evaporated gold film connecting between electrodes of circuit board material cemented along the ends. It was coated with Epithane 343 2-pot Polyurethane clear plastic to protect the fragile gold film, and sealed onto the main plate glass window with the gold surface on the outside, using silicon rubber, so that sealed air space was present between the two sheets of glass. The objective was 100% efficiency in evaporating an ice film which was presumed to be the cause of the fogging of the TV picture in 1987, but in fact the window had turned white due to corrosion by volcanic gases. Fortunately, the Epithane 343 resisted this corrosion efficiently.
Terry Ball tried to cure the faults in the camera which had nearly defeated us in 1986/87, by changing components as described in a manual belatedly supplied by Philips. Unfortunately, the instructions did not apply to our camera, and the original components had to be replaced with the fault uncured. Briefly, if the supply voltage drops momentarily, the camera locks into a stable inoperative mode.
While Terry wrestled with this, Ray Dibble, Steve Lassky and Brian Anderson (surveyor) were shifting the TV transmitter and antenna 70 m closer to Scott Base and adding 0.8 m to the mast so as to improve the ground reflected RF signal, and Susan Ellis was making infrared temperature measurements of the lava lake, and warm ground inside and outside of the craters.
Sunshine and shelter from the wind were important for comfort on the rim, and while Ray, Steve and Brian sweltered, Terry, Susan and their equipment 200 m away got cold, and the infrared measurement became unreliable. A further problem with the TV was that transmissions from the Tait radios near the camera also caused it to lock into inoperative modes, necessitating extra trips back to the summit, opening the camera box, and disconnecting/reconnecting the power. Perhaps it was one too many such actions which weakened the power connection so that it failed on 6 December after we had descended. We were greatly indebted to Bill McIntosh of S081, who found and cured the fault for us.
The type LX06002D infrasonic microphones at EI and CON were replaced by type LX0503A barometric pressure sensors to cure intermittent operation. Over most of the first half of 1987, the microphones operated only in the afternoon, seeming to work only in the sunshine, despite the very wide operating conditions of temperature (to −50°C) and pressure. Ice blocking the pin hole pressure ports was a possible cause, although the microphones had been entirely sealed inside condoms in 1985/86 to prevent this, and also corrosion by volcanic gas. The condoms were still in excellent condition, but the resistance of the strain gauge bridge in the microphones had changed for some reason. No explanation for the intermittent operation could be found, but it appears that the page 17 reliable life on Erebus of these comparatively cheap devices is only about 2 years. We already knew that expensive microphones would have a short life, so cheap ones are still the best.
At E1 we also reinstalled the long period horizontal seismometer provided by NIPR in 1986/87 in a niche in the lava dike, 2 m from the previous site, in order to reduce the tilting which had stopped it operating. The component was again radial to the lava lake.
19 November to 7 January 1988: Recording and Analysis of Data at Scott Base
It was the task of Susan Ellis, the longest staying member of the team, to videotape the Erebus TV as many hours a day as possible, and to fast scan each tape, and re-use those with no useful data. The San-ei visual writing seismic recorder was used to help find eruptions on TV. Susan copied each obvious eruption onto an edited videotape, and at the end of her stay, photographed the TV monitor display of the edited videotape with an NTSC camera and recorder provided by NIPR, to get an NTSC copy for Japan and USA. The original recordings were also kept.
The Erebus seismic network was being recorded as part of the normal work of Scott Base, and as each tape was taken off the Sony Data Recorder, Susan replayed it on a second machine at 100x recording speed into a log amplifier and 4 pen chart recorder to get short index charts of time scale 4 mm per hour of the entire 8 day record. Photocopies of these charts of log seismic amplitude will be sent to Drs Kyle, Kienle, and Kaminuma so that they can choose earthquakes for further analysis, or look at statistics of occurrence. (Normally, staff play tapes back at Scott Base × 10 faster, and record the entire waveforms).
For the well recorded explosion earthquakes, and other earthquakes, Susan made normal seismograms of all useful data channels by playing the tapes at recording speed and the chart recorder at 1 mm/s and 10 mm/s. Initially, there were only 6 channels (limited by discriminators) but after Prof. John Schlue of New Mexico Institute of Mining and Metallurgy had carried out some tests with long period seismometers at Abbotts Peak, he brought back the rack of discriminators which NSF had removed from Scott Base on 7 January 1987, and 8 channels were then recorded.
Susan's final task, before she returned to New Zealand on 8 January was to replace the Sony Data Recorder which had been recording Erebus data for 2 years, with the one used for playback, and to pack up the playback equipment for return to NZ and Japan.
7 January 1988 onwards:
Mr Stan Whitfield, the Science Lab Technician in charge of MEEMS at Scott Base for 1988, is continuing to record one or more videotapes each day until the power to the camera/transmitter switches off in the winter night, and to scan them, and re-use tapes page 18 with no recorded eruptions. He is also running the Erebus seismic recorders as for the previous IMESS project. Recordings up to 16 February were sent to NZ by the last flight out.
In New Zealand, Susan read the seismograms of explosion earthquakes to 4 January 1988, and plotted the time distance graphs and TV origin times. She also plotted the infrared temperatures, and compared them with those measured in the 1986/87 season.
The paper on the TV results presented as a poster at IUGG, Vancouver, September 1987 was written up in Japan for publication by the University of Kyoto Disaster Prevention Research Institute, and presented at their Annual Meeting on 3 February. A comparative study of Erebus and Sakurajima results using computer techniques was begun at Sakurajima Volcano Observatory in February 1988. A list of papers published for 1987 is shown in VUWAE publications.
The recognition of families of "identical" earthquakes is a fast developing area in volcanic seismology, because it enables the work of locating earthquake foci to be reduced and/or made more accurate. It is easy to do using digital seismograms, and then the stacked average seismogram with improved signal to noise ratio, and the residual waveform showing how individuals differ from average are easily obtainable.
Already an important difference between Erebus and Sakurajima has been found: Erebus explosion earthquakes are often identical from onset to coda, but Sakurajima ones become different in the coda, presumably because the source volumes of the explosions develop differently, even though they may initiate in the same place, and thus cause earthquakes with identical onsets.
Management of Science in the Ross Dependency
In regard to the MEEMS project, NZARP coped very well with the special time limitations, transport problems to and on Erebus, and co-operation with NSF season. The political problems of 3 way co-operation between RDRC, NSF, and NIPR are not yet resolved, and seem to revolve around the US requests for equitable opportunities for trade and scientific exchange with Japan.
Although the different financing and accounting procedures in those two countries make accurate comparison difficult, I believe that the Japanese investment in IMESS equalled that of USA, and that as Drs. Kienle, Kyle and Dibble spent a total of 7 months page 19 at NIPR as invited and supported scientists, with as yet no similar invitations to NIPR scientists from either NZ or USA, the Erebus program should be excluded from this confrontation.
The major problem of co-operation in IMESS was the delay in distributing data due to the intensive work of playback, and the time advantage that the institution doing the playback enjoyed. My good relationships with all the parties involved have survived the problem, and I am hopeful that our continued co-operation on a scientist to scientist basis will help reduce the present international and institutional problems.
There was not a single person who was less than helpful in avoiding delays and crises with the 1987/88 programme, but special mention must be made of Jim Barker, Garth Varcoe, Cass Roper, the OIC and Base Mechanic of Scott Base, and VXE6 Helicopter Squadron for giving us the priority and good will needed to make my extremely tight timetable for Erebus and the Sakurajima Volcano Observatory possible. I assure them all that it was very worthwhile.
I also thank my team of Susan Ellis, Terry Ball and Steven Lassky for their dedication, Brian Anderson, and Bill McIntosh for assistance on the mountain, Phil Kyle for co-operating in our descent from Erebus, and for supporting the continuation of the seismic programme with NSF, and Katsu Kaminuma for his full and generous support of the programme, even though he did not get permission to come to Scott Base this season.