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Victoria University Antarctic Research Expedition Science and Logistics Reports 1981-82: VUWAE 26

Erebus Studies and IMESS (K4) - R. Dibble

Erebus Studies and IMESS (K4) - R. Dibble.

Additional data on seismic velocities within and below Erebus were recorded on the IMESS seismic telemetry net (Fig. 1). During epidentre determinations from recordings made early in 1981, Kienle et al. (1981) confirmed the near surface velocity of 1.5km/s determined by the writer in 1975. From recordings of 3 distant earthquakes in November/December 1981 (Table 1), the writer has calculated velocities along the ray paths between sea level and the telemetry station elevations on the assumption that the velocity structure below the Scott Base seismograph is identical to that below sea level under Erebus. Of 8 determinations shown on Fig. 1, 6 lie between 3 and 5.7km/s, and have a mean of 4.5 ± 1.0km/s. The other two values of 1.5 and 15km/s were discarded as anomalous.

Velocities below sea level were determined with the aid of a large seismic shot fired at 04h34m54.48s UT on 23 November 1981 by L.D. McGinniss party. The shot point was at 165.720°E, 77.835°S, and it was recorded at Scott Base, Hoopers Shoulder (166.90°E, 77.538°S, 1900m), Abbott Peak (166.90°E, 77.460°S, 1793m), Bomb (c. 167.43°E, 77.512°S, 1800m), and Terror (c. 168.54°E, 77.518°S, 3230m). Summit (167.15°E, 77.532°S, 3794m) was not operating at the time. The time versus slant-distance graph (Fig. 2) shows an apparent velocity across the net of 7.2km/s with a time intercept of 1.60s. This is more likely to be a downdip than an updip velocity due to isostatic subsidence of the volcanic pile, and indicates oceanic rather than continental type crust under Ross Island.

Table 1. Data for 3 distant earthquakes recorded on the IMESS telemetry net in December 1981. dt (P) is the observed P-time difference t(station) - t(Scott Base); Va is apparent P-velocity across the net from J-B Tables; Ve is the computed velocity within Mount Erebus.

Table 1. Data for 3 distant earthquakes recorded on the IMESS telemetry net in December 1981. dt (P) is the observed P-time difference t(station) - t(Scott Base); Va is apparent P-velocity across the net from J-B Tables; Ve is the computed velocity within Mount Erebus.

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Fifty five eruptive events observed by eye and ear between 27 November and 12 December 1981 are listed in Table 2. Most of these were also recorded on the tape seismograph and infrasonic recorder operated in the summit hut at the levels listed. The larger eruptions again had the characteristic infrasonic signature reported in 1978/79 and 1980/81, and this allowed a further eleven eruptions to be identified instrumentally. The relation between recorded seismic and infrasonic levels are shown in Fig. 3. For a given seismic level, the infrasonic levels (which are related to eruption intensity) varied over a range of 30dB, indicating variation in the partition of energy between seismic and eruptive phenomena.

To check the party's impression that eruptions tended to occur in the early morning, a diagram was constructed of the frequency of occurrence of recorded earthquakes with the time of day (Fig. 4). This was done separately for large earthquakes (which are usually accompanied by eruptions), medium and small earthquakes, and compared with the average diurnal gravity tide (predominantly a diurnal tide of solar origin). There was indeed a peak in large earthquakes between 2 and 6 hours NZST which is just after the maximum tidal gravity (i.e. low earth tide). No obvious correlation with tide exists for medium earthquakes, but small earthquakes were most frequent between 12 and 14 hours NZST which corresponds with minimum tidal gravity (high earth tide). The correlations are of doubtful significance, however, because with the exception of an early morning peak in large earthquakes in December 1974, no correlations were obvious during the other four observation periods.

Further progress was made in the study of earthquake size versus frequency of occurrence. Figure 5 shows that the b-value for small to medium earthquakes in November/December 1981 was 1.6, and not significantly different from that in the 1980/81 season. For the large earthquakes the b-value for combined 80/81 and 81/82 data is only 0.6, indicating that frequency decreases less rapidly with increasing size for eruptive events than for the smaller volcanic earthquakes. Probably there is a mixture of two populations rather than a transition from one slope to the other, and the previous interpretation of a preferred size and upper limit to explosive eruptions of Erebus can no longer be sustained. The concept of different slopes for b-type and eruption earthquakes is important for risk assessment at other volcanoes.

References

Kyle, P.R., Dibble, R.R., Giggenbach, W.F., Keys, J.H., 1982: Volcanic activity associated with the anorthoclase phonolite lava lake, Mt. Erebus, Antarctica, In Craddock (Ed.), Antarctic Geoscience, University of Wisconsin Press, IUGS Series B - 4.

Kienle, J., Kyle, P.R., Estes, S., Takanami, T., Dibble, R.R., Submitted 1981: Seismicity of Mt. Erebus 1980/81, Antarctic Journal of the United States National Science Foundation.

Takanami, T., Terai, K., Osada, N., Kienle, J., Estes, S., Kyle, P.R., Dibble, R.R., 1981: Earthquake Observations at Mt. Erebus, Antarctica. Part 1 (in Japanese), Seismological Society of Japan, Proceedings of the Annual General Meeting, October 1981.

Takanami, T., 1981: Earthquake Observations at the summit of Mt. Erebus, Antarctica (in Japanese), Kyokucki (Polar News, Japan Polar Research Association), 33: 52-7.

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Figure 2: Relation between recorded levels of seismic signals and infrasonic signals of eruptions at the summit of Erebus. For a given seismic level, the range of infrasonic level is 25 dB.

Figure 2: Relation between recorded levels of seismic signals and infrasonic signals of eruptions at the summit of Erebus. For a given seismic level, the range of infrasonic level is 25 dB.

Figure 3: Average diurnal variation of earthquake occurrence rate at Erebus summit between 29 November and 10 December 1981. Large earthquakes (>600 W) were not frequent between 2 and 6 hours NZST, while small earthquakes (0.6-6 W) were most frequent between 10 and 18 hours NZST and correlated with the mean diurnal gravity tide.

Figure 3: Average diurnal variation of earthquake occurrence rate at Erebus summit between 29 November and 10 December 1981. Large earthquakes (>600 W) were not frequent between 2 and 6 hours NZST, while small earthquakes (0.6-6 W) were most frequent between 10 and 18 hours NZST and correlated with the mean diurnal gravity tide.

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Figure 4: Earthquake occurrence rate versus size at the summit of Erebus. The levels and slopes of the graph are similar in the 80/81 and 81/82 seasons, and show different b-values for small earthquakes and for the larger earthquakes which are usually accompanied by eruptions.

Figure 4: Earthquake occurrence rate versus size at the summit of Erebus. The levels and slopes of the graph are similar in the 80/81 and 81/82 seasons, and show different b-values for small earthquakes and for the larger earthquakes which are usually accompanied by eruptions.

Figure 5: Travel time graph of seismic waves from a large explosion fired by L.D. McGinnis in McMurdo Sound which was well-recorded on the IMESS net.

Figure 5: Travel time graph of seismic waves from a large explosion fired by L.D. McGinnis in McMurdo Sound which was well-recorded on the IMESS net.

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Table 2. Eruptions of Erebus between 27 November and 12 December 1981.

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PLATE II: Sphincter corer ready to drop (left), and resulting core (below). (See next page).

PLATE II: Sphincter corer ready to drop (left), and resulting core (below). (See next page).