PAGEOPH, Vol. 125 No. 6 (1987)

0033-4553/87/060993-1651.50 +0.20/0 O 1987 Birkh/iuser Verlag, Basel

Seismic A c t i v i t y o f E r e b u s V o l c a n o , A n t a r c t i c a KATSUTADA KAMINUMA 1

Abstract--Mount Erebus is presently the only Antarctic volcano with sustained eruptive activity in the past few years. It is located on Ross Island and a convecting anorthoclase phonolite lava lake has occupied the summit crater of Mount Erebus from January 1973 to September 1984. A program to monitor the seismic activity of Mount Erebus named IMESS was started in December 1980 as an international cooperative program among Japan, the United States and New Zealand. A new volcanic episode began on 13 September, t984 and continued until December. Our main observations from the seismic activity from 1982-1985 are as follows: (1) The average numbers of earthquakes which occurred around Mount Erebus in 1982, 1983 and January-August 1984 were 64, 134 and 146 events per day, respectively. Several earthquake swarms occurred each year. (2) The average number of earthquakes in 1985 is 23 events per day, with only one earthquake swarm. (3) A remarkable decrease of the background seismicity is recognized before and after the September 1984 activity. (4) Only a few earthquakes were located in the area surrounding Erebus mountain after the September 1984 activity. A magma reservoir is estimated to be located in the southwest area beneath the Erebus summit, based on the hypocenter distributions of earthquakes.

Key words: Antarctica, Erebus Volcano, seismicity.

I. Introduction

Mount Erebus (3794 meters), located on Ross Island at 77~ 167~ is the only Antarctic volcano with sustained eruptive activity in the past few years. A convecting anorthoclase phonolite lava lake was discovered in the inner crater (200meter diameter, 95-meter depth) within the main crater of 500-600 meter and 150meter depth on 6 January 1973 (GIGGENBACHet al., 1973). The lava lake was present until September 1984, when a new volcanic episode occurred. Strombolian eruptions of anorthoclase phonolite lava occurred several times per day from the lava lake and surrounding explosive vents during the period between January 1973 and September i984. A program to continuously monitor the seismic activity of Mount Erebus and to identify its mechanism of eruptions was begun in December 1980 as an international 1 National Institute of Polar Research, 9-10, Kaga 1-chome, Itabashi-ku, Tokyo 173, Japan.

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Katsutada Kaminuma

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Figure 1 The configuration of the radio-telemetered IMESS seismic network on Ross Island. The abbreviations of the stations are as follows: Abbott Peak (ABB),The Erebus Summit (ERE; operated until 13 September, 1984), Hoopers Shoulder (HOO), Bomb (BOM), Mount Terror (TER), Lower Fang (LFA), Three Sisters Cone (TSC), Truncated Cone (TRC), Cape Barne (CBA), Crash Site (CRA) and Scott Base (SBA).

cooperative program among Japan, the United States and New Zealand, called the 'International Mount Erebus Seismic Study (IMESS)'. A tripartite array with three vertical, short-period seismographs was installed on Mount Erebus in December 1980. These stations were linked by radio-telemetry to Scott Base of New Zealand, at 77~ 166~ about 38 kilometers south from the Erebus summit (KIENLE et aL, 1981; TArZANAMTet al., 1983a, b). This net was expanded by two stations in the 1981-1982 field season, and by five additional stations in the 19831984 and 1984-1985 field seasons as shown in Figure 1 (KIENLZ et aL, 1982, 1983, 1984; Sr~IBUYAet al., 1983; UEKI et al., 1984; BABA et aL, 1985). The summit station (ERE in Figure 1) was destroyed by the September 1984 eruptions and the observations at ERE were terminated. All seismic signals were transmitted to Scott Base and recorded on magnetic tapes of a 14-channel FM data recorder and on a longterm chart recorder. At various times, infrasonic signals and magnetic induction signals at the Erebus summit were also transmitted to Scott Base for monitoring explosions and explosive gas charge from the lava lake (SmBuYA et aL, 1983; DIBBLE et al., 1984; KAMINtJMA et aL, 1985a). The space-time behavior of earthquakes occurring in and around Mount Erebus

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in 1980-1983 has been studied previously by K~ENLEet al. (1981, 1982), TA~ANAMTet al. (1983a, b), SmBUYA et at. (1983), UEKI et al. (1984) and KA~INUMA (1986). Numerous microearthquakes were associated with the Erebus magma system while some microearthquakes of seemingly tectonic origin were located on and around Ross Island (Rowe and KIENLE, 1986). The present paper is focused to identify the difference of seismic activities before and after the new volcanic episode in September 1984.

2. Seismicity in 1982-1984

The daily earthquake counts at Hoopers Shoulder Station (HOO in Figure 1) are given in Figures 2a and 2b. Only the events with S - P time less than 10 s and amplitude greater than 4 mm at HOO were counted and illustrated in Figure 2. Teleseisms and local earthquakes, which were estimated to occur outside Ross Island, were not counted. An example of seismograms of these two types of earthquake is given in Figure 3. Seismograms of earthquake and icequake swarms are given in Figure 4. Some typical volcanic earthquakes are shown with symbol (V) and some typical icequakes with symbol (I) in the figure. As shown in Figure 4, icequakes are characterized by higher frequency and shorter duration waveforms, making them relatively easy to distinguish from volcanic earthquakes. Only volcanic earthquakes were counted and shown in Figure 2. The daily number of earthquakes from 1982-1984 varied from 20 to over 250. The average daily number of earthquakes in 1982 was 64, and those in 1983 and 1984 were 134 and 146, respectively. Until the end of 1984, all stations of the network were interrupted during the Antarctic winter because solar panels were used to supply power. However more batteries at HOO were installed in December 1984, and HOO remained operational through the winter of 1985. Earthquake swarms frequently occur in and around the Erebus volcano. Sixteen earthquake swarms were detected during the 30-month period from 1982-1984, as shown with 82-A, -B and so on in Figures 2a and 2b (KAMINUMA e t at., 1986). An empirical definition of an earthquake swarm has been used: The earthquake swarm is defined here as a group of more than 250 events occurring within 24 hours, and the events occurring within a few days of a relatively large number of earthquakes on either side of the 250-event day. Swarm 82-C was the first such swarm studied in this area (KAMrNUMAet al., 1985b). This swarm was characterized by a very sudden onset and very rapid decrease of the activity. This pattern is very similar to the hourly earthquake frequency of the main shock-after shock type. All events of this swarm were located in the northwestern flank of Mount Erebus. Swarms 84-B and -F are characterized by a large number of earthquakes and long duration of activities, however their locations were

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DALLY NUMBER OF EARTHQUAKES 82-B

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not determined because each event was very small and recorded at only one or two stations. The hypocenter distributions of earthquakes for the period from February 1982 to May 1984 are shown in Figure 5. The P-wave structure used for the hypocenter determination is given in Table 1, The velocities below 8 km are based on the results of explosion seismic experiments in the McMurdo Sound (McGINNIS, 1980). 254

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DAILY NUMBER OF EARTHQUAKES 84-F

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earthquakes were located; vertical and horizontal location errors are estimated to be less than 3 km (KAMINUMAet al., 1985a). Many earthquakes are clustered 1-2 km northeast from the summit station in Figure 5. These earthquakes were observed to accompany the volcanic explosions.

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HOOPER

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Figure 3 An example of seismograms of teleseisms and local earthquakes on the monitoring chart of the long-term recorder at Hoopers Shoulder Station. These types of events were not included in the counts of volcanic earthquakes.

Table 1 P-Wave velocity model of Mount Erebus (Depth of '0' is the summit of Mount Erebus), Depth (kin)

V (kin/s)

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1.25 1.62 1.99 2.36 2.73 3.16 4.64 5.66 6.61

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3. The New Activity in September I984 A new volcanic eruptive episode started at 05h08 m (UT) on 13 September, 1984 with a number of large explosions. These events were recorded not only by the IMESS network but also by the WWSSN seismographs at Scott Base and a tidal gravimeter at the Amundsen-Scott South Pole Station, about 1400 kilometers from Mount Erebus. Previous explosions at the summit had only been recorded by the stations on the volcano (ERE, LFA, ABB, HOO, etc.). The daily number of earthquakes and of large explosions counted at Hoopers Shoulder Station (HOO) from 16 and 13 September, respectively are shown in Figure 6. The large explosions are defined in this paper as the events which were recorded

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Vol. 125, 1987

Seismic Activity of Erebus Volcano, Antarctica

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at Scott Base. Before 15 September, the IMESS network was not operating due to a flat battery. The daily number of explosions on 13-15 September in Figure 6 was counted on the WWSSN seismograms at Scott Base. From September 16 to 21, the station transmitted seismic signals less than 20 hours a day because solar panels provided insufficient power after the long winter. Dashed lines of the daily number of earthquakes in Figure 6 show the estimated daily numbers calculated from the seismic recording hours. Arrows show the days when the exact daily number could

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not be counted because of extreme microseisms, but the activity was essentially continuous. The number of events per day decreased strongly two weeks into the sequence and remained at a nearly constant level for the following two weeks. The earthquake activity continued, at this !evel fo{ the following 2.5 months. The activity quickly decreased :to less than 20 events pe r day at the end of December 1984. The explosions counted and illustrated in Figure 6 were the only large events recorded at Scott Base. The number of explosions itself is nearly the same during the above 2.5 months, however the magnitude of each explosion became too small in November and December to be recorded at Scott Base. Figure 7 shows an example of a volcanic explosion earthquake recorded at six stations (LFA, BOM, I2IOO, ABB, TER and SBA). This explosion occurred at about 00h32m10 s, 17 September 1984. The nearest station to this event was Lower Fang Station ( L F A ) w h i c h was 3.5 km away. Amplitude at the stations on the mountain area is off scale. The spectra of seismic waves at each station was obtained by digital processing. The spectra of BOM, H O O , ABB and T E R were taken from the first 60 seconds of seismogram, 'and those of LFA from the first 70 seconds. As the characteristic of the observation system at each station was the same, no instrumental correction was made for the spectra. An example of the spectra of an event that occurred at 22h42 m on 17 September is given in Figure 8. The spectra at each station has a sharp peak around 2.5 Hz. Location estimates could be determined for only twelve events in September and October, and other events were recorded at two or three stations. The accuracy of the location is the same as described in Section 2.

Vol. 125, 1987

Seismic Activityof Erebus Volcano,Antarctica

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The daily number of earthquakes and large explosions decreased to 3-15 after 3 December, as shown in Figure 6, and a new type of explosion started to occur after 6 December 1984. An example of the seismograms from such an explosion is shown in Figure 9. The waveform of this explosion is characterized by large amplitudes at the stations near the summit (TSC, LFA, TRC) and small amplitudes on the slope. Onset is indistinct and tremor-like with a longer duration (30-60 s). The event in Figure 9 was observed by Dr. P. Kyle of New Mexico Institute of Mining and Technology. He observed some explosions at the crater rim which he described as a 'gas jetting eruption' (personal communication). Although several stations were operating in September, October, November and December, only 14 explosions, including 12 in Figure 8, could be located reliably because of insufficient initial phase arrivals at the stations on the slope.

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Vol. 125, 1987

Seismic Activity of Erebus Volcano, Antarctica

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4. Seismicity in 1985 After adding more battery reserve to the Hoopers Shoulder Station in December 1984, the seismic signals at H O O started to be transmitted through the entire year. The earthquakes decreased suddenly to less than 20 per day after December 1984. Even though there was swarm-like activity, the average daily number of earthquakes in 1985 was 23 events per day, which is less than 2 0 ~ of those in the previous three years. Only one swarm (85-A) occurred in July. However, earthquakes decreased to

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less than 10 per day by August. After September there were some days when no earthquakes were recorded at all. Figure 10 denotes the location of earthquakes during the sixteen month period from September 1984 to December 1985. Note that when comparing Figure 10 with Figure 5, the number of earthquakes in Figure 10 is actually larger than that of Figure 5. The large number of stations operating in 1985 allowed most of the events to be located. Thus even though the seismicity rate was lower, more events could be located in 1985 than in the entire previous time span. The clear difference between Figure 5 and Figure 10 is that earthquakes in 1985 are concentrated in the Erebus mountain area. Before September 1984, many earthquakes were located outside of the mountain area. Almost all earthquakes in 1985 clustered near the summit; when the 4 kilometer diameter circle, at depths of 2 to 8 kilometer.

5. S w a r m Activities

As can be seen in Figure 2, earthquake swarms frequently occurred in the Erebus mountain area from 1982-1984, but only one swarm is recognized in 1985. Thus not only the background seismicity but also the swarm activity is decreased after the September 1984 activity. KAMINUMA et al. (1986) pointed out, based on the number of earthquakes and duration of activities, that swarm 84-B and -F might be precursor phenomena for the September 1984 activity. The decrease of the swarm activity in 1985 supports their results. Finally it is an interesting fact that swarms occurred most frequently in March and April, and June and July. The largest number of earthquake swarms for each year occurred in June (1983) or July (1982, 1984, 1985). To explain this fact must be one of the important items for future studies.

6. Discussion

An aseismic zone is recognized in the southwest area beneath the summit on the vertical profile in Figure 5. This aseismic zone was also reported by our previous papers (SHmvYA et al., 1983, UEKI et al., 1984, KAMINUNAet al., 1985b and BABAet al., 1985). We speculate that this aseismic zone might be a magma reservoir which was the magma source for the summit lava lake. To emphasize the idea of the magma reservoir, it is reported by our previous paper (BABAet al., 1985) that the spectra of seismic waves which passed through the aseismic zone has peak s in the lower frequency range of 1-3 Hz, as compared with waves passed through the other areas. From the aseismic zone, the scale of the

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magma reservoir is estimated to be approximately 2 kilometer in diameter, and 4 kilometer in vertical length. The earthquakes located between the summit and the upper part of the reservoir are mostly accompanied with the volcanic eruptions. The two swarms 84-B and -F are characterized by having longer duration and larger total number of earthquakes than other swarms which are shown in Figure 2, and KAMINUMAet al. (1986) pointed out that the two swarms seem to be precursor activities of the new volcanic episode on 13 September 1984. Before the September 1984 episode, we speculated that new magma must have moved from the deeper part of the crust and/or the upper mantle to the reservoir. The two swarms 84-B and -F might mark this event. As shown in Figure 2b, and Figure 10 the number of earthquakes decreased suddenly after the end of December 1984, and almost all earthquakes in 1985 clustered near the Erebus Mountain area. These facts suggest that the inner pressure of the reservoir had decreased and the tectonic stress in the Erebus mountain area had decreased to a state of very low seismicity. The scale and the location of the reservoir are estimated only roughly at this stage. To improve knowledge of the crustal structure, seven explosion seismic experiments at four different blast sites were carried out on Mount Erebus by the IMESS group in December 1984 (KAMINtJMA,1985). These data are being analyzed. After we obtain the detailed structure, the locations of all events should be redetermined, based on the new obtained structure. We believe that the following items will be studied using the IMESS data in the near future; (1) the structure of Mount Erebus, including the exact location of a magma reservoir, (2) more accurate earthquake locations using the structure, and (3) the mechanism of an eruption process of Mount Erebus.

7. Acknowledgement

Prof. Kienle, Geophysical Institute, the University of Alaska, has the responsibility to establish and maintain all seismic stations. Dr. Dibble, Victoria University of Wellington, has installed the infrasonic and magnetic induction sensor at the summit. We, Japanese party, have the responsibility to install and maintain seismic recorders at Scott Base and to make visual seismograms from the seismic magnetic tapes for the participants from the United States and New Zealand. Dr. P. Kyle of the New Mexico Institute of Mining and Technology, has always assisted the IMESS group for maintaining the stations and the operations for the seismic observation during their stay in McMurdo Station. The laboratory technicians of the New Zealand wintering parties in Scott Base have maintained the IMESS seismic recorders through the winter season since December 1980. The author expresses his sincere thanks to all participants mentioned above for their kind international cooperation, not only in the many operations in Antarctica but also the research works in each country.

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T h e research has b e e n s u p p o r t e d b y the N a t i o n a l I n s t i t u t e of P o l a r Research, J a p a n , the U n i t e d States, the N a t i o n a l Science F o u n d a t i o n , V i c t o r i a U n i v e r s i t y of Wellington, and Antarctic Research, N e w Z e a l a n d .

Division, Department

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