Download A SHORT NOTE ON : BASIC BEHAVIOR OF THE MOUNT

A SHORT NOTE ON :
BASIC BEHAVIOR OF THE MOUNT GAMALAMA'S ERUPTION
Lilik Hendrajaya1), Surono2) and Gunawan Handayani1)
1)Earth Physics Laboratory, Dept. of Physics
Institut Teknologi Bandung
2)Volcanological Survey of Indonesia, Bandung
Abstrak
Dikemukakan suatu hipotesis bahwa jika kantung magma suatu gunungapi sedang dalam
kondisi penuh, maka gempa tektonik di daerah tersebut dapat memicu terjadinya erupsi.
Secara mekanika, ini cukup beralasan mengingat bahwa kantung magma dan isinya berada di
dalam kerak bumi yang mengalami tekanan yang berasal dari dorongan kerak samudra yang
bersubduksi. Pengamatan dilakukan di Gunung Gamalama di Pulau Ternate sebelah barat
Pulah Halmahera, Maluku Utara. Pada daerah ini diharapkan terdapat kondisi tekanan
tektonik yang kuat. Hasil pengamatan menunjukkan bahwa picuan gempa tektonik terhadap
erupsi vulkanik dapat teramati. Hasil lainyya yang penting adalah permasalahan proses
terjadinya pengulangan erupsi dan pengetahuan mekanisme erupsi.
Abstract
Hypothesis that the tectonic force release through earthquake will trigger eruption of near by
volcano when its magma pocket is full of magma, is put forward. It is mechanically reasonable
due to the fact that both magma and the pocket are inside the earth crust which suffers
compression stress resulted from the pushing force of oceanic crust in the subduction zone.
Observation is conducted at Mount Gamalama in the island of Ternate west of the Halmahera
Island, North Molluca. It is expected to have strong pushing force condition. The result shows
that the triggering phenomenon is present. The other important results are the understanding
of process to produce repeated volcanic eruptions and the knowledge of eruption mechanism.
1. INTRODUCTION
Gamalama volcano is one of 129 active volcanoes in Indonesia. It can be categorized
as a strato type volcano and is located on Ternate Island, west Halmahera Island of Molucca
Island. Its exact coordinate at 0o48' northern latitude and 127o69.5' eastern longitude with its
top elevation of 1715 m above sea level2).
The tectonic setting of the Gamalama volcanoes shows that it is located on subduction
zone of Pacific-South Asia plates. Figure 1 shows hypocenter distribution of earthquakes
occuring in this region. In this position, it is hypothesized that the magma formation is due to
melting resulted from friction of the Pacific plate that moves downward beneath the SouthAsia plate4), proposed a subduction zone model that describes the tectonic setting around
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Halmahera Island. Figure 2 presents the subduction zone model between Sngihe arc and
Halmahera arc based on earthquake hypocenter distribution as presented in Figure 1.
Regarding the position of Gamalama in this tectonic setting, it is hypothesized that if
the volcano pocket (chamber) is full of magma then any release of tectonic force through
earthquake in surrounding area may trigger volcanic eruption. This can be understood, as a
matter of fact the magma filled pocket in the crusth crust may suffer compression stress from
the pushing force of subducting oceanic plate. The body of Gamalama volcano consists of
basalt rock and andesitic pyroxenite rock which also contains olivine and diabase2).
This history Gamalama eruption indicates that the main eruption of Gamalama can be
described by explosive eruption followed by lava flows2,3,5). The most recent eruption
occurred on August 5, 1994. This eruption can be categorized as magmatic eruption from the
main crater. The following is a short analysis of Gamalama volcano repose period eruption
pattern and its eruption indicator before the August 1994 eruption.
2. ERUPTION REPOSE PERIODS
Hendrajaya1) in his analysis pointed out that repose periods of main eruptions of some
active volcanos in Indonesia showed important patterns. From these patterns, one can derive
important clues about the nature of the magma chambers of the volcanos. In order to have a
meaningful pattern of repose periods, one has to be selective in picking the main eruptions to
be plotted in the repose periods. Only strong magmatic eruptions have to be plotted.
From its story (of eruption), the main eruptions of Gamalama were marked by
explosive ones followed by lava flows2,3,5). For these main eruptions, their repose periods are
plotted in Figure 3. It can be seen from Figure 3 that in the last decade, the eruption intervals
were almost constant, which were about 26 years. Every main eruption was followed by other
smaller eruptions or lava flows that could extend until 5 years before the activies stopped and
the process of build up of energy and magma started again. It seems that the main eruption
was a process to open magma conduit and the following eruptions were processes of clearing
the magma pocket.
On April 25, 1990, an explosive eruption occurred that was followed by lava flows.
Since the, tremor activities increased until May 21, 1993 where another magmatic eruption
happened. The most recent magmatic explosion occurred on October 15, 1994. From the
pattern of the magmatic explosions, it can be seen that Gamalama volcano has a repose period
of 26 years during which its single magma pocket is refilled.
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3. SEISMIC OBSERVATIONAL SYSTEM
Seismic data was collected from Gamalama seismic stations since 1988. The common
analysis of the seismic data includes counting the number of recorded earthquakes, either
tectonics of volcanics, using a PS-2 seismograph, which is a single-vertical-component
seismograph.
A more detailed observation was carried out during the period of September 13 to
September 30, 1993. Five modern seismograph HADES 110 with 1 MByte hard disk (made in
France) were used. Time synchronization was done using a standard Omega watch which
could receive signals from the Reunion Island. This watch synchronized and measured time
with a resolution of 1/1000 second at every seismic stations. This triggering method used the
ratio of signal amplitudes of two different periods :
STA amplitudeaverageof short time period
=
LTA amplitudeaverageof long time period
This triggering method prevents large-amplitude-noise to trigger the HADES 110, since noise
generally has a quasi-constant energy. So only a real earthquake with a sudden shock energy
can trigger the HADES 110. In order to enter all parameters such as sampling rate, earthquake
signal duration, STA/LTA constant, date, time etc., a portable personal computer (386 SX, 33
MHz, 120 Mb) was used. Considering the frequency of 100 Hz and record duration of 2048
second. The time recorded was the East Indonesia Time.
The five seismic stations along with their area names, equipment codes, geographical
coordinates, elevations and station names are following :
1. Batu Hangus, 114, 0o50.25' northern lattitude, 127o21.21' eastern longitude, 135 meters,
BANG.
2. Tolire, 117, 0o50.13' northern lattitude, 127o17.80' eastern longitude, 58 meters, TOLI.
3. Fitu, 116, 0o46.16' northern lattitude, 127o20.01' eastern longitude, 140 meters, FITU.
4. Dobi, 119, 0o47.38' northern lattitude, 127o17.81' eastern longitude, 155 meters, DOBI.
5. Marikubu, 121, 0o47.71' northern lattitude, 127o21.20' eastern longitude, 360 meters,
MARI. Later, this station was moved to 127o21.29' eastern longitude (MAR2) to obtain
betted data.
4. SEISMIC DATA ANALYSIS
During the period of September 13 to September 30, 1993, some tectonics and
volcanic earthquakes were recorded, which can be categorized as :
• earthquakes recorded by only one station
• earthquakes recorded by more than two station
Using those data the following analysis were carried out :
1. Manual determination of first arrivals of P. and S waves.
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Preliminary determination of earthquake hypocenter without taking into account structural
model of seismic rock velocities i.e. assuming the seismic waves propagate in a
homogeneous material.
Plotting the hypocenters horizontally and vertically
Spectral analysis
4.1. Results of Analysis
Figure 4 sgows an example of typical earthquake recorded at 4 stations DOBI, BANG,
MAR2 and FITU. Figure 5 presents the plot of its corresponding hypocenter at 2.4oN and
127.11o E in the depth of 10.14 kilometers. The typical result of displacement spectral
analysis of thus particular data, without equipment and gain corrections, was presented in
Figure 6a, 6b, 6c, 6d. It can be seen that typical frequency spectrum shows a flat part with a
corner frequency at about 9 Hz.
The hipocenter distribution which is plotted with respect to time can be used to predict
the migration of the earthquake hypocenters. Figure 7a, 7b and 7c present the location and
distribution of the hypocenters of the volcanic earthquakes of Gamalama which occurred at
different times.
Figure 8a, 8b, 8c, 8d and 8e illustrate the monthly occurrence number of the
earthquakes from 1988 to 1992. On February 12, 1988, a magmatic eruption occurred which
was preceded by many tectonic earthquakes. It seems that the eruption did not throw out all
the material in the magma pocket, so on April 25, 1990, when another tectonic earthquake
occurred, a magmatic eruption was triggered. It can be seen that after resting for 26 years
since 1962 the magma pocket was full and tectonic earthquakes could trigger a magmatic
eruption. However if the magma pocket had already been empty, the tectonic earthquakes
could only trigger phreatic eruptions producing smoke and vapor.
5. CONCLUSION
This short note shows that there is interrelationship between tectonic activities and the
occurrences of magmatic eruptions of Gamalama volcano. The data show that apparently the
Gamalama's eruptions behavior followed a 26 year repose period and the conduit has no
significant resistance against the increasing magma pressure to produce eruption.
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6. ACKNOWLEDGMENTS
For this study the support by the Directorate General of Higher Education Competitive
Research Grant Scheme is highly appreciated. The authors thank the Volcanological Survey of
Indonesia, for providing data information and for the chance to utilize the observation system.
References
1. Hendrajaya, L., "Repose Period of Volcanic Eruption, Does it Mean Anything?", Buletin
Geologi, v.23, No.1, 1-15, (1993).
2. Koesoemadinata, K. (Ed), "Data Dasar Gunungapi Indonesia", Direktorat Vulkanologi,
819 pp, 1979.
3.
4.
5.
Pardiyanto, L., Suratman, and Bauan, I.N., "Laporan Sementara Erupsi G. Gamalama 25
April 1990", Dit. Vulkanologi, K90-1, 1990, unpublished.
Sumanjuntak, T.O., "Neogene Tectonics and Orogenesis of Indonesia", Jurnal Geologi
dan Sumberdaya Mineral, v.11, no. 2, 1-31, (1993).
Suratman, "Peletusan G. Gamalama September 1980", Proc. PIT IX HAGI, 1980.
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Figure 1. Distribution of tectonic earthquake in the Sangir-Halmahera region (after
Simanjuntak4)
Figure 2. Subduction zone in the Sangir-Halmahera region through NW-SE Cross
Section (after Simanjuntak4)
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Figure 3. Repose period of magnetic eruptions of Mount Gamalama (data taken from
Kusumadinata2) and observational reports)
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Figure 4. Typical seismograms records at stations DOBI, BANG, MAR2 and FITU
Figure 5. Example of preliminary determination of earthquake hypocenter for an event from
2.4oN and 127.11oE with depth of 10.14 km.
Figure 6a
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Figure 6b.
Figure 6c
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Figure 6d.
Figures
6a, 6b, 6c, 6d. The typical results of displacement spectral analyses of a volcanic earthquake
record at DOBI, BANG, MAR2 and FITU without equipment and gain corrections.
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Figure 7a. Epicenter distribution of volcanic earthquakes on September 14, 1993.
Figure 7b. Hypocenter distribution of volcanic earthquakes on September 14, 1993.
Figure 7c.Hypocenter distribution of volcanic earthquakes on September 14, 1993.
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Figure 8a. Monthly number of earthquakes in 1988.
Figure 8b. Monthly number of earthquakes, in 1989.
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Figure 8c. Monthly number of earthquakes, in 1990.
Figure 8d. Monthly number of earthquakes, in 1991.
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Figure 8e. Monthly number of earthquakes, in 1992.