The Nyiragongo volcano in eastern Zaire was chosen in 1991 as the African Decade Volcano by IAVCEI as an activity of International Decade of Natural Disaster Reduction (IDNDR) Program. This volcano presents the highest potential threat to human lives among the many East African active volcanoes. Recently, this potential has been much enhanced by several factors such as; (1) The simultaneously increasing volcanic activities at Nyiragongo and Nyamuragira. (2) The abnormal increasing population fleeing from the neighbouring country Rwanda. (3) The lack of mitigation plan by the local government. After about 12 years quiescence since 1982, the eruption started at the summit crater of Nyiragongo on June 23, 1994. The geophysical observation was commissioned by UNHCR. The minimum baseline monitoring has been urgently carried out at the requests of UNHCR at Geneva and of the Ministry of Foreign Affairs, Japan.
Observation
Five seismic stations, Kunene (KNN), Rusayo (RSY), Kibati (KBT), Mt. Goma (MTG) and Airport (APT), were operated at the southern part of Nyiragongo and Nyamuragira volcanoes. Each station was equipped with the three components seismometer of f02 Hz and the wireless telemetering transmitter. The recording station was equipped by three PC's with MO disk driver, GPS clock and multi-penrecoder. The portable bubble type tilt meter were also deployed at three stations. Nyiragongo's fumarolic area was monitored by a thermister thermometer with data logging system. The preliminary data analyses such as hypocenter location, variations of tremor amplitudes have been routinely performed at Goma Volcano Observatory, in collaboration with colleagues of C. R. S. N., Zaire. The information of volcanic activity has been transmitted to UNHCR/Goma and the local government. The observation has been continuing since November 13, 1994.
Summary of Volcanic Activity
(1) Whenever volcanic tremors are observed at the station at KBT or RSY, the eruptive activity in the lava-lake of Nyiragongo has been always recognized by an eye-witness. After the initial episode of eruption in the lava-lake of Nyiragongo, we observed at least 37 eruptive episodes in the period from June, 1994 to February, 1995. The most significant features in the episodic pattern are not only the intermittent appearance of active periods but also the change of their tempo that happened at the middle of December, 1994. Since January, 1995, the intermittency has declined and continuous features of tremors with small amplitudes became predominant, which suggests that dike-like conduit that has been held open by excess reservoir pressure changed to the open conduit system, in which lava can lift without resistance. If this speculation is correct, in near future, Nyiragongo may show nearly steady state lava lake activity in the summit, which has been seen in the past.
Fig. 18.@Map of located hypocenter of the A-type earthquakes from November to December, 1994. A predominant feature is the clustering at around the summit of Nyiragongo. The most of thd earthquakes are located at a depth around 1 km. The cross is the telemetering seismic station. The open square indicates the major refugee camp.(2) The volcanic earthquakes were classified into two types on the basis of their waveform. The so-called A-type event is characterized by clear P and S phases having rather high frequency content. The C-type is by a small P and obscure S phases with low frequency component. The seismic activity of the A-type events shows rather concentrated features at the depth of about 15 km just beneath the Nyiragongo crater. The A-type earthquakes with very shallow focal depths were not located closely to the Nyiragongo crater, indicating that there were effectively no brittle failures of rock on the mountainside caused by excess magma pressure.
Fig. 19.@Lava lake activity in the summit of Nyiragongo. (Oct. 14, 1994)(3) The pattern of continuously recorded tilt vector indicates that Nyiragongo has slowly inflated since the beginning of eruption. This inflation was probably caused by the storage of excess magma in the very shallow reservoir or in the lava lake. In the initial stages of the volcanic activity, the tilt disturbances (or tilt tremors) were occasionally observed for several days. The amplitudes of this disturbance is 2-3 micron radian and its period ranges from several seconds to 1000 seconds.
(4) The temporal changes of surface level of the lava-lake is compiled for the period between June 23, 1994 and March 15, 1995. The average magma supply rate was estimated to be 1.3~10 5m^3/day or 1.5 m^3/sec.
Fig. 20.@Red glow over the top of the Nyiragogo (Oct. 15, 1994)(5) The following scenario is still most probable at this moment. If hydrostatic pressure exceeds the breaking strength of surrounding rocks at Nyiragongo, the huge amount of lava that has persisted in the summit crater will drain suddenly into the flank through fissure (s), of which direction (s) is not defined. The sudden rise in the lava level in the summit crater and associated fissuring at Nyiragongo are the most dangerous feature to the refugees and residents around Nyiragongo. (6) In order to detect the precursory phenomena, it is important not only to maintain the monitoring system operated at the ground surface but also to add the monitoring of temporal variation of lava level at the summit.
(written by Hiroyuki Hamaguchi)
Go back