Grant-in-Aid for Overseas Scientific Survey.
|Research Institution||Tohoku University|
HAMAGUCHI Hiroyuki Faculty of Science, Tohoku University, Professor, 理学部, 教授 (20004385)
KAVOTA K.S. C. R. S. N(ザイール), 研究員
WAFULA Mifun ザイール, C. R. S. N., 研究員
ZANA Ndonton ザイール, C. R. S. N., 所長
西村 太志 東北大学, 理学部, 学振特別研究員 (40222187)
森田 裕一 東北大学, 理学部, 助手 (30220073)
林 信太郎 秋田大学, 教育学部, 助教授 (90180968)
笠原 稔 北海道大学, 理学部, 助教授 (40001846)
田中 和夫 弘前大学, 理学部, 教授 (40003511)
KAVOTHA K.S. ザイール, C. R. S. N., 研究員
MIFUNDU Wafu ザイール, C. R. S. N., 研究員
NDONTONI Zan ザイール, C. R. S. N., 所長
NISHIMURA Takeshi Faculty of Science, Tohoku University, Researcher
HAYASHI Shintaro Faculty of Education, Akita University, Associate Professor
KAVOTHA K. S. C. R. S. N. Lwiro Centre, Zaire, Researcher
WAFULA Mifundu C. R. S. N. Lwiro Centre, Zaire, Researcher
MORITA Yuichi Faculty of Science, Tohoku University, Research Associate
ZANA Ndontoni C. R. S. N. Lwiro Centre, Zaire, Directer
TANAKA Kazuo Faculty of Science, Hirosaki University, Professor
KASAHARA Minoru Faculty of Science, Hokkaido University, Associate Professor
|Project Fiscal Year
1990 – 1991
Completed(Fiscal Year 1991)
|Budget Amount *help
¥20,000,000 (Direct Cost : ¥20,000,000)
Fiscal Year 1991 : ¥8,000,000 (Direct Cost : ¥8,000,000)
Fiscal Year 1990 : ¥12,000,000 (Direct Cost : ¥12,000,000)
|Keywords||Volcanic eruption / Nyamuragira volcano / Zaire / Crustal movement / Magma chamber / Mantle structure / East African Rift Valley / Hotspot volcano / Magma / 東アフリカ地溝帯 / 火山噴火 / ザイ-ル / ニウムラギラ火山 / マグマ溜り / 結晶分化 / マントル / 活火山 / ニアムラギラ火山 / 広帯域地震観測 / 火山活動 / ホットスポット火山 / マグマ分化 / 火山性地震|
This study is divided into two parts : The one is to study the heterogeneity of the deep mantle and the core-mantle boundary beneath the African plate. The other is the geophysical surveys of active hotspot volcano Nyamuragira and Nyiragongo in the western branch of the East African Rift Valley. The field studies in 1990 were successfully carried out in Zaire, however those in 1991 have been interrupted by the social unrest that occurred on September 23, 1991. The followings are the brief summary of the present study.
(1) Observation of broad-band seismometer : The broad-band seismometers (CMG-3) were installed at Lwiro seismographic station (LWI), C. R. S. N., Bukavu, Zaire in 1990 and at Binza meteorological station (BNZ), Kinshasa in 1991. However, the seismometer at BNZ have been operated incompletely because the installation was interrupted by the social unrest. It can not be discussed the results for lack of data. We studied the heterogeneity in the mantle and core by using the da
ta of WWSSN and GDSN. The following results were obtained ; (a) the core-mantle boundary was bumped several kilometers just beneath the African continent, (b) the S velocity in the D" layer was 3-5% slower beneath Africa than the standard model such as PREM. (3) The temperatures on the surface of the core was estimated to be several ten mili-kelvin higher in the A-hemisphere including the Africa and Atlantic regions than in the P-hemisphere including the Pacific region.
(2) Volcanic earthquake and tremor : In addition to the four seismic stations of C. R. S. N., the eight temporary stations were installed in and around volcano Nyiragongo and Nyamuragira in 1990. A moderate size earthquake occurred at the SE part of Nyiragongo volcano on September 20, 1990, which induced a swarm of small earthquakes at the axis of the rift valley. The low frequency volcanic earthquakes accompanying the strombolian eruption at Nyamuragira volcano were located at the depth of 0.2-0.5 km just beneath the active vent. These events were explained by the downward single forces having 10^4-10^5 Newtons and 0.15 sec pulse width at a source.
(3) Crustal movements : The geodetic measurements at eight base lines crossing the caldera rim of Nyamuragira revealed 15-36 cm dilatations during the period of 1989 and 1990. This dilatations were due to the opening of the fissure genarated by the 1989 eruption in the caldera. On the other hand, the geodetic measurements at the base lines crossing the rift valley showed no significant changes, suggesting that the recent crustal movements were limited in the active volcano area.
(4) Magma activity : Temporal chemical variations at Nyamuragira were examined on two different time scale ; 1976-1899 (seven eruptions) and 23 days during the 1986 eruption. In both cases, lava has become more mafic with time, indicating that a zoned magma body, with less mafic magma at the top, is being sampled at sequentially greater depths over the course of time.
(5) Recent eruption : After two years and one month dormancy, Nyamuragira began to erupt on September 20, 1991 from a fissure with the strike of N30E at about 15 km NE from summit. A swarm of earthquakes for 3 days preceded the eruption by one week. The new cone grew up 60-70 meter high with basal dimension of 400 x 300 meters during a week.
It should be concluded, from what has been summarized above, that the present study contributed significantly to understand the intraplate hotspot activity in the African continent. Less