Geophysical Study on the Hotspot Volcanoes in the African Continent.

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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 … More 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