Geophysical Study on Hotspot Volcanisim in African Continent

点击翻译按钮获取中文摘要

(1) We analyzed SmKS and PKP waves recorded by the GDSN in order to survey the lateral heterogeneity in the lower mantle and at the top of the Earth's core. The spherical harmonic analysis revealed that the hemishperical (degree 1) pattern in the heterogeneities predominants at the top of the Earth's outer core and inner core beneath the African continent.(2) We developed the small and low-power data logger with the precise GPS clock. This equipments were applied successfully to the field observations in the volcanic regions in Africa.(3) The Nyiragongo volcano in eastern Zaire presents the highest potential threat to human lives in Goma region, partly because of the abnormal increasing population fleeing from Rwanda and partly because of the lack of mitigation plan by the local government.(4) The recent volcano-magmatic activity of Nyiragongo and Nyamuragira are summarized as follows ;(a) After the initial episode of eruption in the lava-lake, 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 ocnduit changed to the open conduit system.(b) The seismic activity shows rather concentrated features at the depth of about 15 km just beneath the Nyiragongo carter. The earthquakes with very shallow focal depths were not located closely to the Nyiragongo crater.(c) 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.