Geophysical Study on Hotspot Volcanisim in African Continent (Summary)

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Although there are many hotspot volcanoes in the African plate, the Nyiragongo volcano in the eastern Zaire presents the highest potential threat to the local habitants. This potential has been recently much enhanced by (a) the starting volcanic eruptions at Nyiragongo on June 23,1994, and (b) the abnormal increasing population of about 1 million refugee fleeing from Rwanda on the middle of July, 1994. We have installed the seismo-volcanic monitoring systems around the volcano. Based on the instrumental observations, we showed that the seismic swarm activities have been high stages in the period between 1994 and 1996 and that they were located at the NE,NW and SE parts of Nyiragongo volcano. Recently, the number of felt earthquakes increased gradually around the volcanic area. We have continued to monitor the change of lava-lake level in the crater of Nyiragongo since August, 1994. We showed that the level of lava lake has ascending with a constant rate of 28 cm/day until July 1995 and that the supply rate of basaltic magma was estimated to be 2.2x10^5m^3/day. This ascend was stopped around October, 1995. The color of lake water in Kivulake changed at the northern part of the lake since February, 1996, which suggests the existence of submarine volcanic activity on the lake bottom. The above mentioned our observations clearly show that the present volcanic activity of Nyiragongo is still very active stages in all aspects.We also investigated the heterogeneity and anisotropy in the inner core. The heterogeneity in the core was examined by using differential travel times of PKP(BC)-PKP(DF). The geographical pattern of the travel time residuals was found to be a manifestation of degree one heterogeneity in the outermost about 500 km of the inner core. Spherical harmonics analysis showed that the P velocities were fast in the region from 43ﾟ E to 177ﾟ E and slow in the region from 183ﾟ W to 43ﾟ E.Geophysical implications of this pattern are briefly discussed.