Geophysical Study on Hotspot Volcanisim in African Continent (Summary)

尽管非洲板块有许多热点火山，但扎伊尔东部的尼拉贡戈火山对当地居民的潜在威胁最大。由于(a) 1994年6月23日尼拉贡戈火山开始爆发，以及(b) 1994年7月中旬从卢旺达逃离的大约100万难民人口异常增加，这种潜力最近大大加强。我们在火山周围安装了地震-火山监测系统。仪器观测结果表明，1994 ~ 1996年尼拉贡戈火山地震群活动处于高位，分布在尼拉贡戈火山的NE、NW和SE部分。最近，在火山地区附近，感觉到的地震数量逐渐增加。自1994年8月以来，我们继续监测尼拉贡戈火山口熔岩湖水位的变化。结果表明，到1995年7月，熔岩湖水位以28 cm/d的恒定速率上升，玄武岩岩浆供给速率估计为2.2x10^5m^3/d。这种上升在1995年10月左右停止了。1996年2月以来，基瓦拉克湖北部湖水的颜色发生了变化，这表明湖底存在海底火山活动。上述观测结果清楚地表明，尼拉贡戈火山目前在各方面仍处于非常活跃的阶段。我们还研究了内核的非均质性和各向异性。通过PKP(BC)-PKP（DF）的差分行程时间来检验核心的异质性。旅行时残差的地理分布在距内核最外层约500 km处表现为一级异质性。球谐分析表明，在43 ~ 177个区域P速度较快，在183 ~ 43个区域P速度较慢，并简要讨论了该模式的地球物理意义。

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.