Geophysical Study on Hotspot Volcanisim in African Continent

(1)分析了GDSN记录的SmKS和PKP波，考察了下地幔和地核顶部的横向非均质性。球谐分析表明，非均质性的半周期（1度）模式在地球外核顶部和非洲大陆之下的内核中占优势。(2)研制了具有精确GPS时钟的小型低功耗数据记录仪。该设备已成功地应用于非洲火山地区的野外观测。(3)扎伊尔东部的尼拉贡戈火山对戈马地区的人类生命构成了最大的潜在威胁，部分原因是逃离卢旺达的人口异常增加，部分原因是当地政府缺乏缓解计划。(4)尼拉贡戈和尼亚穆拉吉拉最近的火山岩浆活动总结如下：(a)在1994年6月至1995年2月期间，我们观测到熔岩湖首次喷发后，至少有37次喷发。这种偶发型的最显著特征不仅是活跃期的间歇性出现，而且是1994年12月中旬发生的活跃期节奏的变化。1995年1月以来，断续性减弱，以小振幅连续型为主，表明堤状管道系统向明渠系统转变。(b)地震活动在尼拉贡戈河正下方约15公里处显示出相当集中的特征。震源深度非常浅的地震并不靠近尼拉贡戈火山口。(c)连续记录的倾斜矢量模式表明，尼拉贡戈火山自喷发开始以来一直在缓慢膨胀。这种膨胀可能是由于过量的岩浆储存在非常浅的储层或熔岩湖中造成的。

(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.