Geophysical Study on the Hotspot Volcanoes in the African Continent.

这项研究分为两部分：一是研究非洲板块深部地幔和核幔边界的不均一性。另一项是对东非裂谷西支的活动热点火山尼亚穆拉吉拉和尼拉贡戈进行地球物理调查。1990年在扎伊尔成功地进行了实地考察，但1991年的实地考察因1991年9月23日发生的社会动乱而中断。(1)宽带地震仪观测：1990年在扎伊尔布卡武Lwiro地震台站和1991年在金沙萨宾扎气象台安装了CMG-3型宽带地震仪。然而，由于社会动乱导致安装中断，BNZ的地震仪一直没有完全运行。由于缺乏数据，不能讨论结果。利用da…研究了地幔和地核中的非均质性。更多关于WWSSN和GDSN的信息。得到了以下结果：(A)核-地幔边界在非洲大陆下方几公里处隆起，(B)D“层中的S速度在非洲下方比标准模型(如PREM)慢3-5%。(3)包括非洲和大西洋地区在内的A半球的地核表面温度估计比包括太平洋地区在内的P半球的温度高出几十毫尔文。(2)火山地震和地震：除中国北半球的4个地震台外，1990年在尼拉贡戈和尼亚穆拉吉拉火山及其周围安装了8个临时台站。1990年9月20日，尼拉贡戈火山东南部发生中等规模地震，在裂谷轴线诱发了一系列小震。伴随着Nyamuragira火山的Stombolian喷发而发生的低频火山地震位于活动喷口下方0.2-0.5公里处。这些事件是由震源处具有10^4-10^5牛顿和0.15秒脉冲宽度的向下单力解释的。(3)地壳运动：1989年至1990年期间，在穿过尼亚穆拉吉拉火山口边缘的8条基线上进行的大地测量显示，地壳膨胀了15-36厘米。这种扩张是由于1989年火山口喷发造成的裂缝的打开。另一方面，穿过裂谷的基线上的大地测量没有显示出明显的变化，表明活火山地区最近的地壳运动是有限的。(4)岩浆活动：在两个不同的时间尺度上研究了尼亚穆拉吉拉的化学时间变化；1976-1899年(7次喷发)和1986年喷发期间的23天。在这两个例子中，熔岩都随着时间的推移变得更具镁铁质，这表明随着时间的推移，一个带状的岩浆体，顶部的镁铁质岩浆较少，随着时间的推移，正在顺序地在更深的地方取样。(5)最近的喷发：经过两年零一个月的休眠，Nyamuragira于1991年9月20日开始从N30E走向的裂缝中喷发，该裂缝位于山顶东北约15公里处。在火山喷发前一周，一群地震持续了3天。新的锥体在一周的时间里长出了60-70米高，底部尺寸为400×300米。从上面的总结可以得出结论，本研究对了解非洲大陆的板内热点活动有重要贡献。较少

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